GHSA-x2qc-cmh9-f4hf
MEDIUMGHSA-x2qc-cmh9-f4hf is a medium-severity (CVSS 4.3) CWE-248 vulnerability in deno. O3 Security confirms whether GHSA-x2qc-cmh9-f4hf is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Deno: Denial of service via non-ASCII bytes in WebSocket response headers
Blast Radius
denoReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects crates.io packages — download data is not available via public APIs for these ecosystems.
Description
Summary
A Deno program that opens a client WebSocket connection could be crashed by
the remote server. While handling the WebSocket handshake response, Deno parsed
the Sec-WebSocket-Protocol and Sec-WebSocket-Extensions response headers in
a way that assumed their bytes were always printable ASCII. A response header
containing non-visible-ASCII bytes (0x80-0xFF) caused a panic that aborted
the entire Deno process.
Details
When establishing a client WebSocket connection, Deno read the
Sec-WebSocket-Protocol and Sec-WebSocket-Extensions headers from the
server's 101 Switching Protocols response and converted them to strings
without handling the failure case. HeaderValue::to_str() returns an error for
any value containing bytes outside the visible-ASCII range, so a header carrying
such bytes triggered an unrecoverable error during conversion.
Because the client initiates the outbound connection, the handshake response is
fully controlled by the server. A server that returns bytes such as 0xFF 0xFE
in either header could therefore crash any client that connected to it.
This is purely an availability issue. There is no information disclosure and no memory-safety impact; the only effect is termination of the current process.
Impact
Remote denial of service. Any Deno application that establishes WebSocket
connections to untrusted or potentially-compromised endpoints could be
terminated by the remote peer. Exploitation requires the victim application to
initiate the outbound WebSocket connection. An attacker who controls the
WebSocket endpoint, or who can man-in-the-middle a plaintext ws:// connection,
could trigger the crash. The effect is confined to crashing the process that
opened the connection.
Patch
The issue is fixed in Deno 2.7.5. The header values are now parsed with
graceful fallbacks: values that cannot be represented as ASCII strings are
skipped instead of aborting the process. A regression test covers a server that
returns non-ASCII bytes in Sec-WebSocket-Protocol.
Users should upgrade to Deno 2.7.5 or later.
Workarounds
Until you can upgrade, only connect to trusted WebSocket endpoints and prefer
wss:// (TLS) over ws://, which prevents a network man-in-the-middle from
injecting malicious header bytes into the handshake response.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | deno | all versions | 2.7.5 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for deno. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update deno to 2.7.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-x2qc-cmh9-f4hf is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-x2qc-cmh9-f4hf is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-x2qc-cmh9-f4hf. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-x2qc-cmh9-f4hf in your dependencies?
O3 detects GHSA-x2qc-cmh9-f4hf across crates.io dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.