diff --git a/crates/e2e_test/src/security_boundary_test.rs b/crates/e2e_test/src/security_boundary_test.rs
index 34d026c12..0315422ff 100644
--- a/crates/e2e_test/src/security_boundary_test.rs
+++ b/crates/e2e_test/src/security_boundary_test.rs
@@ -21,11 +21,13 @@
 //! - SSRF prevention (internal/private endpoints rejected for tiering)
 //! - Race condition handling (concurrent writes converge without corruption)
 
-use crate::common::{RustFSTestEnvironment, awscurl_put, init_logging, require_awscurl};
+use crate::common::{RustFSTestEnvironment, init_logging, signed_s3_request};
 use aws_sdk_s3::error::ProvideErrorMetadata;
 use aws_sdk_s3::primitives::ByteStream;
 use aws_sdk_s3::types::{CompletedMultipartUpload, CompletedPart, Tag, Tagging};
 use std::error::Error;
+use std::time::Duration;
+use tokio::net::TcpListener;
 
 /// Oversized tagging payloads must be rejected by the per-object tag limit.
 ///
@@ -74,8 +76,12 @@ async fn test_large_xml_body_rejection() -> Result<(), Box<dyn Error + Send + Sy
     let _ = client.delete_object().bucket(&bucket_name).key(object_key).send().await;
     let _ = client.delete_bucket().bucket(&bucket_name).send().await;
 
-    assert!(result.is_err(), "Server must reject an oversized tagging payload, but it was accepted");
-    let err = result.expect_err("checked is_err above");
+    let err = result.expect_err("Server must reject an oversized tagging payload");
+    assert_eq!(
+        err.raw_response().map(|response| response.status().as_u16()),
+        Some(400),
+        "Oversized tagging should return HTTP 400, got: {err:?}"
+    );
     let code = err.as_service_error().and_then(|e| e.code());
     assert_eq!(
         code,
@@ -146,6 +152,11 @@ async fn test_excessive_multipart_parts() -> Result<(), Box<dyn Error + Send + S
     let _ = client.delete_bucket().bucket(&bucket_name).send().await;
 
     let error = result.expect_err("server should reject a completion part number above 10000");
+    assert_eq!(
+        error.raw_response().map(|response| response.status().as_u16()),
+        Some(400),
+        "part-limit rejection must return HTTP 400, got: {error:?}"
+    );
     let service_error = error
         .as_service_error()
         .expect("part-limit rejection must be an S3 service error");
@@ -219,15 +230,14 @@ async fn test_concurrent_object_operations() -> Result<(), Box<dyn Error + Send
     // After deletion, the object must be absent.
     client.delete_object().bucket(&bucket_name).key(key).send().await?;
     let get_after_delete = client.get_object().bucket(&bucket_name).key(key).send().await;
-    assert!(get_after_delete.is_err(), "Object must be absent after delete");
-    let code = get_after_delete
-        .err()
-        .and_then(|e| e.as_service_error().and_then(|se| se.code()).map(str::to_string));
+    let error = get_after_delete.expect_err("Object must be absent after delete");
     assert_eq!(
-        code.as_deref(),
-        Some("NoSuchKey"),
-        "GET after delete should return NoSuchKey, got {code:?}"
+        error.raw_response().map(|response| response.status().as_u16()),
+        Some(404),
+        "GET after delete should return HTTP 404, got: {error:?}"
     );
+    let code = error.as_service_error().and_then(|service_error| service_error.code());
+    assert_eq!(code, Some("NoSuchKey"), "GET after delete should return NoSuchKey, got: {error:?}");
 
     let _ = client.delete_bucket().bucket(&bucket_name).send().await;
 
@@ -235,35 +245,31 @@ async fn test_concurrent_object_operations() -> Result<(), Box<dyn Error + Send
     Ok(())
 }
 
-/// Internal/private endpoints must be rejected as remote tier backends (SSRF).
+/// Internal/private endpoints must be rejected as remote tier backends (SSRF)
+/// before RustFS opens a connection to them.
 ///
 /// This issues a real admin AddTier call (`PUT /rustfs/admin/v3/tier`) for each
-/// internal/private endpoint and asserts the server rejects it (non-2xx, so the
-/// signed request helper returns an error). An internal endpoint must never be
-/// accepted as a tier backend. The rejection may originate from explicit
-/// SSRF/internal-address filtering or from the backend connectivity/credential
-/// validation performed during AddTier; either way the security-relevant
-/// outcome — the internal endpoint is not accepted — is asserted here.
-///
-/// The admin API is exercised via signed `awscurl` requests, matching the
-/// pattern used by the other admin-API E2E tests in this crate. The full E2E
-/// lane installs and verifies the pinned `awscurl` prerequisite.
+/// internal/private endpoint and requires the exact validation response. A
+/// loopback listener additionally proves the literal loopback case is rejected
+/// without an outbound connection; a generic connectivity failure is not
+/// sufficient evidence of SSRF protection.
 #[tokio::test]
 async fn test_tiering_url_validation() -> Result<(), Box<dyn Error + Send + Sync>> {
     init_logging();
-    require_awscurl()?;
     let mut env = RustFSTestEnvironment::new().await?;
     env.start_rustfs_server(vec![]).await?;
 
     let tier_url = format!("{}/rustfs/admin/v3/tier", env.url);
+    let sentinel = TcpListener::bind("127.0.0.1:0").await?;
+    let sentinel_endpoint = format!("http://{}", sentinel.local_addr()?);
     let internal_endpoints = [
-        "http://127.0.0.1:8080",
-        "http://localhost:8080",
-        "http://169.254.169.254", // cloud instance metadata endpoint
-        "http://[::1]:8080",
+        (sentinel_endpoint.as_str(), true),
+        ("http://localhost:8080", false),
+        ("http://169.254.169.254", false), // cloud instance metadata endpoint
+        ("http://[::1]:8080", false),
     ];
 
-    for endpoint in internal_endpoints {
+    for (endpoint, checks_connection_attempt) in internal_endpoints {
         // AddTier expects an uppercase tier name and a backend configuration.
         let body = serde_json::json!({
             "type": "s3",
@@ -280,11 +286,30 @@ async fn test_tiering_url_validation() -> Result<(), Box<dyn Error + Send + Sync
         })
         .to_string();
 
-        let result = awscurl_put(&tier_url, &body, &env.access_key, &env.secret_key).await;
+        let response = signed_s3_request(
+            http::Method::PUT,
+            &tier_url,
+            Some(body),
+            Some("application/json"),
+            &env.access_key,
+            &env.secret_key,
+        )
+        .await?;
+        let status = response.status();
+        let response_body = response.text().await?;
+        assert_eq!(status, 400, "AddTier must reject internal endpoint {endpoint} with HTTP 400");
         assert!(
-            result.is_err(),
-            "AddTier must reject internal endpoint {endpoint}, but it was accepted: {result:?}"
+            response_body.contains("<Code>InvalidArgument</Code>") && response_body.contains("tier endpoint is not allowed"),
+            "AddTier must reject internal endpoint {endpoint} during URL validation, got: {response_body}"
         );
+
+        if checks_connection_attempt {
+            match tokio::time::timeout(Duration::from_secs(1), sentinel.accept()).await {
+                Err(_) => {}
+                Ok(Ok((_, peer))) => panic!("SSRF guard connected to loopback endpoint {endpoint} from {peer}"),
+                Ok(Err(error)) => return Err(error.into()),
+            }
+        }
     }
 
     env.stop_server();
diff --git a/rustfs/src/admin/handlers/tier.rs b/rustfs/src/admin/handlers/tier.rs
index 03cd43714..81b8dd7d9 100644
--- a/rustfs/src/admin/handlers/tier.rs
+++ b/rustfs/src/admin/handlers/tier.rs
@@ -17,8 +17,9 @@ use crate::admin::runtime_sources::object_store_from_extensions;
 use crate::admin::storage_api::runtime_sources::TierConfigMgr;
 use crate::admin::storage_api::tier::{
     AdminError, DailyAllTierStats, ERR_TIER_ALREADY_EXISTS, ERR_TIER_BACKEND_IN_USE, ERR_TIER_BACKEND_NOT_EMPTY,
-    ERR_TIER_CONNECT_ERR, ERR_TIER_INVALID_CREDENTIALS, ERR_TIER_MISSING_CREDENTIALS, ERR_TIER_NAME_NOT_UPPERCASE,
-    ERR_TIER_NOT_FOUND, ERR_TIER_RESERVED_NAME, TierConfig, TierConfigUpdateError, TierCreds, TierType,
+    ERR_TIER_CONNECT_ERR, ERR_TIER_INVALID_CONFIG, ERR_TIER_INVALID_CREDENTIALS, ERR_TIER_MISSING_CREDENTIALS,
+    ERR_TIER_NAME_NOT_UPPERCASE, ERR_TIER_NOT_FOUND, ERR_TIER_RESERVED_NAME, TierConfig, TierConfigUpdateError, TierCreds,
+    TierType,
 };
 use crate::{
     admin::runtime_sources::{current_daily_tier_stats, current_notification_system, current_tier_config_handle},
@@ -391,6 +392,8 @@ impl Operation for AddTier {
                     S3ErrorCode::Custom("TierConnectError".into()),
                     "tier connectivity check failed",
                 ))
+            } else if err.code == ERR_TIER_INVALID_CONFIG.code {
+                Err(S3Error::with_message(S3ErrorCode::InvalidArgument, err.message))
             } else if err.code == ERR_TIER_INVALID_CREDENTIALS.code {
                 Err(S3Error::with_message(S3ErrorCode::Custom(err.code.clone().into()), err.message))
             } else {
diff --git a/rustfs/src/admin/storage_api.rs b/rustfs/src/admin/storage_api.rs
index df72db34d..fdb6b8a4b 100644
--- a/rustfs/src/admin/storage_api.rs
+++ b/rustfs/src/admin/storage_api.rs
@@ -817,6 +817,7 @@ pub(crate) static ERR_TIER_MISSING_CREDENTIALS: AdminErrorRef =
 pub(crate) static ERR_TIER_ALREADY_EXISTS: AdminErrorRef =
     AdminErrorRef(|| &ecstore_tier::tier_handlers::ERR_TIER_ALREADY_EXISTS);
 pub(crate) static ERR_TIER_CONNECT_ERR: AdminErrorRef = AdminErrorRef(|| &ecstore_tier::tier_handlers::ERR_TIER_CONNECT_ERR);
+pub(crate) static ERR_TIER_INVALID_CONFIG: AdminErrorRef = AdminErrorRef(|| &ecstore_tier::tier::ERR_TIER_INVALID_CONFIG);
 pub(crate) static ERR_TIER_INVALID_CREDENTIALS: AdminErrorRef =
     AdminErrorRef(|| &ecstore_tier::tier_handlers::ERR_TIER_INVALID_CREDENTIALS);
 pub(crate) static ERR_TIER_NAME_NOT_UPPERCASE: AdminErrorRef =
@@ -963,7 +964,8 @@ pub(crate) mod s3 {
 pub(crate) mod tier {
     pub(crate) use super::{
         AdminError, DailyAllTierStats, ERR_TIER_ALREADY_EXISTS, ERR_TIER_BACKEND_IN_USE, ERR_TIER_BACKEND_NOT_EMPTY,
-        ERR_TIER_CONNECT_ERR, ERR_TIER_INVALID_CREDENTIALS, ERR_TIER_MISSING_CREDENTIALS, ERR_TIER_NAME_NOT_UPPERCASE,
-        ERR_TIER_NOT_FOUND, ERR_TIER_RESERVED_NAME, TierConfig, TierConfigUpdateError, TierCreds, TierType,
+        ERR_TIER_CONNECT_ERR, ERR_TIER_INVALID_CONFIG, ERR_TIER_INVALID_CREDENTIALS, ERR_TIER_MISSING_CREDENTIALS,
+        ERR_TIER_NAME_NOT_UPPERCASE, ERR_TIER_NOT_FOUND, ERR_TIER_RESERVED_NAME, TierConfig, TierConfigUpdateError, TierCreds,
+        TierType,
     };
 }