initial wip

2021-12-22 19:42:00 +01:00 · 2021-12-22 19:42:00 +01:00 · 3c558d9e5b
commit 3c558d9e5b
20 changed files with 1056 additions and 0 deletions
--- a/.gitignore
+++ b/.gitignore
@ -0,0 +1,2 @@
+/target
+Cargo.lock
--- a/Cargo.toml
+++ b/Cargo.toml
@ -0,0 +1,19 @@
+[package]
+name = "tower-helmet"
+description = "Helps with securing your tower servers with various HTTP headers "
+version = "0.1.0"
+authors = ["Atrox <hello@atrox.dev>"]
+edition = "2018"
+license = "MIT"
+repository = "https://github.com/atrox/tower-helmet"
+homepage = "https://github.com/atrox/tower-helmet"
+categories = ["asynchronous", "network-programming", "web-programming"]
+keywords = ["http", "tower", "security", "service", "header"]
+
+[dependencies]
+futures = "0.3.18"
+http = "0.2.5"
+pin-project-lite = "0.2.7"
+tower-layer = "0.3.1"
+tower-service = "0.3.1"
+lazy_static = "1.4.0"
--- a/22
+++ b/22
@ -0,0 +1,22 @@
+The MIT License
+
+Copyright (c) 2021 Atrox <hello@atrox.dev>
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+'Software'), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
--- a/README.md
+++ b/README.md
@ -0,0 +1,40 @@
+# tower-helmet
+
+this is still very **work in progress**
+
+a port of the beautiful [helmet.js](https://github.com/helmetjs/helmet) in the javascript world.
+
+`tower-helmet` helps you secure your tower server by setting various HTTP headers. _It's not a silver bullet_, but it can help!
+
+You can find a list of all available headers under the [header] module. By default (with [HelmetLayer::default]) **all of them** are enabled.
+Please take a good look at [ContentSecurityPolicy]. Most of the time you will need to adapt this one to your needs.
+
+# Examples
+
+```rust
+use tower_helmet::header::{ContentSecurityPolicy, ExpectCt, XFrameOptions};
+use tower_helmet::HelmetLayer;
+
+// default layer with all security headers active
+let layer = HelmetLayer::default();
+
+// default layer with customizations applied
+let mut directives = HashMap::new();
+directives.insert("default-src", vec!["'self'", "https://example.com"]);
+directives.insert("img-src", vec!["'self'", "data:", "https://example.com"]);
+directives.insert("script-src", vec!["'self'", "'unsafe-inline'", "https://example.com"]);
+let csp = ContentSecurityPolicy {
+  directives,
+  ..Default::default()
+};
+
+let layer = HelmetLayer::default()
+    .disable_strict_transport_security()
+    .disable_cross_origin_embedder_policy()
+    .content_security_policy(csp);
+
+// completely blank layer, selectively enable and add headers
+let layer = HelmetLayer::new()
+  .x_frame_options(XFrameOptions::SameOrigin)
+  .expect_ct(ExpectCt::default());
+```
--- a/src/header/content_security_policy.rs
+++ b/src/header/content_security_policy.rs
@ -0,0 +1,135 @@
+use crate::IntoHeader;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::HeaderValue;
+use lazy_static::lazy_static;
+use std::collections::HashMap;
+
+lazy_static! {
+    static ref DEFAULT_DIRECTIVES: HashMap<&'static str, Vec<&'static str>> = {
+        let mut m = HashMap::new();
+        m.insert("default-src", vec!["'self'"]);
+        m.insert("base-uri", vec!["'self'"]);
+        m.insert("block-all-mixed-content", vec![]);
+        m.insert("font-src", vec!["'self'", "https:", "data:"]);
+        m.insert("frame-ancestors", vec!["'self'"]);
+        m.insert("img-src", vec!["'self'", "data:"]);
+        m.insert("object-src", vec!["'none'"]);
+        m.insert("script-src", vec!["'self'"]);
+        m.insert("script-src-attr", vec!["'none'"]);
+        m.insert("style-src", vec!["'self'", "https:", "'unsafe-inline'"]);
+        m.insert("upgrade-insecure-requests", vec![]);
+        m
+    };
+}
+
+/// `ContentSecurityPolicy` sets the `Content-Security-Policy` header which helps mitigate cross-site scripting attacks, among other things.
+/// See [MDN's introductory article on Content Security Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP).
+///
+/// This middleware performs very little validation. You should rely on CSP checkers like [CSP Evaluator](https://csp-evaluator.withgoogle.com/) instead.
+///
+/// If no directive is supplied and `use_defaults` is `true`, the following policy is set (whitespace added for readability):
+/// ```text
+/// default-src 'self';
+/// base-uri 'self';
+/// block-all-mixed-content;
+/// font-src 'self' https: data:;
+/// frame-ancestors 'self';
+/// img-src 'self' data:;
+/// object-src 'none';
+/// script-src 'self';
+/// script-src-attr 'none';
+/// style-src 'self' https: 'unsafe-inline';
+/// upgrade-insecure-requests
+/// ```
+///
+/// Examples:
+/// TODO
+pub struct ContentSecurityPolicy<'a> {
+    pub use_defaults: bool,
+    /// Each key is the directive name in kebab case (such as `default-src`).
+    /// Each value is a vector of strings for that directive
+    pub directives: HashMap<&'a str, Vec<&'a str>>,
+    /// If `true`, [the `Content-Security-Policy-Report-Only` header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy-Report-Only) will be set instead.
+    pub report_only: bool,
+}
+
+impl ContentSecurityPolicy<'static> {
+    /// Returns the default directives
+    ///
+    /// ```text
+    /// default-src 'self';
+    /// base-uri 'self';
+    /// block-all-mixed-content;
+    /// font-src 'self' https: data:;
+    /// frame-ancestors 'self';
+    /// img-src 'self' data:;
+    /// object-src 'none';
+    /// script-src 'self';
+    /// script-src-attr 'none';
+    /// style-src 'self' https: 'unsafe-inline';
+    /// upgrade-insecure-requests
+    /// ```
+    pub fn default_directives() -> &'static HashMap<&'static str, Vec<&'static str>> {
+        &DEFAULT_DIRECTIVES
+    }
+}
+
+impl<'a> Default for ContentSecurityPolicy<'a> {
+    fn default() -> Self {
+        ContentSecurityPolicy {
+            use_defaults: true,
+            directives: HashMap::new(),
+            report_only: false,
+        }
+    }
+}
+
+impl<'a> IntoHeader for ContentSecurityPolicy<'a> {
+    fn header_name(&self) -> HeaderName {
+        if self.report_only {
+            http::header::CONTENT_SECURITY_POLICY_REPORT_ONLY
+        } else {
+            http::header::CONTENT_SECURITY_POLICY
+        }
+    }
+
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue> {
+        let directives = if self.use_defaults {
+            if self.directives.is_empty() {
+                DEFAULT_DIRECTIVES.clone()
+            } else {
+                let mut directives = DEFAULT_DIRECTIVES.clone();
+                directives.extend(self.directives.clone().into_iter());
+
+                directives
+            }
+        } else {
+            self.directives.clone()
+        };
+
+        let header = directives
+            .iter()
+            .map(|(key, values)| format!("{} {}", key, values.join(" ")))
+            .collect::<Vec<String>>()
+            .join("; ");
+
+        HeaderValue::from_str(header.trim())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_wow() {
+        for (k, v) in DEFAULT_DIRECTIVES.iter() {
+            println!("{}:{:?}", k, v)
+        }
+
+        let csp = ContentSecurityPolicy::default();
+
+        assert_eq!(csp.header_name(), "Content-Security-Policy");
+        assert_eq!(csp.header_value().unwrap(), "frame-ancestors 'self'; object-src 'none'; style-src 'self' https: 'unsafe-inline'; default-src 'self'; img-src 'self' data:; script-src-attr 'none'; upgrade-insecure-requests ; script-src 'self'; block-all-mixed-content ; base-uri 'self'; font-src 'self' https: data:");
+    }
+}
--- a/src/header/cross_origin_embedder_policy.rs
+++ b/src/header/cross_origin_embedder_policy.rs
@ -0,0 +1,23 @@
+use crate::IntoHeader;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::HeaderValue;
+
+/// `CrossOriginEmbedderPolicy` sets the `Cross-Origin-Embedder-Policy` header to `require-corp`.
+/// See [MDN's article on this header](https://developer.cdn.mozilla.net/en-US/docs/Web/HTTP/Headers/Cross-Origin-Embedder-Policy) for more.
+pub struct CrossOriginEmbedderPolicy;
+
+impl Default for CrossOriginEmbedderPolicy {
+    fn default() -> Self {
+        CrossOriginEmbedderPolicy
+    }
+}
+
+impl IntoHeader for CrossOriginEmbedderPolicy {
+    fn header_name(&self) -> HeaderName {
+        HeaderName::from_static("cross-origin-embedder-policy")
+    }
+
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue> {
+        HeaderValue::from_str("require-corp")
+    }
+}
--- a/src/header/cross_origin_opener_policy.rs
+++ b/src/header/cross_origin_opener_policy.rs
@ -0,0 +1,40 @@
+use crate::IntoHeader;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::HeaderValue;
+use std::fmt::{Display, Formatter};
+
+/// `CrossOriginOpenerPolicy` sets the `Cross-Origin-Opener-Policy` header.
+/// For more, see [MDN's article on this header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cross-Origin-Opener-Policy).
+pub enum CrossOriginOpenerPolicy {
+    UnsafeNone,
+    SameOriginAllowPopups,
+    SameOrigin,
+}
+
+impl Display for CrossOriginOpenerPolicy {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        let s = match self {
+            CrossOriginOpenerPolicy::UnsafeNone => "unsafe-none",
+            CrossOriginOpenerPolicy::SameOriginAllowPopups => "same-origin-allow-popups",
+            CrossOriginOpenerPolicy::SameOrigin => "same-origin",
+        };
+
+        write!(f, "{}", s)
+    }
+}
+
+impl Default for CrossOriginOpenerPolicy {
+    fn default() -> Self {
+        CrossOriginOpenerPolicy::SameOrigin
+    }
+}
+
+impl IntoHeader for CrossOriginOpenerPolicy {
+    fn header_name(&self) -> HeaderName {
+        HeaderName::from_static("cross-origin-opener-policy")
+    }
+
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue> {
+        HeaderValue::from_str(self.to_string().as_str())
+    }
+}
--- a/src/header/cross_origin_resource_policy.rs
+++ b/src/header/cross_origin_resource_policy.rs
@ -0,0 +1,40 @@
+use crate::IntoHeader;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::HeaderValue;
+use std::fmt::{Display, Formatter};
+
+/// `CrossOriginResourcePolicy` sets the `Cross-Origin-Resource-Policy` header.
+/// For more, see ["Consider deploying Cross-Origin Resource Policy](https://resourcepolicy.fyi/) and [MDN's article on this header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cross-Origin-Resource-Policy).
+pub enum CrossOriginResourcePolicy {
+    SameSite,
+    SameOrigin,
+    CrossOrigin,
+}
+
+impl Display for CrossOriginResourcePolicy {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        let s = match self {
+            CrossOriginResourcePolicy::SameSite => "same-site",
+            CrossOriginResourcePolicy::SameOrigin => "same-origin",
+            CrossOriginResourcePolicy::CrossOrigin => "cross-origin",
+        };
+
+        write!(f, "{}", s)
+    }
+}
+
+impl Default for CrossOriginResourcePolicy {
+    fn default() -> Self {
+        CrossOriginResourcePolicy::SameOrigin
+    }
+}
+
+impl IntoHeader for CrossOriginResourcePolicy {
+    fn header_name(&self) -> HeaderName {
+        HeaderName::from_static("cross-origin-resource-policy")
+    }
+
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue> {
+        HeaderValue::from_str(self.to_string().as_str())
+    }
+}
--- a/src/header/expect_ct.rs
+++ b/src/header/expect_ct.rs
@ -0,0 +1,45 @@
+use crate::IntoHeader;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::HeaderValue;
+use std::time::Duration;
+
+/// `ExpectCt` sets the `Expect-CT` header which helps mitigate misissued SSL certificates.
+/// See [MDN's article on Certificate Transparency](https://developer.mozilla.org/en-US/docs/Web/Security/Certificate_Transparency) and the [`Expect-CT` header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Expect-CT) for more.
+pub struct ExpectCt {
+    /// `max_age` is the number of seconds to expect Certificate Transparency.
+    pub max_age: Duration,
+    /// If `true`, the user agent (usually a browser) should refuse future connections that violate its Certificate Transparency policy.
+    pub enforce: bool,
+    /// If set, complying user agents will report Certificate Transparency failures to this URL.
+    pub report_uri: Option<String>,
+}
+
+impl Default for ExpectCt {
+    fn default() -> Self {
+        ExpectCt {
+            max_age: Duration::from_secs(0),
+            enforce: false,
+            report_uri: None,
+        }
+    }
+}
+
+impl IntoHeader for ExpectCt {
+    fn header_name(&self) -> HeaderName {
+        HeaderName::from_static("expect-ct")
+    }
+
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue> {
+        let mut directives = vec![format!("max-age={}", self.max_age.as_secs())];
+
+        if self.enforce {
+            directives.push("enforce".to_owned());
+        }
+
+        if let Some(report_uri) = self.report_uri.as_ref() {
+            directives.push(format!("report-uri={}", report_uri));
+        }
+
+        HeaderValue::from_str(directives.join(", ").as_str())
+    }
+}
--- a/src/header/mod.rs
+++ b/src/header/mod.rs
@ -0,0 +1,31 @@
+mod content_security_policy;
+mod cross_origin_embedder_policy;
+mod cross_origin_opener_policy;
+mod cross_origin_resource_policy;
+mod expect_ct;
+mod origin_agent_cluster;
+mod referrer_policy;
+mod strict_transport_security;
+mod x_content_type_options;
+mod x_dns_prefetch_control;
+mod x_download_options;
+mod x_frame_options;
+mod x_permitted_cross_domain_policies;
+mod x_xss_protection;
+
+pub use self::{
+    content_security_policy::ContentSecurityPolicy,
+    cross_origin_embedder_policy::CrossOriginEmbedderPolicy,
+    cross_origin_opener_policy::CrossOriginOpenerPolicy,
+    cross_origin_resource_policy::CrossOriginResourcePolicy,
+    expect_ct::ExpectCt,
+    origin_agent_cluster::OriginAgentCluster,
+    referrer_policy::{ReferrerPolicy, ReferrerPolicyValue},
+    strict_transport_security::StrictTransportSecurity,
+    x_content_type_options::XContentTypeOptions,
+    x_dns_prefetch_control::XDnsPrefetchControl,
+    x_download_options::XDownloadOptions,
+    x_frame_options::XFrameOptions,
+    x_permitted_cross_domain_policies::XPermittedCrossDomainPolicies,
+    x_xss_protection::XXSSProtection,
+};
--- a/src/header/origin_agent_cluster.rs
+++ b/src/header/origin_agent_cluster.rs
@ -0,0 +1,23 @@
+use crate::IntoHeader;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::HeaderValue;
+
+/// `OriginAgentCluster` sets the `Origin-Agent-Cluster` header, which provides a mechanism to allow web applications to isolate their origins.
+/// Read more about it [in the spec](https://whatpr.org/html/6214/origin.html#origin-keyed-agent-clusters).
+pub struct OriginAgentCluster;
+
+impl Default for OriginAgentCluster {
+    fn default() -> Self {
+        OriginAgentCluster
+    }
+}
+
+impl IntoHeader for OriginAgentCluster {
+    fn header_name(&self) -> HeaderName {
+        HeaderName::from_static("origin-agent-cluster")
+    }
+
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue> {
+        HeaderValue::from_str("?1")
+    }
+}
--- a/src/header/referrer_policy.rs
+++ b/src/header/referrer_policy.rs
@ -0,0 +1,51 @@
+use crate::IntoHeader;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::HeaderValue;
+use std::fmt::{Display, Formatter};
+
+/// `ReferrerPolicy` sets the `Referrer-Policy` header which controls what information is set in [the `Referer` header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referer).
+/// See ["Referer header: privacy and security concerns"](https://developer.mozilla.org/en-US/docs/Web/Security/Referer_header:_privacy_and_security_concerns) and [the header's documentation](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referrer-Policy) on MDN for more.
+pub struct ReferrerPolicy(pub Vec<ReferrerPolicyValue>);
+
+pub enum ReferrerPolicyValue {
+    NoReferrer,
+    NoReferrerWhenDowngrade,
+    Origin,
+    OriginWhenCrossOrigin,
+    SameOrigin,
+    StrictOrigin,
+    StrictOriginWhenCrossOrigin,
+}
+
+impl Display for ReferrerPolicyValue {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        let s = match self {
+            ReferrerPolicyValue::NoReferrer => "no-referrer",
+            ReferrerPolicyValue::NoReferrerWhenDowngrade => "no-referrer-when-downgrade",
+            ReferrerPolicyValue::Origin => "origin",
+            ReferrerPolicyValue::OriginWhenCrossOrigin => "origin-when-cross-origin",
+            ReferrerPolicyValue::SameOrigin => "same-origin",
+            ReferrerPolicyValue::StrictOrigin => "strict-origin",
+            ReferrerPolicyValue::StrictOriginWhenCrossOrigin => "strict-origin-when-cross-origin",
+        };
+
+        write!(f, "{}", s)
+    }
+}
+
+impl Default for ReferrerPolicy {
+    fn default() -> Self {
+        ReferrerPolicy(vec![ReferrerPolicyValue::NoReferrer])
+    }
+}
+
+impl IntoHeader for ReferrerPolicy {
+    fn header_name(&self) -> HeaderName {
+        http::header::REFERRER_POLICY
+    }
+
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue> {
+        let s: Vec<String> = self.0.iter().map(|v| v.to_string()).collect();
+        HeaderValue::from_str(s.join(",").as_str())
+    }
+}
--- a/src/header/strict_transport_security.rs
+++ b/src/header/strict_transport_security.rs
@ -0,0 +1,46 @@
+use crate::IntoHeader;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::HeaderValue;
+use std::time::Duration;
+
+/// `StrictTransportSecurity` sets the `Strict-Transport-Security` header which tells browsers to prefer HTTPS over insecure HTTP.
+/// See [the documentation on MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Strict-Transport-Security) for more.
+pub struct StrictTransportSecurity {
+    /// `max_age` is the number of seconds browsers should remember to prefer HTTPS. It defaults to `15552000`, which is 180 days.
+    pub max_age: Duration,
+    /// `include_subdomains` dictates whether to include the `includeSubDomains` directive, which makes this policy extend to subdomains. It defaults to `true`.
+    pub include_subdomains: bool,
+    /// If true, it adds the `preload` directive, expressing intent to add your HSTS policy to browsers.
+    /// See [the "Preloading Strict Transport Security" section on MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Strict-Transport-Security#Preloading_Strict_Transport_Security) for more.
+    pub preload: bool,
+}
+
+impl Default for StrictTransportSecurity {
+    fn default() -> Self {
+        StrictTransportSecurity {
+            max_age: Duration::from_secs(15552000),
+            include_subdomains: true,
+            preload: false,
+        }
+    }
+}
+
+impl IntoHeader for StrictTransportSecurity {
+    fn header_name(&self) -> HeaderName {
+        http::header::STRICT_TRANSPORT_SECURITY
+    }
+
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue> {
+        let mut directives = vec![format!("max-age={}", self.max_age.as_secs())];
+
+        if self.include_subdomains {
+            directives.push("includeSubdomains".to_owned());
+        }
+
+        if self.preload {
+            directives.push("preload".to_owned());
+        }
+
+        HeaderValue::from_str(directives.join("; ").as_str())
+    }
+}
--- a/src/header/x_content_type_options.rs
+++ b/src/header/x_content_type_options.rs
@ -0,0 +1,24 @@
+use crate::IntoHeader;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::HeaderValue;
+
+/// `XContentTypeOptions` sets the `X-Content-Type-Options` header to `nosniff`.
+/// This mitigates [MIME type sniffing](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types#MIME_sniffing) which can cause security vulnerabilities.
+/// See [documentation for this header on MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Content-Type-Options) for more.
+pub struct XContentTypeOptions;
+
+impl Default for XContentTypeOptions {
+    fn default() -> Self {
+        XContentTypeOptions
+    }
+}
+
+impl IntoHeader for XContentTypeOptions {
+    fn header_name(&self) -> HeaderName {
+        http::header::X_CONTENT_TYPE_OPTIONS
+    }
+
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue> {
+        HeaderValue::from_str("nosniff")
+    }
+}
--- a/src/header/x_dns_prefetch_control.rs
+++ b/src/header/x_dns_prefetch_control.rs
@ -0,0 +1,21 @@
+use crate::IntoHeader;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::HeaderValue;
+
+/// `XDnsPrefetchControl` sets the `X-DNS-Prefetch-Control` header to help control DNS prefetching, which can improve user privacy at the expense of performance.
+/// See [documentation on MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-DNS-Prefetch-Control) for more.
+#[derive(Default)]
+pub struct XDnsPrefetchControl(
+    /// Is indictating whether to enable DNS prefetching.
+    pub bool,
+);
+
+impl IntoHeader for XDnsPrefetchControl {
+    fn header_name(&self) -> HeaderName {
+        http::header::X_DNS_PREFETCH_CONTROL
+    }
+
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue> {
+        HeaderValue::from_str(if self.0 { "on" } else { "off" })
+    }
+}
--- a/src/header/x_download_options.rs
+++ b/src/header/x_download_options.rs
@ -0,0 +1,24 @@
+use crate::IntoHeader;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::HeaderValue;
+
+/// `XDownloadOptions` sets the `X-Download-Options` header, which is specific to Internet Explorer 8.
+/// It forces potentially-unsafe downloads to be saved, mitigating execution of HTML in your site's context.
+/// For more, see [this old post on MSDN](https://docs.microsoft.com/en-us/archive/blogs/ie/ie8-security-part-v-comprehensive-protection).
+pub struct XDownloadOptions;
+
+impl Default for XDownloadOptions {
+    fn default() -> Self {
+        XDownloadOptions
+    }
+}
+
+impl IntoHeader for XDownloadOptions {
+    fn header_name(&self) -> HeaderName {
+        HeaderName::from_static("x-download-options")
+    }
+
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue> {
+        HeaderValue::from_str("noopen")
+    }
+}
--- a/src/header/x_frame_options.rs
+++ b/src/header/x_frame_options.rs
@ -0,0 +1,41 @@
+use crate::IntoHeader;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::HeaderValue;
+use std::fmt::{Display, Formatter};
+
+/// `XFrameOptions` sets the `X-Frame-Options` header to help you mitigate [clickjacking attacks](https://en.wikipedia.org/wiki/Clickjacking).
+/// This header is superseded by [the `frame-ancestors` Content Security Policy directive](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/frame-ancestors) but is still useful on old browsers.
+/// For more, see `helmet.contentSecurityPolicy`, as well as [the documentation on MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options).
+///
+/// `DENY` or `SAMEORIGIN`. (A legacy directive, `ALLOW-FROM`, is not supported by this crate. [Read more here.](https://github.com/helmetjs/helmet/wiki/How-to-use-X%E2%80%93Frame%E2%80%93Options's-%60ALLOW%E2%80%93FROM%60-directive))
+pub enum XFrameOptions {
+    Deny,
+    SameOrigin,
+}
+
+impl Display for XFrameOptions {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        let s = match self {
+            XFrameOptions::SameOrigin => "SAMEORIGIN",
+            XFrameOptions::Deny => "DENY",
+        };
+
+        write!(f, "{}", s)
+    }
+}
+
+impl Default for XFrameOptions {
+    fn default() -> Self {
+        XFrameOptions::SameOrigin
+    }
+}
+
+impl IntoHeader for XFrameOptions {
+    fn header_name(&self) -> HeaderName {
+        http::header::X_FRAME_OPTIONS
+    }
+
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue> {
+        HeaderValue::from_str(self.to_string().as_str())
+    }
+}
--- a/src/header/x_permitted_cross_domain_policies.rs
+++ b/src/header/x_permitted_cross_domain_policies.rs
@ -0,0 +1,42 @@
+use crate::IntoHeader;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::HeaderValue;
+use std::fmt::{Display, Formatter};
+
+/// `XPermittedCrossDomainPolicies` sets the `X-Permitted-Cross-Domain-Policies` header, which tells some clients (mostly Adobe products) your domain's policy for loading cross-domain content.
+/// See [the description on OWASP](https://owasp.org/www-project-secure-headers/) for more.
+pub enum XPermittedCrossDomainPolicies {
+    None,
+    MasterOnly,
+    ByContentType,
+    All,
+}
+
+impl Display for XPermittedCrossDomainPolicies {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        let s = match self {
+            XPermittedCrossDomainPolicies::None => "none",
+            XPermittedCrossDomainPolicies::MasterOnly => "master-only",
+            XPermittedCrossDomainPolicies::ByContentType => "by-content-type",
+            XPermittedCrossDomainPolicies::All => "all",
+        };
+
+        write!(f, "{}", s)
+    }
+}
+
+impl Default for XPermittedCrossDomainPolicies {
+    fn default() -> Self {
+        XPermittedCrossDomainPolicies::None
+    }
+}
+
+impl IntoHeader for XPermittedCrossDomainPolicies {
+    fn header_name(&self) -> HeaderName {
+        HeaderName::from_static("x-permitted-cross-domain-policies")
+    }
+
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue> {
+        HeaderValue::from_str(self.to_string().as_str())
+    }
+}
--- a/src/header/x_xss_protection.rs
+++ b/src/header/x_xss_protection.rs
@ -0,0 +1,23 @@
+use crate::IntoHeader;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::HeaderValue;
+
+/// `XXSSProtection` disables browsers' buggy cross-site scripting filter by setting the `X-XSS-Protection` header to `0`.
+/// See [discussion about disabling the header here](https://github.com/helmetjs/helmet/issues/230) and [documentation on MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-XSS-Protection).
+pub struct XXSSProtection;
+
+impl Default for XXSSProtection {
+    fn default() -> Self {
+        XXSSProtection
+    }
+}
+
+impl IntoHeader for XXSSProtection {
+    fn header_name(&self) -> HeaderName {
+        http::header::X_XSS_PROTECTION
+    }
+
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue> {
+        HeaderValue::from_str("0")
+    }
+}
--- a/src/lib.rs
+++ b/src/lib.rs
@ -0,0 +1,364 @@
+//! # Overview
+//!
+//! `tower-helmet` helps you secure your tower server by setting various HTTP headers. _It's not a silver bullet_, but it can help!
+//!
+//! You can find a list of all available headers under the [header] module. By default (with [HelmetLayer::default]) **all of them** are enabled.
+//! Please take a good look at [ContentSecurityPolicy]. Most of the time you will need to adapt this one to your needs.
+//!
+//! # Examples
+//!
+//! ```
+//! use tower_helmet::header::{ContentSecurityPolicy, ExpectCt, XFrameOptions};
+//! use tower_helmet::HelmetLayer;
+//!
+//! // default layer with all security headers active
+//! let layer = HelmetLayer::default();
+//!
+//! // default layer with customizations applied
+//! let mut directives = HashMap::new();
+//! directives.insert("default-src", vec!["'self'", "https://example.com"]);
+//! directives.insert("img-src", vec!["'self'", "data:", "https://example.com"]);
+//! directives.insert("script-src", vec!["'self'", "'unsafe-inline'", "https://example.com"]);
+//! let csp = ContentSecurityPolicy {
+//!   directives,
+//!   ..Default::default()
+//! };
+//!
+//! let layer = HelmetLayer::default()
+//!     .disable_strict_transport_security()
+//!     .disable_cross_origin_embedder_policy()
+//!     .content_security_policy(csp);
+//!
+//! // completely blank layer, selectively enable and add headers
+//! let layer = HelmetLayer::new()
+//!   .x_frame_options(XFrameOptions::SameOrigin)
+//!   .expect_ct(ExpectCt::default());
+//! ```
+pub mod header;
+
+use futures::ready;
+use http::header::{HeaderName, InvalidHeaderValue};
+use http::{HeaderMap, HeaderValue, Request, Response};
+use pin_project_lite::pin_project;
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use tower_layer::Layer;
+use tower_service::Service;
+
+use header::{
+    ContentSecurityPolicy, CrossOriginEmbedderPolicy, CrossOriginOpenerPolicy,
+    CrossOriginResourcePolicy, ExpectCt, OriginAgentCluster, ReferrerPolicy,
+    StrictTransportSecurity, XContentTypeOptions, XDnsPrefetchControl, XDownloadOptions,
+    XFrameOptions, XPermittedCrossDomainPolicies, XXSSProtection,
+};
+
+trait IntoHeader {
+    fn header_name(&self) -> HeaderName;
+    fn header_value(&self) -> Result<HeaderValue, InvalidHeaderValue>;
+}
+
+/// HelmetLayer
+pub struct HelmetLayer<'a> {
+    content_security_policy: Option<ContentSecurityPolicy<'a>>,
+    cross_origin_embedder_policy: Option<CrossOriginEmbedderPolicy>,
+    cross_origin_opener_policy: Option<CrossOriginOpenerPolicy>,
+    cross_origin_resource_policy: Option<CrossOriginResourcePolicy>,
+    expect_ct: Option<ExpectCt>,
+    origin_agent_cluster: Option<OriginAgentCluster>,
+    referrer_policy: Option<ReferrerPolicy>,
+    strict_transport_security: Option<StrictTransportSecurity>,
+    x_content_type_options: Option<XContentTypeOptions>,
+    x_dns_prefetch_control: Option<XDnsPrefetchControl>,
+    x_download_options: Option<XDownloadOptions>,
+    x_frame_options: Option<XFrameOptions>,
+    x_permitted_cross_domain_policies: Option<XPermittedCrossDomainPolicies>,
+    x_xss_protection: Option<XXSSProtection>,
+}
+
+impl<'a> Default for HelmetLayer<'a> {
+    fn default() -> Self {
+        HelmetLayer {
+            content_security_policy: Some(ContentSecurityPolicy::default()),
+            cross_origin_embedder_policy: Some(CrossOriginEmbedderPolicy::default()),
+            cross_origin_opener_policy: Some(CrossOriginOpenerPolicy::default()),
+            cross_origin_resource_policy: Some(CrossOriginResourcePolicy::default()),
+            expect_ct: Some(ExpectCt::default()),
+            origin_agent_cluster: Some(OriginAgentCluster::default()),
+            referrer_policy: Some(ReferrerPolicy::default()),
+            strict_transport_security: Some(StrictTransportSecurity::default()),
+            x_content_type_options: Some(XContentTypeOptions::default()),
+            x_dns_prefetch_control: Some(XDnsPrefetchControl::default()),
+            x_download_options: Some(XDownloadOptions::default()),
+            x_frame_options: Some(XFrameOptions::default()),
+            x_permitted_cross_domain_policies: Some(XPermittedCrossDomainPolicies::default()),
+            x_xss_protection: Some(XXSSProtection::default()),
+        }
+    }
+}
+
+impl<'a> HelmetLayer<'a> {
+    pub fn new() -> Self {
+        HelmetLayer {
+            content_security_policy: None,
+            cross_origin_embedder_policy: None,
+            cross_origin_opener_policy: None,
+            cross_origin_resource_policy: None,
+            expect_ct: None,
+            origin_agent_cluster: None,
+            referrer_policy: None,
+            strict_transport_security: None,
+            x_content_type_options: None,
+            x_dns_prefetch_control: None,
+            x_download_options: None,
+            x_frame_options: None,
+            x_permitted_cross_domain_policies: None,
+            x_xss_protection: None,
+        }
+    }
+
+    pub fn recommended_defaults() -> Self {
+        HelmetLayer::default()
+    }
+
+    pub fn content_security_policy(&mut self, v: ContentSecurityPolicy<'a>) -> &mut Self {
+        self.content_security_policy = Some(v);
+        self
+    }
+    pub fn disable_content_security_policy(&mut self) -> &mut Self {
+        self.content_security_policy = None;
+        self
+    }
+
+    pub fn cross_origin_embedder_policy(&mut self, v: CrossOriginEmbedderPolicy) -> &mut Self {
+        self.cross_origin_embedder_policy = Some(v);
+        self
+    }
+    pub fn disable_cross_origin_embedder_policy(&mut self) -> &mut Self {
+        self.cross_origin_embedder_policy = None;
+        self
+    }
+
+    pub fn cross_origin_opener_policy(&mut self, v: CrossOriginOpenerPolicy) -> &mut Self {
+        self.cross_origin_opener_policy = Some(v);
+        self
+    }
+    pub fn disable_cross_origin_opener_policy(&mut self) -> &mut Self {
+        self.cross_origin_opener_policy = None;
+        self
+    }
+
+    pub fn cross_origin_resource_policy(&mut self, v: CrossOriginResourcePolicy) -> &mut Self {
+        self.cross_origin_resource_policy = Some(v);
+        self
+    }
+    pub fn disable_cross_origin_resource_policy(&mut self) -> &mut Self {
+        self.cross_origin_resource_policy = None;
+        self
+    }
+
+    pub fn expect_ct(&mut self, v: ExpectCt) -> &mut Self {
+        self.expect_ct = Some(v);
+        self
+    }
+    pub fn disable_expect_ct(&mut self) -> &mut Self {
+        self.expect_ct = None;
+        self
+    }
+
+    pub fn origin_agent_cluster(&mut self, v: OriginAgentCluster) -> &mut Self {
+        self.origin_agent_cluster = Some(v);
+        self
+    }
+    pub fn disable_origin_agent_cluster(&mut self) -> &mut Self {
+        self.origin_agent_cluster = None;
+        self
+    }
+
+    pub fn referrer_policy(&mut self, v: ReferrerPolicy) -> &mut Self {
+        self.referrer_policy = Some(v);
+        self
+    }
+    pub fn disable_referrer_policy(&mut self) -> &mut Self {
+        self.referrer_policy = None;
+        self
+    }
+
+    pub fn strict_transport_security(&mut self, v: StrictTransportSecurity) -> &mut Self {
+        self.strict_transport_security = Some(v);
+        self
+    }
+    pub fn disable_strict_transport_security(&mut self) -> &mut Self {
+        self.strict_transport_security = None;
+        self
+    }
+
+    pub fn x_content_type_options(&mut self, v: XContentTypeOptions) -> &mut Self {
+        self.x_content_type_options = Some(v);
+        self
+    }
+    pub fn disable_x_content_type_options(&mut self) -> &mut Self {
+        self.x_content_type_options = None;
+        self
+    }
+
+    pub fn x_dns_prefetch_control(&mut self, v: XDnsPrefetchControl) -> &mut Self {
+        self.x_dns_prefetch_control = Some(v);
+        self
+    }
+    pub fn disable_x_dns_prefetch_control(&mut self) -> &mut Self {
+        self.x_dns_prefetch_control = None;
+        self
+    }
+
+    pub fn x_download_options(&mut self, v: XDownloadOptions) -> &mut Self {
+        self.x_download_options = Some(v);
+        self
+    }
+    pub fn disable_x_download_options(&mut self) -> &mut Self {
+        self.x_download_options = None;
+        self
+    }
+
+    pub fn x_frame_options(&mut self, v: XFrameOptions) -> &mut Self {
+        self.x_frame_options = Some(v);
+        self
+    }
+    pub fn disable_x_frame_options(&mut self) -> &mut Self {
+        self.x_frame_options = None;
+        self
+    }
+
+    pub fn x_permitted_cross_domain_policies(
+        &mut self,
+        v: XPermittedCrossDomainPolicies,
+    ) -> &mut Self {
+        self.x_permitted_cross_domain_policies = Some(v);
+        self
+    }
+    pub fn disable_x_permitted_cross_domain_policies(&mut self) -> &mut Self {
+        self.x_permitted_cross_domain_policies = None;
+        self
+    }
+
+    pub fn x_xss_protection(&mut self, v: XXSSProtection) -> &mut Self {
+        self.x_xss_protection = Some(v);
+        self
+    }
+    pub fn disable_x_xss_protection(&mut self) -> &mut Self {
+        self.x_xss_protection = None;
+        self
+    }
+}
+
+impl<'a, S> Layer<S> for HelmetLayer<'a> {
+    type Service = HelmetService<S>;
+
+    fn layer(&self, service: S) -> Self::Service {
+        let mut headers = HeaderMap::new();
+
+        if let Some(h) = &self.content_security_policy {
+            headers.insert(h.header_name(), h.header_value().unwrap());
+        }
+        if let Some(h) = &self.cross_origin_embedder_policy {
+            headers.insert(h.header_name(), h.header_value().unwrap());
+        }
+        if let Some(h) = &self.cross_origin_opener_policy {
+            headers.insert(h.header_name(), h.header_value().unwrap());
+        }
+        if let Some(h) = &self.cross_origin_resource_policy {
+            headers.insert(h.header_name(), h.header_value().unwrap());
+        }
+        if let Some(h) = &self.expect_ct {
+            headers.insert(h.header_name(), h.header_value().unwrap());
+        }
+        if let Some(h) = &self.origin_agent_cluster {
+            headers.insert(h.header_name(), h.header_value().unwrap());
+        }
+        if let Some(h) = &self.referrer_policy {
+            headers.insert(h.header_name(), h.header_value().unwrap());
+        }
+        if let Some(h) = &self.strict_transport_security {
+            headers.insert(h.header_name(), h.header_value().unwrap());
+        }
+        if let Some(h) = &self.x_content_type_options {
+            headers.insert(h.header_name(), h.header_value().unwrap());
+        }
+        if let Some(h) = &self.x_dns_prefetch_control {
+            headers.insert(h.header_name(), h.header_value().unwrap());
+        }
+        if let Some(h) = &self.x_download_options {
+            headers.insert(h.header_name(), h.header_value().unwrap());
+        }
+        if let Some(h) = &self.x_frame_options {
+            headers.insert(h.header_name(), h.header_value().unwrap());
+        }
+        if let Some(h) = &self.x_permitted_cross_domain_policies {
+            headers.insert(h.header_name(), h.header_value().unwrap());
+        }
+        if let Some(h) = &self.x_xss_protection {
+            headers.insert(h.header_name(), h.header_value().unwrap());
+        }
+
+        HelmetService {
+            inner: service,
+            headers,
+        }
+    }
+}
+
+#[derive(Clone)]
+pub struct HelmetService<S> {
+    inner: S,
+    headers: HeaderMap,
+}
+
+impl<ReqBody, ResBody, S> Service<Request<ReqBody>> for HelmetService<S>
+where
+    S: Service<Request<ReqBody>, Response = Response<ResBody>>,
+{
+    type Response = S::Response;
+    type Error = S::Error;
+    type Future = ResponseFuture<S::Future>;
+
+    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.inner.poll_ready(cx)
+    }
+
+    fn call(&mut self, request: Request<ReqBody>) -> Self::Future {
+        ResponseFuture {
+            future: self.inner.call(request),
+            headers: self.headers.clone(),
+        }
+    }
+}
+
+pin_project! {
+    /// Response future for [`HelmetService`].
+    #[derive(Debug)]
+    pub struct ResponseFuture<F> {
+        #[pin]
+        future: F,
+
+        headers: HeaderMap,
+    }
+}
+
+impl<F, ResBody, E> Future for ResponseFuture<F>
+where
+    F: Future<Output = Result<Response<ResBody>, E>>,
+{
+    type Output = F::Output;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = self.project();
+        let mut res: Response<ResBody> = ready!(this.future.poll(cx)?);
+        let headers = res.headers_mut();
+
+        for (name, value) in this.headers.iter() {
+            headers.insert(name, value.clone());
+        }
+
+        Poll::Ready(Ok(res))
+    }
+}