GPA to return 404 if nested percent encoding characters detected.

cspell.json

@@ -248,6 +248,7 @@
     "guiddef",
     "handleapi",
     "hdr",
+    "hexdigit",
     "HEXINT",
     "hklm",
     "hlist",
@@ -374,6 +375,7 @@
     "npidof",
     "nprintf",
     "NSG",
+    "nstance",
     "nsudo",
     "ntdll",
     "ntimeout",

proxy_agent/src/proxy/proxy_connection.rs

@@ -303,6 +303,12 @@ impl HttpConnectionContext {
         path_has_traversal(self.url.path())
     }
 
+    /// Checks if the request path contains nested percent-encoding, which can be used to bypass
+    /// certain security checks. Returns `true` if nested percent-encoding is detected.
+    pub fn has_nested_percent_encoding(&self) -> bool {
+        path_has_nested_percent_encoding(self.url.path())
+    }
+
     pub fn log(&mut self, logger_level: LoggerLevel, message: String) {
         if config::get_enable_http_proxy_trace() {
             self.add_stage(message.clone());
@@ -448,9 +454,22 @@ fn path_has_traversal(raw_path: &str) -> bool {
     decoded.contains("..")
 }
 
+/// Detect nested percent-encoding without changing request handling.
+/// Every deeper encoding exposes a valid `%HH` sequence after one decode.
+fn path_has_nested_percent_encoding(raw_path: &str) -> bool {
+    let decoded = percent_encoding::percent_decode_str(raw_path).decode_utf8_lossy();
+    contains_percent_encoded_byte(decoded.as_bytes())
+}
+
+fn contains_percent_encoded_byte(value: &[u8]) -> bool {
+    value.windows(3).any(|window| {
+        window[0] == b'%' && window[1].is_ascii_hexdigit() && window[2].is_ascii_hexdigit()
+    })
+}
+
 #[cfg(test)]
 mod tests {
-    use super::path_has_traversal;
+    use super::{path_has_nested_percent_encoding, path_has_traversal};
 
     #[test]
     fn clean_paths_are_not_traversal() {
@@ -532,4 +551,33 @@ mod tests {
             );
         }
     }
+
+    #[test]
+    fn nested_percent_encoding_tests() {
+        for p in [
+            "/foo/%252e%252e/bar",
+            "/foo/%252E%252e/bar",
+            "/foo/%25252e%25252e/bar",
+            "/foo/%25%32%65%25%32%65/bar",
+            "/metadata%252fidentity/oauth2/token",
+            "/metadata/%253Fapi-version=2021-02-01",
+        ] {
+            assert!(
+                path_has_nested_percent_encoding(p),
+                "expected nested percent encoding to be detected: {p:?}"
+            );
+        }
+
+        for p in [
+            "/metadata/%69nstance",
+            "/metadata/value%25done",
+            "/metadata/value%252",
+            "/metadata/value%25zz",
+        ] {
+            assert!(
+                !path_has_nested_percent_encoding(p),
+                "expected unambiguous encoding to be allowed: {p:?}"
+            );
+        }
+    }
 }

proxy_agent/src/proxy/proxy_server.rs

@@ -439,6 +439,18 @@ impl ProxyServer {
             return Ok(Self::method_not_allowed_response());
         }
 
+        if http_connection_context.has_nested_percent_encoding() {
+            // If the proxied request contains nested percent encoding characters, we will return 404 Not Found to avoid potential security issues.
+            self.log_connection_summary(
+                &mut http_connection_context,
+                StatusCode::NOT_FOUND,
+                false,
+                "Nested percent encoding found in the request, return NOT FOUND!".to_string(),
+            )
+            .await;
+            return Ok(Self::closed_response(StatusCode::NOT_FOUND));
+        }
+
         if http_connection_context.contains_traversal_characters() {
             // If the proxied request contains traversal characters, we will return 403 Forbidden to avoid potential security issues.
             self.log_connection_summary(
@@ -1330,6 +1342,39 @@ mod tests {
             "response.status must be FORBIDDEN."
         );
 
+        // Match IIS request-filtering behavior for double and deeper
+        // percent encoding without recursively decoding the request.
+        for path in [
+            "/test/%252e%252e/",
+            "/test/%25252e%25252e/",
+            "/test/%25%32%65%25%32%65/",
+        ] {
+            let endpoint = hyper_client::HostEndpoint::new(host, port, path);
+            let request = hyper_client::build_request(
+                Method::GET,
+                &endpoint,
+                &HashMap::new(),
+                None,
+                key_keeper_shared_state
+                    .get_current_key_guid()
+                    .await
+                    .unwrap_or(None),
+                key_keeper_shared_state
+                    .get_current_key_value()
+                    .await
+                    .unwrap_or(None),
+            )
+            .unwrap();
+            let response = hyper_client::send_request(host, port, request, logger::write_warning)
+                .await
+                .unwrap();
+            assert_eq!(
+                http::StatusCode::NOT_FOUND,
+                response.status(),
+                "nested percent-encoded path {path:?} must return NOT_FOUND"
+            );
+        }
+
         // test large request body
         let body = vec![88u8; super::REQUEST_BODY_LOW_LIMIT_SIZE + 1];
         let request = hyper_client::build_request(