Add domain allowlist to block SSRF via first-party proxy redirects

.env.dev

@@ -8,3 +8,11 @@ TRUSTED_SERVER__SYNTHETIC__OPID_STORE=opid_store
 # [proxy]
 # Disable TLS certificate verification for local dev with self-signed certs
 # TRUSTED_SERVER__PROXY__CERTIFICATE_CHECK=false
+#
+# Restrict first-party proxy redirect targets to an allowlist (JSON array or indexed form).
+# Leave unset in local dev; configure in production to prevent SSRF via redirect chains
+# initiated by signed first-party proxy URLs.
+# TRUSTED_SERVER__PROXY__ALLOWED_DOMAINS='["*.doubleclick.net","*.googlesyndication.com"]'
+# Or using indexed form:
+# TRUSTED_SERVER__PROXY__ALLOWED_DOMAINS__0='*.doubleclick.net'
+# TRUSTED_SERVER__PROXY__ALLOWED_DOMAINS__1='*.googlesyndication.com'

crates/trusted-server-core/src/proxy.rs

@@ -479,6 +479,35 @@ fn append_synthetic_id(req: &Request, target_url_parsed: &mut url::Url) {
     }
 }
 
+/// Returns `true` when a redirect to `host` should be followed.
+///
+/// When `allowed_domains` is empty every host is permitted (open mode).
+/// When non-empty the host must match at least one pattern via [`is_host_allowed`].
+fn redirect_is_permitted(allowed_domains: &[String], host: &str) -> bool {
+    allowed_domains.is_empty() || allowed_domains.iter().any(|p| is_host_allowed(host, p))
+}
+
+/// Returns `true` if `host` is permitted by `pattern`.
+///
+/// - `"example.com"` matches exactly `example.com`.
+/// - `"*.example.com"` matches `example.com` and any subdomain at any depth.
+///
+/// Comparison is case-insensitive. The wildcard check requires a dot boundary,
+/// so `"*.example.com"` does **not** match `"evil-example.com"`.
+fn is_host_allowed(host: &str, pattern: &str) -> bool {
+    let host = host.to_ascii_lowercase();
+    let pattern = pattern.to_ascii_lowercase();
+
+    if let Some(suffix) = pattern.strip_prefix("*.") {
+        host == suffix
+            || host
+                .strip_suffix(suffix)
+                .is_some_and(|rest| rest.ends_with('.'))
+    } else {
+        host == pattern
+    }
+}
+
 async fn proxy_with_redirects(
     settings: &Settings,
     req: &Request,
@@ -584,6 +613,17 @@ async fn proxy_with_redirects(
             return finalize_response(settings, req, &current_url, beresp, stream_passthrough);
         }
 
+        let next_host = next_url.host_str().unwrap_or("");
+        if !redirect_is_permitted(&settings.proxy.allowed_domains, next_host) {
+            log::warn!(
+                "redirect to `{}` blocked: host not in proxy allowed_domains",
+                next_host
+            );
+            return Err(Report::new(TrustedServerError::Proxy {
+                message: format!("redirect to `{next_host}` is not permitted"),
+            }));
+        }
+
         log::info!(
             "following redirect {} => {} (status {})",
             current_url,
@@ -1108,8 +1148,9 @@ fn reconstruct_and_validate_signed_target(
 mod tests {
     use super::{
         copy_proxy_forward_headers, handle_first_party_click, handle_first_party_proxy,
-        handle_first_party_proxy_rebuild, handle_first_party_proxy_sign,
-        reconstruct_and_validate_signed_target, ProxyRequestConfig, SUPPORTED_ENCODINGS,
+        handle_first_party_proxy_rebuild, handle_first_party_proxy_sign, is_host_allowed,
+        reconstruct_and_validate_signed_target, redirect_is_permitted, ProxyRequestConfig,
+        SUPPORTED_ENCODINGS,
     };
     use crate::error::{IntoHttpResponse, TrustedServerError};
     use crate::test_support::tests::create_test_settings;
@@ -1834,4 +1875,177 @@ mod tests {
             body
         );
     }
+
+    // --- is_host_allowed ---
+
+    #[test]
+    fn exact_match() {
+        assert!(
+            is_host_allowed("example.com", "example.com"),
+            "should match exact domain"
+        );
+    }
+
+    #[test]
+    fn exact_no_match() {
+        assert!(
+            !is_host_allowed("other.com", "example.com"),
+            "should not match different domain"
+        );
+    }
+
+    #[test]
+    fn wildcard_subdomain() {
+        assert!(
+            is_host_allowed("ad.example.com", "*.example.com"),
+            "should match direct subdomain"
+        );
+    }
+
+    #[test]
+    fn wildcard_deep_subdomain() {
+        assert!(
+            is_host_allowed("a.b.example.com", "*.example.com"),
+            "should match deep subdomain"
+        );
+    }
+
+    #[test]
+    fn wildcard_apex_match() {
+        assert!(
+            is_host_allowed("example.com", "*.example.com"),
+            "wildcard should also match apex domain"
+        );
+    }
+
+    #[test]
+    fn wildcard_no_boundary_bypass() {
+        assert!(
+            !is_host_allowed("evil-example.com", "*.example.com"),
+            "should not match host that lacks dot boundary"
+        );
+    }
+
+    #[test]
+    fn case_insensitive_host() {
+        assert!(
+            is_host_allowed("AD.EXAMPLE.COM", "*.example.com"),
+            "should match uppercase host"
+        );
+    }
+
+    #[test]
+    fn case_insensitive_pattern() {
+        assert!(
+            is_host_allowed("ad.example.com", "*.EXAMPLE.COM"),
+            "should match uppercase pattern"
+        );
+    }
+
+    // --- redirect allowlist enforcement (logic tests via is_host_allowed) ---
+
+    #[test]
+    fn redirect_allowed_exact() {
+        let allowed = ["ad.example.com".to_string()];
+        assert!(
+            allowed.iter().any(|p| is_host_allowed("ad.example.com", p)),
+            "should permit exact-match host"
+        );
+    }
+
+    #[test]
+    fn redirect_allowed_wildcard() {
+        let allowed = ["*.example.com".to_string()];
+        assert!(
+            allowed
+                .iter()
+                .any(|p| is_host_allowed("sub.example.com", p)),
+            "should permit wildcard-matched host"
+        );
+    }
+
+    #[test]
+    fn redirect_blocked() {
+        let allowed = ["*.example.com".to_string()];
+        assert!(
+            !allowed.iter().any(|p| is_host_allowed("evil.com", p)),
+            "should block host not in allowlist"
+        );
+    }
+
+    #[test]
+    fn redirect_empty_allowlist_permits_any() {
+        // The guard at proxy_with_redirects checks `!allowed_domains.is_empty()`
+        // before calling is_host_allowed, so no host is ever blocked when the
+        // list is empty. Verify the combined condition is false for any host.
+        let allowed: [String; 0] = [];
+        let would_block =
+            !allowed.is_empty() && !allowed.iter().any(|p| is_host_allowed("evil.com", p));
+        assert!(
+            !would_block,
+            "empty allowlist should not block any redirect host"
+        );
+    }
+
+    #[test]
+    fn redirect_bypass_attempt() {
+        let allowed = ["*.example.com".to_string()];
+        assert!(
+            !allowed
+                .iter()
+                .any(|p| is_host_allowed("evil-example.com", p)),
+            "should block dot-boundary bypass attempt"
+        );
+    }
+
+    // --- redirect_is_permitted (full guard: empty-list bypass + is_host_allowed) ---
+
+    #[test]
+    fn redirect_chain_allowed_when_host_matches_allowlist() {
+        let allowed = vec!["ad.example.com".to_string(), "cdn.example.com".to_string()];
+        assert!(
+            redirect_is_permitted(&allowed, "ad.example.com"),
+            "should permit redirect to exact-match host"
+        );
+        assert!(
+            redirect_is_permitted(&allowed, "cdn.example.com"),
+            "should permit redirect to second allowed host"
+        );
+    }
+
+    #[test]
+    fn redirect_chain_allowed_when_host_matches_wildcard() {
+        let allowed = vec!["*.example.com".to_string()];
+        assert!(
+            redirect_is_permitted(&allowed, "sub.example.com"),
+            "should permit redirect to wildcard-matched subdomain"
+        );
+    }
+
+    #[test]
+    fn redirect_chain_blocked_when_host_not_in_allowlist() {
+        let allowed = vec!["ad.example.com".to_string()];
+        assert!(
+            !redirect_is_permitted(&allowed, "evil.com"),
+            "should block redirect to host not in allowlist"
+        );
+    }
+
+    #[test]
+    fn redirect_chain_allowed_when_allowlist_is_empty() {
+        let allowed: Vec<String> = vec![];
+        assert!(
+            redirect_is_permitted(&allowed, "any-host.com"),
+            "should allow any redirect when allowlist is empty (open mode)"
+        );
+    }
+
+    #[test]
+    fn redirect_chain_blocked_when_host_is_empty() {
+        let allowed = vec!["example.com".to_string()];
+        assert!(
+            !redirect_is_permitted(&allowed, ""),
+            "should block redirect with empty host when allowlist is non-empty"
+        );
+    }
 }