Uncaught Exception (due to a data race) leads to process termination in WaitressGHSA-f5x9-8jwc-25rw
MEDIUMFix: Pylons/waitress#377GHSA-f5x9-8jwc-25rw is a medium-severity (CVSS 6.5) CWE-248 vulnerability in waitress. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for waitress — see the affected versions and patch details below.
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.
Exploitation and automatability from CISA’s SSVC triage for GHSA-f5x9-8jwc-25rw.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
waitressReal-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
Impact
Waitress may terminate early due to a thread closing a socket while the main thread is about to call select(). This will lead to the main thread raising an exception that is not handled and then causing the entire application to be killed.
Patches
This issue has been fixed in Waitress 2.1.2 by no longer allowing the WSGI thread to close the socket, instead it is always delegated to the main thread.
Workarounds
There is no work-around, however users using waitress behind a reverse proxy server are less likely to have issues if the reverse proxy always reads the full response.
For more information
If you have any questions or comments about this advisory:
- Open an issue in https://github.com/Pylons/waitress/issues (if not sensitive or security related)
- email the Pylons Security mailing list: [email protected] (if security related)
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | waitress | ≥ 2.1.0&&< 2.1.2 | 2.1.2pip install --upgrade 'waitress==2.1.2' |
Affected Products
waitressagendalessResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for waitress, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update waitress to 2.1.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-f5x9-8jwc-25rw 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.
Frequently Asked Questions
Is GHSA-f5x9-8jwc-25rw in your dependencies?
Find it across PyPI, including transitive dependencies.