diff --git a/Cargo.lock b/Cargo.lock
index 51837298..de9dd4ea 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2827,6 +2827,7 @@ dependencies = [
  "serde",
  "serde_json",
  "sha2 0.10.9",
+ "subtle",
  "temp-env",
  "tokio",
  "tokio-test",
diff --git a/Cargo.toml b/Cargo.toml
index 424c357d..00655e1e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -82,6 +82,7 @@ regex = "1.12.3"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0.149"
 sha2 = "0.10.9"
+subtle = "2.6"
 temp-env = "0.3.6"
 tokio = { version = "1.49", features = ["sync", "macros", "io-util", "rt", "time"] }
 tokio-test = "0.4"
diff --git a/crates/trusted-server-core/Cargo.toml b/crates/trusted-server-core/Cargo.toml
index 76b1106d..6a321fd2 100644
--- a/crates/trusted-server-core/Cargo.toml
+++ b/crates/trusted-server-core/Cargo.toml
@@ -41,6 +41,7 @@ regex = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
 sha2 = { workspace = true }
+subtle = { workspace = true }
 tokio = { workspace = true }
 toml = { workspace = true }
 trusted-server-js = { path = "../js" }
diff --git a/crates/trusted-server-core/src/auth.rs b/crates/trusted-server-core/src/auth.rs
index 816891f6..e23fd06d 100644
--- a/crates/trusted-server-core/src/auth.rs
+++ b/crates/trusted-server-core/src/auth.rs
@@ -1,6 +1,8 @@
 use base64::{engine::general_purpose::STANDARD, Engine as _};
 use fastly::http::{header, StatusCode};
 use fastly::{Request, Response};
+use sha2::{Digest as _, Sha256};
+use subtle::ConstantTimeEq as _;
 
 use crate::settings::Settings;
 
@@ -30,7 +32,19 @@ pub fn enforce_basic_auth(settings: &Settings, req: &Request) -> Option<Response
         None => return Some(unauthorized_response()),
     };
 
-    if handler.username == username && handler.password == password {
+    // Hash before comparing to normalise lengths — `ct_eq` on raw byte slices
+    // short-circuits when lengths differ, which would leak credential length.
+    // SHA-256 produces fixed-size digests so the comparison is truly constant-time.
+    //
+    // Note: constant-time guarantees are best-effort on WASM targets because the
+    // runtime optimiser/JIT may re-introduce variable-time paths. This is an
+    // inherent limitation of all constant-time code in managed runtimes.
+    let username_match = Sha256::digest(handler.username.expose().as_bytes())
+        .ct_eq(&Sha256::digest(username.as_bytes()));
+    let password_match = Sha256::digest(handler.password.expose().as_bytes())
+        .ct_eq(&Sha256::digest(password.as_bytes()));
+
+    if bool::from(username_match & password_match) {
         None
     } else {
         Some(unauthorized_response())
@@ -76,16 +90,11 @@ mod tests {
     use base64::engine::general_purpose::STANDARD;
     use fastly::http::{header, Method};
 
-    use crate::test_support::tests::crate_test_settings_str;
-
-    fn settings_with_handlers() -> Settings {
-        let config = crate_test_settings_str();
-        Settings::from_toml(&config).expect("should parse settings with handlers")
-    }
+    use crate::test_support::tests::create_test_settings;
 
     #[test]
     fn no_challenge_for_non_protected_path() {
-        let settings = settings_with_handlers();
+        let settings = create_test_settings();
         let req = Request::new(Method::GET, "https://example.com/open");
 
         assert!(enforce_basic_auth(&settings, &req).is_none());
@@ -93,7 +102,7 @@ mod tests {
 
     #[test]
     fn challenge_when_missing_credentials() {
-        let settings = settings_with_handlers();
+        let settings = create_test_settings();
         let req = Request::new(Method::GET, "https://example.com/secure");
 
         let response = enforce_basic_auth(&settings, &req).expect("should challenge");
@@ -106,7 +115,7 @@ mod tests {
 
     #[test]
     fn allow_when_credentials_match() {
-        let settings = settings_with_handlers();
+        let settings = create_test_settings();
         let mut req = Request::new(Method::GET, "https://example.com/secure/data");
         let token = STANDARD.encode("user:pass");
         req.set_header(header::AUTHORIZATION, format!("Basic {token}"));
@@ -115,19 +124,20 @@ mod tests {
     }
 
     #[test]
-    fn challenge_when_credentials_mismatch() {
-        let settings = settings_with_handlers();
+    fn challenge_when_both_credentials_wrong() {
+        let settings = create_test_settings();
         let mut req = Request::new(Method::GET, "https://example.com/secure/data");
-        let token = STANDARD.encode("user:wrong");
+        let token = STANDARD.encode("wrong:wrong");
         req.set_header(header::AUTHORIZATION, format!("Basic {token}"));
 
-        let response = enforce_basic_auth(&settings, &req).expect("should challenge");
+        let response = enforce_basic_auth(&settings, &req)
+            .expect("should challenge when both username and password are wrong");
         assert_eq!(response.get_status(), StatusCode::UNAUTHORIZED);
     }
 
     #[test]
     fn challenge_when_scheme_is_not_basic() {
-        let settings = settings_with_handlers();
+        let settings = create_test_settings();
         let mut req = Request::new(Method::GET, "https://example.com/secure");
         req.set_header(header::AUTHORIZATION, "Bearer token");
 
@@ -137,7 +147,7 @@ mod tests {
 
     #[test]
     fn allow_admin_path_with_valid_credentials() {
-        let settings = settings_with_handlers();
+        let settings = create_test_settings();
         let mut req = Request::new(Method::POST, "https://example.com/admin/keys/rotate");
         let token = STANDARD.encode("admin:admin-pass");
         req.set_header(header::AUTHORIZATION, format!("Basic {token}"));
@@ -150,7 +160,7 @@ mod tests {
 
     #[test]
     fn challenge_admin_path_with_wrong_credentials() {
-        let settings = settings_with_handlers();
+        let settings = create_test_settings();
         let mut req = Request::new(Method::POST, "https://example.com/admin/keys/rotate");
         let token = STANDARD.encode("admin:wrong");
         req.set_header(header::AUTHORIZATION, format!("Basic {token}"));
@@ -162,11 +172,35 @@ mod tests {
 
     #[test]
     fn challenge_admin_path_with_missing_credentials() {
-        let settings = settings_with_handlers();
+        let settings = create_test_settings();
         let req = Request::new(Method::POST, "https://example.com/admin/keys/rotate");
 
         let response = enforce_basic_auth(&settings, &req)
             .expect("should challenge admin path with missing credentials");
         assert_eq!(response.get_status(), StatusCode::UNAUTHORIZED);
     }
+
+    #[test]
+    fn challenge_when_username_wrong_password_correct() {
+        let settings = create_test_settings();
+        let mut req = Request::new(Method::GET, "https://example.com/secure/data");
+        let token = STANDARD.encode("wrong:pass");
+        req.set_header(header::AUTHORIZATION, format!("Basic {token}"));
+
+        let response = enforce_basic_auth(&settings, &req)
+            .expect("should challenge when only username is wrong");
+        assert_eq!(response.get_status(), StatusCode::UNAUTHORIZED);
+    }
+
+    #[test]
+    fn challenge_when_username_correct_password_wrong() {
+        let settings = create_test_settings();
+        let mut req = Request::new(Method::GET, "https://example.com/secure/data");
+        let token = STANDARD.encode("user:wrong");
+        req.set_header(header::AUTHORIZATION, format!("Basic {token}"));
+
+        let response = enforce_basic_auth(&settings, &req)
+            .expect("should challenge when only password is wrong");
+        assert_eq!(response.get_status(), StatusCode::UNAUTHORIZED);
+    }
 }