Add JA4/TLS fingerprint debug endpoint at /_ts/debug/ja4

crates/trusted-server-adapter-fastly/src/main.rs

@@ -1,5 +1,5 @@
 use error_stack::Report;
-use fastly::http::Method;
+use fastly::http::{header, Method};
 use fastly::{Request, Response};
 
 use trusted_server_core::auction::endpoints::handle_auction;
@@ -70,6 +70,17 @@ fn main() {
     };
     log::debug!("Settings {settings:?}");
 
+    // Short-circuit the ja4 debug probe before finalize_response so that
+    // Cache-Control: no-store, private cannot be replaced by operator [response_headers].
+    if req.get_method() == Method::GET && req.get_path() == "/_ts/debug/ja4" {
+        if settings.debug.ja4_endpoint_enabled {
+            build_ja4_debug_response(&req).send_to_client();
+        } else {
+            Response::from_status(fastly::http::StatusCode::NOT_FOUND).send_to_client();
+        }
+        return;
+    }
+
     // Build the auction orchestrator once at startup
     let orchestrator = match build_orchestrator(&settings) {
         Ok(o) => o,
@@ -109,6 +120,48 @@ fn main() {
     }
 }
 
+const FALLBACK_UNAVAILABLE: &str = "unavailable";
+const FALLBACK_NOT_SENT: &str = "not sent";
+const FALLBACK_NONE: &str = "none";
+
+// TODO: remove after JA4 evaluation completes — see #645
+fn build_ja4_debug_response(req: &Request) -> Response {
+    let ja4 = req.get_tls_ja4().unwrap_or(FALLBACK_UNAVAILABLE);
+    let h2 = req
+        .get_client_h2_fingerprint()
+        .unwrap_or(FALLBACK_UNAVAILABLE);
+    let cipher = req
+        .get_tls_cipher_openssl_name()
+        .unwrap_or(FALLBACK_UNAVAILABLE);
+    let tls_version = req.get_tls_protocol().unwrap_or(FALLBACK_UNAVAILABLE);
+    let ua = req.get_header_str("user-agent").unwrap_or(FALLBACK_NONE);
+    let ch_mobile = req
+        .get_header_str("sec-ch-ua-mobile")
+        .unwrap_or(FALLBACK_NOT_SENT);
+    let ch_platform = req
+        .get_header_str("sec-ch-ua-platform")
+        .unwrap_or(FALLBACK_NOT_SENT);
+
+    let body = format!(
+        "ja4:         {ja4}\n\
+         h2_fp:       {h2}\n\
+         cipher:      {cipher}\n\
+         tls_version: {tls_version}\n\
+         user-agent:  {ua}\n\
+         ch-mobile:   {ch_mobile}\n\
+         ch-platform: {ch_platform}\n"
+    );
+
+    Response::from_status(fastly::http::StatusCode::OK)
+        .with_header(header::CACHE_CONTROL, "no-store, private")
+        .with_header(
+            header::VARY,
+            "User-Agent, Sec-CH-UA-Mobile, Sec-CH-UA-Platform",
+        )
+        .with_content_type(fastly::mime::TEXT_PLAIN_UTF_8)
+        .with_body(body)
+}
+
 async fn route_request(
     settings: &Settings,
     orchestrator: &AuctionOrchestrator,
@@ -320,3 +373,63 @@ fn finalize_response(settings: &Settings, geo_info: Option<&GeoInfo>, response:
         response.set_header(key, value);
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use fastly::mime;
+
+    #[test]
+    fn ja4_debug_response_uses_plain_text_and_fallback_values() {
+        let req = Request::get("https://example.com/_ts/debug/ja4");
+
+        let mut response = build_ja4_debug_response(&req);
+
+        assert_eq!(
+            response.get_status(),
+            fastly::http::StatusCode::OK,
+            "should return 200 OK"
+        );
+        assert_eq!(
+            response.get_content_type(),
+            Some(mime::TEXT_PLAIN_UTF_8),
+            "should return plain text content"
+        );
+        assert_eq!(
+            response.get_header_str(header::CACHE_CONTROL),
+            Some("no-store, private"),
+            "should disable caching for the debug response"
+        );
+
+        let body = response.take_body_str();
+
+        assert!(
+            body.contains("ja4:         unavailable"),
+            "should include JA4 fallback"
+        );
+        assert!(
+            body.contains("h2_fp:       unavailable"),
+            "should include H2 fingerprint fallback"
+        );
+        assert!(
+            body.contains("cipher:      unavailable"),
+            "should include cipher fallback"
+        );
+        assert!(
+            body.contains("tls_version: unavailable"),
+            "should include TLS version fallback"
+        );
+        assert!(
+            body.contains("user-agent:  none"),
+            "should include user-agent fallback"
+        );
+        assert!(
+            body.contains("ch-mobile:   not sent"),
+            "should include sec-ch-ua-mobile fallback"
+        );
+        assert!(
+            body.contains("ch-platform: not sent"),
+            "should include sec-ch-ua-platform fallback"
+        );
+    }
+}

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

@@ -399,6 +399,18 @@ impl Proxy {
     }
 }
 
+/// Debug-only features. All flags default to `false` (off in production).
+#[derive(Debug, Default, Clone, Deserialize, Serialize)]
+pub struct DebugConfig {
+    /// Expose the JA4/TLS fingerprint debug endpoint at `GET /_ts/debug/ja4`.
+    ///
+    /// When `false` (the default), the endpoint returns 404. Enable only for
+    /// intentional Fastly/browser TLS investigation — the endpoint reflects
+    /// Fastly-observed TLS details that browser JS cannot normally read.
+    #[serde(default)]
+    pub ja4_endpoint_enabled: bool,
+}
+
 #[derive(Debug, Default, Clone, Deserialize, Serialize, Validate)]
 pub struct Settings {
     #[validate(nested)]
@@ -423,6 +435,8 @@ pub struct Settings {
     pub consent: ConsentConfig,
     #[serde(default)]
     pub proxy: Proxy,
+    #[serde(default)]
+    pub debug: DebugConfig,
 }
 
 #[allow(unused)]

trusted-server.toml

@@ -184,6 +184,24 @@ enabled = false
 endpoint = "https://origin-mocktioneer.cdintel.com/adserver/mediate"
 timeout_ms = 1000
 
+# Debug configuration (all flags default to false — do not enable in production)
+# [debug]
+# Enable the JA4/TLS fingerprint debug endpoint at GET /_ts/debug/ja4.
+# Returns a plain-text response with the following fields (Fastly-observed values):
+#   ja4         — JA4 TLS client fingerprint
+#   h2_fp       — HTTP/2 client fingerprint
+#   cipher      — TLS cipher suite (OpenSSL name)
+#   tls_version — TLS protocol version
+#   user-agent  — User-Agent request header
+#   ch-mobile   — Sec-CH-UA-Mobile client hint
+#   ch-platform — Sec-CH-UA-Platform client hint
+# Fastly TLS/fingerprint fields fall back to "unavailable"; client hints fall back
+# to "not sent"; user-agent falls back to "none" when absent.
+# Response always carries Cache-Control: no-store, private.
+# IMPORTANT: This endpoint reflects TLS details that browser JS cannot normally read.
+# Disable after investigation is complete.
+# ja4_endpoint_enabled = false
+
 # Map auction-request context keys to mediation URL query parameters.
 # Each key is a context key from the JS client; the value becomes the
 # query parameter name. Arrays are joined with commas.