diff --git a/crates/fetch/src/http.rs b/crates/fetch/src/http.rs
index c95478388a..1002e5205e 100644
--- a/crates/fetch/src/http.rs
+++ b/crates/fetch/src/http.rs
@@ -519,7 +519,8 @@ impl Response {
                 if *left == 0 {
                     let line = self.conn.line(1024, limits)?;
                     let hex = line.split(';').next().unwrap_or_default().trim();
-                    let size = u64::from_str_radix(hex, 16).map_err(|_| HttpError::Protocol(format!("bad chunk size `{hex}`")))?;
+                    let size = if hex.bytes().all(|b| b.is_ascii_hexdigit()) { u64::from_str_radix(hex, 16).ok() } else { None }
+                        .ok_or_else(|| HttpError::Protocol(format!("bad chunk size `{hex}`")))?;
                     if size == 0 {
                         // trailers, up to the empty line
                         let mut n = 0;
diff --git a/crates/fetch/src/tests.rs b/crates/fetch/src/tests.rs
index 08f869dcca..e1e7a5679d 100644
--- a/crates/fetch/src/tests.rs
+++ b/crates/fetch/src/tests.rs
@@ -86,6 +86,9 @@ fn handle(mut s: TcpStream, hits: &AtomicUsize) {
             b.extend_from_slice(&[7u8; 5000]);
             send(&mut s, "200 OK", "", &b)
         }
+        "signedchunk" => {
+            let _ = s.write_all(b"HTTP/1.1 200 OK\r\nTransfer-Encoding: chunked\r\nConnection: close\r\n\r\n+5\r\nhello\r\n0\r\n\r\n");
+        }
         "chunked" => {
             let _ = s.write_all(b"HTTP/1.1 200 OK\r\nTransfer-Encoding: chunked\r\nConnection: close\r\n\r\n");
             for c in body.chunks(70_001) {
@@ -249,6 +252,16 @@ fn a_416_or_a_bad_complete_part_starts_over() {
     let _ = std::fs::remove_dir_all(&dir);
 }
 
+#[test]
+fn signed_chunk_size_is_rejected() {
+    let srv = server();
+    let dir = tmp("signedchunk");
+    let f = FileSpec { name: "vocab.json", size: None, sha256: None, max: 1 << 20 };
+    let r = run(&[f], &[format!("{}/signedchunk", srv.base)], &dir, &AtomicBool::new(false)).0;
+    assert!(r.is_err(), "a chunk size of `+5` was accepted");
+    let _ = std::fs::remove_dir_all(&dir);
+}
+
 #[test]
 fn chunked_bodies_and_unpinned_files() {
     let srv = server();