diff --git a/Cargo.lock b/Cargo.lock
index 617f54b45..e769854d8 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -10355,7 +10355,7 @@ checksum = "9774ba4a74de5f7b1c1451ed6cd5285a32eddb5cccb8cc655a4e50009e06477f"
 [[package]]
 name = "s3s"
 version = "0.14.1"
-source = "git+https://github.com/cxymds/s3s.git?rev=afa1796ec64cd1e1c78dfee46c4b4b72bbc7b39d#afa1796ec64cd1e1c78dfee46c4b4b72bbc7b39d"
+source = "git+https://github.com/cxymds/s3s.git?rev=fe3941d91fa1c69956f209a9145995c9f0235bff#fe3941d91fa1c69956f209a9145995c9f0235bff"
 dependencies = [
  "arc-swap",
  "arrayvec",
diff --git a/Cargo.toml b/Cargo.toml
index 3329a6e39..8f0a1db8b 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -288,7 +288,7 @@ redis = { version = "1.4.1" }
 rustix = { version = "1.1.4" }
 rust-embed = { version = "8.12.0" }
 rustc-hash = { version = "2.1.3" }
-s3s = { git = "https://github.com/cxymds/s3s.git", rev = "afa1796ec64cd1e1c78dfee46c4b4b72bbc7b39d" }
+s3s = { git = "https://github.com/cxymds/s3s.git", rev = "fe3941d91fa1c69956f209a9145995c9f0235bff" }
 serial_test = "4.0.1"
 shadow-rs = { default-features = false, version = "2.0.0" }
 siphasher = "1.0.3"
diff --git a/deny.toml b/deny.toml
index 0929d8659..91524f4cb 100644
--- a/deny.toml
+++ b/deny.toml
@@ -40,7 +40,7 @@ allow-git = [
     # SigV4 payload-checksum fix until it is available in a crates.io release.
     # owner: marshawcoco  review: 2026-10
     "https://github.com/s3s-project/s3s.git",
-    # Presigned URL expiry validation pending upstream submission.
+    # Presigned expiry and constant-time authentication fixes pending upstream merge.
     # owner: cxymds  review: 2026-10
     "https://github.com/cxymds/s3s.git",
     "https://github.com/apache/datafusion.git",
diff --git a/rustfs/src/admin/router.rs b/rustfs/src/admin/router.rs
index 869f636ee..352c42f7d 100644
--- a/rustfs/src/admin/router.rs
+++ b/rustfs/src/admin/router.rs
@@ -33,7 +33,7 @@ use crate::admin::runtime_sources::{
 };
 use crate::admin::storage_api::access::{ReqInfo, authorize_request, spawn_traced};
 use crate::admin::storage_api::contract::bucket::{BucketOperations, BucketOptions};
-use crate::auth::{check_key_valid, get_session_token};
+use crate::auth::{check_key_valid, constant_time_eq, get_session_token};
 use crate::error::ApiError;
 use crate::license::license_check;
 use crate::server::{
@@ -905,7 +905,9 @@ fn validate_object_lambda_response_auth_headers(headers: &HeaderMap, output_rout
         .and_then(|value| value.to_str().ok())
         .map(str::trim);
 
-    if route == Some(output_route) && token == Some(output_token) {
+    if route.is_some_and(|route| constant_time_eq(route, output_route))
+        && token.is_some_and(|token| constant_time_eq(token, output_token))
+    {
         return Ok(());
     }
 
@@ -4351,6 +4353,39 @@ mod tests {
         let err = validate_object_lambda_response_auth_headers(&mismatched, "route-123", "token-456")
             .expect_err("mismatched auth headers should fail");
         assert_eq!(err.code(), &S3ErrorCode::InvalidRequest);
+
+        for (route, token) in [
+            ("Route-123", "token-456"),
+            ("route-124", "token-456"),
+            ("route-1234", "token-456"),
+            ("route-123", "Token-456"),
+            ("route-123", "token-457"),
+            ("route-123", "token-4567"),
+        ] {
+            let mut headers = HeaderMap::new();
+            headers.insert(
+                "x-amz-request-route",
+                HeaderValue::try_from(route).expect("test route must be a valid header"),
+            );
+            headers.insert(
+                "x-amz-request-token",
+                HeaderValue::try_from(token).expect("test token must be a valid header"),
+            );
+            assert!(
+                validate_object_lambda_response_auth_headers(&headers, "route-123", "token-456").is_err(),
+                "first-byte, last-byte, and length mismatches must fail: {route}/{token}"
+            );
+        }
+    }
+
+    #[test]
+    fn object_lambda_auth_headers_use_constant_time_helper() {
+        let source = include_str!("router.rs");
+        let production = source.split_once("#[cfg(test)]").map_or(source, |(production, _)| production);
+        assert!(!production.contains("route == Some(output_route)"));
+        assert!(!production.contains("token == Some(output_token)"));
+        assert!(production.contains("constant_time_eq(route, output_route)"));
+        assert!(production.contains("constant_time_eq(token, output_token)"));
     }
 
     #[test]
diff --git a/rustfs/src/auth.rs b/rustfs/src/auth.rs
index 731430790..298c39fa5 100644
--- a/rustfs/src/auth.rs
+++ b/rustfs/src/auth.rs
@@ -452,7 +452,7 @@ pub fn check_claims_from_token(token: &str, cred: &Credentials) -> S3Result<Hash
         return Err(s3_error!(InvalidRequest, "invalid token2"));
     }
 
-    if !cred.is_service_account() && cred.is_temp() && token != cred.session_token {
+    if !cred.is_service_account() && cred.is_temp() && !constant_time_eq(token, &cred.session_token) {
         return Err(s3_error!(InvalidRequest, "invalid token3"));
     }
 
@@ -1801,9 +1801,10 @@ mod tests {
     #[test]
     fn test_constant_time_eq() {
         assert!(constant_time_eq("test", "test"));
-        assert!(!constant_time_eq("test", "Test"));
-        assert!(!constant_time_eq("test", "test1"));
-        assert!(!constant_time_eq("test1", "test"));
+        assert!(!constant_time_eq("Test", "test"), "first-byte mismatch must fail");
+        assert!(!constant_time_eq("tesu", "test"), "last-byte mismatch must fail");
+        assert!(!constant_time_eq("test", "test1"), "longer candidate must fail");
+        assert!(!constant_time_eq("test1", "test"), "shorter candidate must fail");
         assert!(!constant_time_eq("", "test"));
         assert!(constant_time_eq("", ""));
 
@@ -1815,6 +1816,18 @@ mod tests {
         assert!(!constant_time_eq(key1, key3));
     }
 
+    #[test]
+    fn session_token_comparison_uses_constant_time_helper() {
+        let source = include_str!("auth.rs");
+        let production = source.split_once("#[cfg(test)]").map_or(source, |(production, _)| production);
+        let ordinary_comparison = ["token ", "!=", " cred.session_token"].concat();
+        assert!(
+            !production.contains(&ordinary_comparison),
+            "temporary session tokens must not use ordinary string comparison"
+        );
+        assert!(production.contains("!constant_time_eq(token, &cred.session_token)"));
+    }
+
     #[test]
     fn test_get_condition_values_source_ip() {
         let mut headers = HeaderMap::new();