feat: Add HMAC-SHA256 webhook signature validation for Slack (#588)

* feat: Add HMAC-SHA256 webhook signature validation for Slack

* review fixes

Cargo.lock

@@ -2853,6 +2853,7 @@ dependencies = [
  "futures",
  "hex",
  "hkdf",
+ "hmac",
  "html-to-markdown-rs",
  "http-body-util",
  "hyper 1.8.1",

Cargo.toml

@@ -128,6 +128,7 @@ wasmparser = "0.220"  # WASM binary parsing for validation
 # Cryptography for secrets management
 aes-gcm = "0.10"
 hkdf = "0.12"
+hmac = "0.12"
 sha2 = "0.10"
 blake3 = "1"
 rand = "0.8"

channels-src/slack/slack.capabilities.json

@@ -44,6 +44,9 @@
       "emit_rate_limit": {
         "messages_per_minute": 100,
         "messages_per_hour": 5000
+      },
+      "webhook": {
+        "hmac_secret_name": "slack_signing_secret"
       }
     }
   },

src/channels/wasm/loader.rs

@@ -277,6 +277,13 @@ impl LoadedChannel {
             .and_then(|f| f.signature_key_secret_name().map(|s| s.to_string()))
     }
 
+    /// Get the HMAC-SHA256 signing secret name from capabilities.
+    pub fn hmac_secret_name(&self) -> Option<String> {
+        self.capabilities_file
+            .as_ref()
+            .and_then(|f| f.hmac_secret_name().map(|s| s.to_string()))
+    }
+
     /// Get the webhook secret name from capabilities.
     pub fn webhook_secret_name(&self) -> String {
         self.capabilities_file

src/channels/wasm/router.rs

@@ -44,6 +44,8 @@ pub struct WasmChannelRouter {
     secret_headers: RwLock<HashMap<String, String>>,
     /// Ed25519 public keys for signature verification by channel name (hex-encoded).
     signature_keys: RwLock<HashMap<String, String>>,
+    /// HMAC-SHA256 signing secrets for signature verification by channel name (Slack-style).
+    hmac_secrets: RwLock<HashMap<String, String>>,
 }
 
 impl WasmChannelRouter {
@@ -55,6 +57,7 @@ impl WasmChannelRouter {
             secrets: RwLock::new(HashMap::new()),
             secret_headers: RwLock::new(HashMap::new()),
             signature_keys: RwLock::new(HashMap::new()),
+            hmac_secrets: RwLock::new(HashMap::new()),
         }
     }
 
@@ -134,6 +137,7 @@ impl WasmChannelRouter {
         self.secrets.write().await.remove(channel_name);
         self.secret_headers.write().await.remove(channel_name);
         self.signature_keys.write().await.remove(channel_name);
+        self.hmac_secrets.write().await.remove(channel_name);
 
         // Remove all paths for this channel
         self.path_to_channel
@@ -208,6 +212,24 @@ impl WasmChannelRouter {
     pub async fn get_signature_key(&self, channel_name: &str) -> Option<String> {
         self.signature_keys.read().await.get(channel_name).cloned()
     }
+
+    /// Register an HMAC-SHA256 signing secret for signature verification.
+    ///
+    /// Channels with a registered secret will have Slack-style HMAC-SHA256
+    /// signature validation performed before forwarding to WASM.
+    pub async fn register_hmac_secret(&self, channel_name: &str, secret: &str) {
+        self.hmac_secrets
+            .write()
+            .await
+            .insert(channel_name.to_string(), secret.to_string());
+    }
+
+    /// Get the HMAC signing secret for a channel.
+    ///
+    /// Returns `None` if no secret is registered (no HMAC check needed).
+    pub async fn get_hmac_secret(&self, channel_name: &str) -> Option<String> {
+        self.hmac_secrets.read().await.get(channel_name).cloned()
+    }
 }
 
 impl Default for WasmChannelRouter {
@@ -427,6 +449,57 @@ async fn webhook_handler(
         }
     }
 
+    // HMAC-SHA256 signature verification (Slack-style)
+    if let Some(hmac_secret) = state.router.get_hmac_secret(channel_name).await {
+        let timestamp = headers
+            .get("x-slack-request-timestamp")
+            .and_then(|v| v.to_str().ok());
+        let sig_header = headers
+            .get("x-slack-signature")
+            .and_then(|v| v.to_str().ok());
+
+        match (timestamp, sig_header) {
+            (Some(ts), Some(sig)) => {
+                let now_secs = std::time::SystemTime::now()
+                    .duration_since(std::time::UNIX_EPOCH)
+                    .unwrap_or_default()
+                    .as_secs() as i64;
+
+                if !crate::channels::wasm::signature::verify_slack_signature(
+                    &hmac_secret,
+                    ts,
+                    &body,
+                    sig,
+                    now_secs,
+                ) {
+                    tracing::warn!(
+                        channel = %channel_name,
+                        "HMAC-SHA256 signature verification failed"
+                    );
+                    return (
+                        StatusCode::UNAUTHORIZED,
+                        Json(serde_json::json!({
+                            "error": "Invalid Slack signature"
+                        })),
+                    );
+                }
+                tracing::debug!(channel = %channel_name, "HMAC-SHA256 signature verified");
+            }
+            _ => {
+                tracing::warn!(
+                    channel = %channel_name,
+                    "Slack signature headers missing but secret is registered"
+                );
+                return (
+                    StatusCode::UNAUTHORIZED,
+                    Json(serde_json::json!({
+                        "error": "Missing Slack signature headers"
+                    })),
+                );
+            }
+        }
+    }
+
     // Convert headers to HashMap
     let headers_map: HashMap<String, String> = headers
         .iter()
@@ -731,7 +804,59 @@ mod tests {
         assert_eq!(router.get_secret_header("slack").await, "X-Webhook-Secret");
     }
 
-    // ── Category 3: Router Signature Key Management ─────────────────────
+    // ── Category 3: Router HMAC Secret Management ───────────────────────
+
+    #[tokio::test]
+    async fn test_register_and_get_hmac_secret() {
+        let router = WasmChannelRouter::new();
+        let channel = create_test_channel("slack");
+
+        router.register(channel, vec![], None, None).await;
+
+        let hmac_secret = "my-slack-signing-secret";
+        router.register_hmac_secret("slack", hmac_secret).await;
+
+        let retrieved = router.get_hmac_secret("slack").await;
+        assert_eq!(retrieved, Some(hmac_secret.to_string()));
+    }
+
+    #[tokio::test]
+    async fn test_no_hmac_secret_returns_none() {
+        let router = WasmChannelRouter::new();
+        let channel = create_test_channel("slack");
+        router.register(channel, vec![], None, None).await;
+
+        // Slack has no HMAC secret registered
+        let secret = router.get_hmac_secret("slack").await;
+        assert!(secret.is_none());
+    }
+
+    #[tokio::test]
+    async fn test_unregister_removes_hmac_secret() {
+        let router = WasmChannelRouter::new();
+        let channel = create_test_channel("slack");
+
+        let endpoints = vec![RegisteredEndpoint {
+            channel_name: "slack".to_string(),
+            path: "/webhook/slack".to_string(),
+            methods: vec!["POST".to_string()],
+            require_secret: false,
+        }];
+
+        router.register(channel, endpoints, None, None).await;
+        router.register_hmac_secret("slack", "signing-secret").await;
+
+        // Secret should exist
+        assert!(router.get_hmac_secret("slack").await.is_some());
+
+        // Unregister
+        router.unregister("slack").await;
+
+        // Secret should be gone
+        assert!(router.get_hmac_secret("slack").await.is_none());
+    }
+
+    // ── Category 4: Router Signature Key Management ─────────────────────
 
     #[tokio::test]
     async fn test_register_and_get_signature_key() {
@@ -1163,4 +1288,215 @@ mod tests {
             "Valid secret + valid signature should not return 401"
         );
     }
+
+    // ── HMAC-SHA256 Webhook Signature Tests ────────────────────────────
+
+    /// Helper to create a router with a registered channel at /webhook/slack.
+    async fn setup_slack_router() -> (Arc<WasmChannelRouter>, AxumRouter) {
+        let wasm_router = Arc::new(WasmChannelRouter::new());
+        let channel = create_test_channel("slack");
+
+        let endpoints = vec![RegisteredEndpoint {
+            channel_name: "slack".to_string(),
+            path: "/webhook/slack".to_string(),
+            methods: vec!["POST".to_string()],
+            require_secret: false,
+        }];
+
+        wasm_router.register(channel, endpoints, None, None).await;
+
+        let app = create_wasm_channel_router(wasm_router.clone(), None);
+        (wasm_router, app)
+    }
+
+    /// Helper: compute expected Slack signature for testing.
+    fn slack_signature(signing_secret: &str, timestamp: &str, body: &[u8]) -> String {
+        use hmac::{Hmac, Mac};
+        use sha2::Sha256;
+
+        let mut basestring = Vec::new();
+        basestring.extend_from_slice(b"v0:");
+        basestring.extend_from_slice(timestamp.as_bytes());
+        basestring.push(b':');
+        basestring.extend_from_slice(body);
+
+        let mut mac = Hmac::<Sha256>::new_from_slice(signing_secret.as_bytes()).unwrap();
+        mac.update(&basestring);
+        let computed = mac.finalize().into_bytes();
+        format!("v0={}", hex::encode(computed))
+    }
+
+    #[tokio::test]
+    async fn test_webhook_hmac_rejects_missing_sig_headers() {
+        let (wasm_router, app) = setup_slack_router().await;
+
+        wasm_router
+            .register_hmac_secret("slack", "my-signing-secret")
+            .await;
+
+        // Send request without HMAC signature headers
+        let req = Request::builder()
+            .method("POST")
+            .uri("/webhook/slack")
+            .header("content-type", "application/json")
+            .body(Body::from("token=xyzz0WbapA4vBCDEFasx0q6G"))
+            .unwrap();
+
+        let resp = app.oneshot(req).await.unwrap();
+        assert_eq!(
+            resp.status(),
+            StatusCode::UNAUTHORIZED,
+            "Missing HMAC signature headers should return 401"
+        );
+    }
+
+    #[tokio::test]
+    async fn test_webhook_hmac_rejects_invalid_signature() {
+        let (wasm_router, app) = setup_slack_router().await;
+
+        wasm_router
+            .register_hmac_secret("slack", "my-signing-secret")
+            .await;
+
+        let req = Request::builder()
+            .method("POST")
+            .uri("/webhook/slack")
+            .header("content-type", "application/json")
+            .header("x-slack-request-timestamp", "1234567890")
+            .header("x-slack-signature", "v0=deadbeefdeadbeef")
+            .body(Body::from("token=xyzz0WbapA4vBCDEFasx0q6G"))
+            .unwrap();
+
+        let resp = app.oneshot(req).await.unwrap();
+        assert_eq!(
+            resp.status(),
+            StatusCode::UNAUTHORIZED,
+            "Invalid HMAC signature should return 401"
+        );
+    }
+
+    #[tokio::test]
+    async fn test_webhook_hmac_accepts_valid_signature() {
+        let (wasm_router, app) = setup_slack_router().await;
+
+        let signing_secret = "my-signing-secret";
+        wasm_router
+            .register_hmac_secret("slack", signing_secret)
+            .await;
+
+        let now_secs = std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)
+            .unwrap()
+            .as_secs();
+        let timestamp = now_secs.to_string();
+        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G";
+
+        let signature = slack_signature(signing_secret, &timestamp, body);
+
+        let req = Request::builder()
+            .method("POST")
+            .uri("/webhook/slack")
+            .header("content-type", "application/json")
+            .header("x-slack-request-timestamp", &timestamp)
+            .header("x-slack-signature", &signature)
+            .body(Body::from(&body[..]))
+            .unwrap();
+
+        let resp = app.oneshot(req).await.unwrap();
+        // Should NOT be 401 — signature is valid (may be 500 since no WASM module)
+        assert_ne!(
+            resp.status(),
+            StatusCode::UNAUTHORIZED,
+            "Valid HMAC signature should not return 401"
+        );
+    }
+
+    #[tokio::test]
+    async fn test_webhook_hmac_skips_check_for_no_secret() {
+        let (_wasm_router, app) = setup_slack_router().await;
+
+        // No HMAC secret registered — should not require signature
+        let req = Request::builder()
+            .method("POST")
+            .uri("/webhook/slack")
+            .header("content-type", "application/json")
+            .body(Body::from("token=xyzz0WbapA4vBCDEFasx0q6G"))
+            .unwrap();
+
+        let resp = app.oneshot(req).await.unwrap();
+        // Should NOT be 401 (may be 500 since no WASM module, but not auth failure)
+        assert_ne!(
+            resp.status(),
+            StatusCode::UNAUTHORIZED,
+            "No HMAC secret registered — should skip check"
+        );
+    }
+
+    #[tokio::test]
+    async fn test_webhook_hmac_uses_correct_body() {
+        let (wasm_router, app) = setup_slack_router().await;
+
+        let signing_secret = "my-signing-secret";
+        wasm_router
+            .register_hmac_secret("slack", signing_secret)
+            .await;
+
+        let timestamp = "1234567890";
+        let body_a = b"token=xyzz0WbapA4vBCDEFasx0q6G";
+        let body_b = b"token=MODIFIED";
+
+        // Sign body A
+        let signature = slack_signature(signing_secret, timestamp, body_a);
+
+        // But send body B
+        let req = Request::builder()
+            .method("POST")
+            .uri("/webhook/slack")
+            .header("content-type", "application/json")
+            .header("x-slack-request-timestamp", timestamp)
+            .header("x-slack-signature", &signature)
+            .body(Body::from(&body_b[..]))
+            .unwrap();
+
+        let resp = app.oneshot(req).await.unwrap();
+        assert_eq!(
+            resp.status(),
+            StatusCode::UNAUTHORIZED,
+            "Signature for different body should return 401"
+        );
+    }
+
+    #[tokio::test]
+    async fn test_webhook_hmac_uses_correct_timestamp() {
+        let (wasm_router, app) = setup_slack_router().await;
+
+        let signing_secret = "my-signing-secret";
+        wasm_router
+            .register_hmac_secret("slack", signing_secret)
+            .await;
+
+        let timestamp_a = "1234567890";
+        let timestamp_b = "9999999999";
+        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G";
+
+        // Sign with timestamp A
+        let signature = slack_signature(signing_secret, timestamp_a, body);
+
+        // But send timestamp B in the header
+        let req = Request::builder()
+            .method("POST")
+            .uri("/webhook/slack")
+            .header("content-type", "application/json")
+            .header("x-slack-request-timestamp", timestamp_b)
+            .header("x-slack-signature", &signature)
+            .body(Body::from(&body[..]))
+            .unwrap();
+
+        let resp = app.oneshot(req).await.unwrap();
+        assert_eq!(
+            resp.status(),
+            StatusCode::UNAUTHORIZED,
+            "Signature with mismatched timestamp should return 401"
+        );
+    }
 }

src/channels/wasm/schema.rs

@@ -154,6 +154,18 @@ impl ChannelCapabilitiesFile {
             .and_then(|w| w.signature_key_secret_name.as_deref())
     }
 
+    /// Get the HMAC-SHA256 signing secret name for this channel.
+    ///
+    /// Returns the secret name declared in `webhook.hmac_secret_name`,
+    /// used to look up the HMAC signing secret in the secrets store (Slack-style).
+    pub fn hmac_secret_name(&self) -> Option<&str> {
+        self.capabilities
+            .channel
+            .as_ref()
+            .and_then(|c| c.webhook.as_ref())
+            .and_then(|w| w.hmac_secret_name.as_deref())
+    }
+
     /// Get the webhook secret name for this channel.
     ///
     /// Returns the configured secret name or defaults to "{channel_name}_webhook_secret".
@@ -278,6 +290,10 @@ pub struct WebhookSchema {
     /// for signature verification (e.g., Discord interaction verification).
     #[serde(default)]
     pub signature_key_secret_name: Option<String>,
+
+    /// Secret name in secrets store for HMAC-SHA256 signing (Slack-style).
+    #[serde(default)]
+    pub hmac_secret_name: Option<String>,
 }
 
 /// Setup configuration schema.

src/channels/wasm/signature.rs

@@ -1,9 +1,11 @@
-//! Discord Ed25519 signature verification.
+//! Webhook signature verification (Discord Ed25519 and Slack HMAC-SHA256).
 //!
-//! Validates `X-Signature-Ed25519` and `X-Signature-Timestamp` headers
-//! on incoming Discord interaction webhooks, per Discord's security requirements.
+//! Validates request signatures for incoming webhooks:
+//! - Discord: `X-Signature-Ed25519` and `X-Signature-Timestamp` headers
+//! - Slack: `X-Slack-Signature` and `X-Slack-Request-Timestamp` headers
 //!
 //! See: <https://discord.com/developers/docs/interactions/overview#validating-security-request-headers>
+//! See: <https://api.slack.com/authentication/verifying-requests-from-slack>
 
 /// Verify a Discord interaction signature.
 ///
@@ -50,6 +52,60 @@ pub fn verify_discord_signature(
     verifying_key.verify_strict(&message, &signature).is_ok()
 }
 
+/// Verify a Slack webhook signature using HMAC-SHA256.
+///
+/// Slack signs each webhook request with HMAC-SHA256 using:
+/// - basestring = `"v0:" + timestamp + ":" + body`
+/// - signature = hex-encoded HMAC-SHA256(signing_secret, basestring)
+/// - header = `"v0=" + signature` (in `X-Slack-Signature` header)
+///
+/// Includes staleness check: rejects requests with timestamps older than 5 minutes.
+/// Returns `true` if the signature is valid, `false` on any error
+/// (bad timing, mismatched signature, invalid format, etc.).
+pub fn verify_slack_signature(
+    signing_secret: &str,
+    timestamp: &str,
+    body: &[u8],
+    signature_header: &str,
+    now_secs: i64,
+) -> bool {
+    use hmac::{Hmac, Mac};
+    use sha2::Sha256;
+
+    // 1. Parse and check staleness (5-minute window)
+    let ts: i64 = match timestamp.parse() {
+        Ok(v) => v,
+        Err(_) => return false,
+    };
+    if (now_secs - ts).abs() > 300 {
+        return false;
+    }
+
+    // 2. Build the basestring: "v0:{timestamp}:{body}"
+    let mut basestring = Vec::with_capacity(3 + timestamp.len() + 1 + body.len());
+    basestring.extend_from_slice(b"v0:");
+    basestring.extend_from_slice(timestamp.as_bytes());
+    basestring.push(b':');
+    basestring.extend_from_slice(body);
+
+    // 3. Compute HMAC-SHA256
+    let mut mac = match Hmac::<Sha256>::new_from_slice(signing_secret.as_bytes()) {
+        Ok(m) => m,
+        Err(_) => return false,
+    };
+    mac.update(&basestring);
+    let computed = mac.finalize().into_bytes();
+    let computed_hex = hex::encode(computed);
+    let expected = format!("v0={}", computed_hex);
+
+    // 4. Constant-time compare (avoids timing side-channels)
+    use subtle::ConstantTimeEq;
+    expected
+        .as_bytes()
+        .ct_eq(signature_header.as_bytes())
+        .into()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -338,4 +394,264 @@ mod tests {
             "Negative timestamp should be rejected"
         );
     }
+
+    // ── Category: HMAC-SHA256 Signature Verification (Slack) ────────────
+
+    /// Helper: compute expected Slack signature for a given secret, timestamp, and body.
+    fn sign_slack_message(signing_secret: &str, timestamp: &str, body: &[u8]) -> String {
+        use hmac::{Hmac, Mac};
+        use sha2::Sha256;
+
+        let mut basestring = Vec::new();
+        basestring.extend_from_slice(b"v0:");
+        basestring.extend_from_slice(timestamp.as_bytes());
+        basestring.push(b':');
+        basestring.extend_from_slice(body);
+
+        let mut mac = Hmac::<Sha256>::new_from_slice(signing_secret.as_bytes()).unwrap();
+        mac.update(&basestring);
+        let computed = mac.finalize().into_bytes();
+        format!("v0={}", hex::encode(computed))
+    }
+
+    const SLACK_TEST_TS: i64 = 1234567890;
+
+    #[test]
+    fn test_slack_valid_signature_succeeds() {
+        let signing_secret = "my-signing-secret";
+        let timestamp = "1234567890";
+        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G&team_id=T1DC2JH3J";
+
+        let signature = sign_slack_message(signing_secret, timestamp, body);
+        assert!(verify_slack_signature(
+            signing_secret,
+            timestamp,
+            body,
+            &signature,
+            SLACK_TEST_TS
+        ));
+    }
+
+    #[test]
+    fn test_slack_tampered_body_fails() {
+        let signing_secret = "my-signing-secret";
+        let timestamp = "1234567890";
+        let original_body = b"token=xyzz0WbapA4vBCDEFasx0q6G&team_id=T1DC2JH3J";
+        let tampered_body = b"token=MODIFIED&team_id=T1DC2JH3J";
+
+        let signature = sign_slack_message(signing_secret, timestamp, original_body);
+        assert!(
+            !verify_slack_signature(
+                signing_secret,
+                timestamp,
+                tampered_body,
+                &signature,
+                SLACK_TEST_TS
+            ),
+            "Signature for different body should fail"
+        );
+    }
+
+    #[test]
+    fn test_slack_tampered_timestamp_fails() {
+        let signing_secret = "my-signing-secret";
+        let timestamp = "1234567890";
+        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G&team_id=T1DC2JH3J";
+
+        let signature = sign_slack_message(signing_secret, timestamp, body);
+        assert!(
+            !verify_slack_signature(
+                signing_secret,
+                "9999999999", // Different timestamp in signature
+                body,
+                &signature,
+                SLACK_TEST_TS
+            ),
+            "Signature with wrong timestamp should fail"
+        );
+    }
+
+    #[test]
+    fn test_slack_tampered_signature_fails() {
+        let signing_secret = "my-signing-secret";
+        let timestamp = "1234567890";
+        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G&team_id=T1DC2JH3J";
+
+        let signature = sign_slack_message(signing_secret, timestamp, body);
+        // Flip a byte in the signature hex (change first char after "v0=")
+        let chars: Vec<char> = signature.chars().collect();
+        let mut new_chars = chars.clone();
+        if chars.len() > 3 {
+            new_chars[3] = if chars[3] == 'a' { 'b' } else { 'a' };
+        }
+        let modified_sig: String = new_chars.iter().collect();
+
+        assert!(
+            !verify_slack_signature(
+                signing_secret,
+                timestamp,
+                body,
+                &modified_sig,
+                SLACK_TEST_TS
+            ),
+            "Tampered signature should fail"
+        );
+    }
+
+    #[test]
+    fn test_slack_stale_timestamp_rejected() {
+        let signing_secret = "my-signing-secret";
+        let timestamp = "1234567890";
+        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G";
+
+        let signature = sign_slack_message(signing_secret, timestamp, body);
+        // now_secs is 400 seconds after timestamp — too stale
+        assert!(
+            !verify_slack_signature(
+                signing_secret,
+                timestamp,
+                body,
+                &signature,
+                SLACK_TEST_TS + 400
+            ),
+            "Stale timestamp (400s old) should be rejected"
+        );
+    }
+
+    #[test]
+    fn test_slack_future_timestamp_rejected() {
+        let signing_secret = "my-signing-secret";
+        let timestamp = "1234567890";
+        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G";
+
+        let signature = sign_slack_message(signing_secret, timestamp, body);
+        // now_secs is 400 seconds before timestamp — future
+        assert!(
+            !verify_slack_signature(
+                signing_secret,
+                timestamp,
+                body,
+                &signature,
+                SLACK_TEST_TS - 400
+            ),
+            "Future timestamp (400s ahead) should be rejected"
+        );
+    }
+
+    #[test]
+    fn test_slack_boundary_300s_accepted() {
+        let signing_secret = "my-signing-secret";
+        let timestamp = "1234567890";
+        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G";
+
+        let signature = sign_slack_message(signing_secret, timestamp, body);
+        // Exactly 300 seconds difference — should be accepted
+        assert!(
+            verify_slack_signature(
+                signing_secret,
+                timestamp,
+                body,
+                &signature,
+                SLACK_TEST_TS + 300
+            ),
+            "Timestamp exactly 300s old should be accepted"
+        );
+    }
+
+    #[test]
+    fn test_slack_boundary_301s_rejected() {
+        let signing_secret = "my-signing-secret";
+        let timestamp = "1234567890";
+        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G";
+
+        let signature = sign_slack_message(signing_secret, timestamp, body);
+        // 301 seconds difference — should be rejected
+        assert!(
+            !verify_slack_signature(
+                signing_secret,
+                timestamp,
+                body,
+                &signature,
+                SLACK_TEST_TS + 301
+            ),
+            "Timestamp 301s old should be rejected"
+        );
+    }
+
+    #[test]
+    fn test_slack_non_numeric_timestamp_rejected() {
+        let signing_secret = "my-signing-secret";
+        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G";
+
+        assert!(
+            !verify_slack_signature(signing_secret, "not-a-number", body, "v0=abc123", 0),
+            "Non-numeric timestamp should be rejected"
+        );
+    }
+
+    #[test]
+    fn test_slack_missing_v0_prefix_fails() {
+        let signing_secret = "my-signing-secret";
+        let timestamp = "1234567890";
+        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G";
+
+        let signature = sign_slack_message(signing_secret, timestamp, body);
+        // Remove the "v0=" prefix
+        let bad_sig = signature.strip_prefix("v0=").unwrap_or(&signature);
+
+        assert!(
+            !verify_slack_signature(signing_secret, timestamp, body, bad_sig, SLACK_TEST_TS),
+            "Missing v0= prefix should fail"
+        );
+    }
+
+    #[test]
+    fn test_slack_wrong_signing_secret_fails() {
+        let secret_a = "secret-a";
+        let secret_b = "secret-b";
+        let timestamp = "1234567890";
+        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G";
+
+        let signature = sign_slack_message(secret_a, timestamp, body);
+        // Try to verify with a different secret
+        assert!(
+            !verify_slack_signature(secret_b, timestamp, body, &signature, SLACK_TEST_TS),
+            "Signature from different secret should fail"
+        );
+    }
+
+    #[test]
+    fn test_slack_empty_body_valid() {
+        let signing_secret = "my-signing-secret";
+        let timestamp = "1234567890";
+        let body = b"";
+
+        let signature = sign_slack_message(signing_secret, timestamp, body);
+        assert!(
+            verify_slack_signature(signing_secret, timestamp, body, &signature, SLACK_TEST_TS),
+            "Empty body with valid signature should succeed"
+        );
+    }
+
+    #[test]
+    fn test_slack_negative_timestamp_rejected() {
+        let signing_secret = "my-signing-secret";
+        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G";
+
+        assert!(
+            !verify_slack_signature(signing_secret, "-1", body, "v0=abc123", 0),
+            "Negative timestamp should be rejected"
+        );
+    }
+
+    #[test]
+    fn test_slack_empty_timestamp_rejected() {
+        let signing_secret = "my-signing-secret";
+        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G";
+
+        assert!(
+            !verify_slack_signature(signing_secret, "", body, "v0=abc123", 0),
+            "Empty timestamp should be rejected"
+        );
+    }
 }

src/extensions/manager.rs

@@ -2397,6 +2397,7 @@ impl ExtensionManager {
         let webhook_secret_name = loaded.webhook_secret_name();
         let secret_header = loaded.webhook_secret_header().map(|s| s.to_string());
         let sig_key_secret_name = loaded.signature_key_secret_name();
+        let hmac_secret_name = loaded.hmac_secret_name();
 
         // Get webhook secret from secrets store
         let webhook_secret = self
@@ -2480,6 +2481,21 @@ impl ExtensionManager {
                     }
                 }
             }
+
+            // Register HMAC signing secret if declared in capabilities
+            if let Some(hmac_name) = &hmac_secret_name {
+                match self.secrets.get_decrypted(&self.user_id, hmac_name).await {
+                    Ok(secret) => {
+                        wasm_channel_router
+                            .register_hmac_secret(&channel_name, secret.expose())
+                            .await;
+                        tracing::info!(channel = %channel_name, "Registered HMAC signing secret for hot-activated channel");
+                    }
+                    Err(e) => {
+                        tracing::warn!(channel = %channel_name, error = %e, "HMAC secret not found");
+                    }
+                }
+            }
         }
 
         // Inject credentials
@@ -2587,19 +2603,30 @@ impl ExtensionManager {
             }
         };
 
-        // Also refresh the webhook secret in the router
-        // Load capabilities file to get the correct secret name (may be overridden)
-        let webhook_secret_name = {
+        // Load capabilities file once to extract all secret names
         let cap_path = self
             .wasm_channels_dir
             .join(format!("{}.capabilities.json", name));
-            match tokio::fs::read(&cap_path).await {
-                Ok(bytes) => crate::channels::wasm::ChannelCapabilitiesFile::from_bytes(&bytes)
-                    .map(|f| f.webhook_secret_name())
-                    .unwrap_or_else(|_| format!("{}_webhook_secret", name)),
-                Err(_) => format!("{}_webhook_secret", name),
-            }
+        let capabilities_file = match tokio::fs::read(&cap_path).await {
+            Ok(bytes) => crate::channels::wasm::ChannelCapabilitiesFile::from_bytes(&bytes).ok(),
+            Err(_) => None,
         };
+
+        // Extract all secret names from the capabilities file
+        let webhook_secret_name = capabilities_file
+            .as_ref()
+            .map(|f| f.webhook_secret_name())
+            .unwrap_or_else(|| format!("{}_webhook_secret", name));
+
+        let sig_key_secret_name = capabilities_file
+            .as_ref()
+            .and_then(|f| f.signature_key_secret_name().map(|s| s.to_string()));
+
+        let hmac_secret_name = capabilities_file
+            .as_ref()
+            .and_then(|f| f.hmac_secret_name().map(|s| s.to_string()));
+
+        // Refresh webhook secret
         if let Ok(secret) = self
             .secrets
             .get_decrypted(&self.user_id, &webhook_secret_name)
@@ -2618,18 +2645,7 @@ impl ExtensionManager {
             existing_channel.update_config(config_updates).await;
         }
 
-        // Also refresh signature key in the router
-        let sig_key_secret_name = {
-            let cap_path = self
-                .wasm_channels_dir
-                .join(format!("{}.capabilities.json", name));
-            match tokio::fs::read(&cap_path).await {
-                Ok(bytes) => crate::channels::wasm::ChannelCapabilitiesFile::from_bytes(&bytes)
-                    .ok()
-                    .and_then(|f| f.signature_key_secret_name().map(|s| s.to_string())),
-                Err(_) => None,
-            }
-        };
+        // Refresh signature key
         if let Some(ref sig_key_name) = sig_key_secret_name
             && let Ok(key_secret) = self
                 .secrets
@@ -2649,6 +2665,23 @@ impl ExtensionManager {
             }
         }
 
+        // Refresh HMAC signing secret
+        if let Some(ref hmac_secret_name_ref) = hmac_secret_name {
+            match self
+                .secrets
+                .get_decrypted(&self.user_id, hmac_secret_name_ref)
+                .await
+            {
+                Ok(secret) => {
+                    router.register_hmac_secret(name, secret.expose()).await;
+                    tracing::info!(channel = %name, "Refreshed HMAC signing secret");
+                }
+                Err(e) => {
+                    tracing::warn!(channel = %name, error = %e, "HMAC secret not found");
+                }
+            }
+        }
+
         // Refresh tunnel_url in case it wasn't set at startup
         if let Some(ref tunnel_url) = self.tunnel_url {
             let mut config_updates = std::collections::HashMap::new();

src/main.rs

@@ -950,6 +950,7 @@ async fn setup_wasm_channels(
 
         let secret_name = loaded.webhook_secret_name();
         let sig_key_secret_name = loaded.signature_key_secret_name();
+        let hmac_secret_name = loaded.hmac_secret_name();
 
         let webhook_secret = if let Some(secrets) = secrets_store {
             secrets
@@ -1044,6 +1045,17 @@ async fn setup_wasm_channels(
             }
         }
 
+        // Register HMAC signing secret if declared in capabilities
+        if let Some(ref hmac_secret_name) = hmac_secret_name
+            && let Some(secrets) = secrets_store
+            && let Ok(secret) = secrets.get_decrypted("default", hmac_secret_name).await
+        {
+            wasm_router
+                .register_hmac_secret(&channel_name, secret.expose())
+                .await;
+            tracing::info!(channel = %channel_name, "Registered HMAC signing secret");
+        }
+
         if let Some(secrets) = secrets_store {
             match inject_channel_credentials(&channel_arc, secrets.as_ref(), &channel_name).await {
                 Ok(count) => {