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GHSA-73m2-3pwg-5fgc
No affected components available
Impact
When waitress receives a header that contains invalid characters it will cause the regular expression engine to catastrophically backtrack causing the process to use 100% CPU time and blocking any other interactions.
This would allow an attacker to send a single request with an invalid header and take the service offline.
Invalid header example:
Bad-header: xxxxxxxxxxxxxxx\x10
Increasing the number of x's in the header will increase the amount of time Waitress spends in the regular expression engine.
This issue was introduced in version 1.4.2 when the regular expression was updated to attempt to match the behaviour required by errata associated with RFC7230.
Patches
The regular expression that is used to validate incoming headers has been updated in version 1.4.3, it is recommended that people upgrade to the new version of Waitress as soon as possible.
Workarounds
If you have deployed a reverse proxy in front of Waitress it may already be rejecting requests that include invalid headers.
Thanks
The Pylons Project would like to thank Fil Zembowicz for reaching out and disclosing this vulnerability!
References
Catastrophic backtracking explained: https://www.regular-expressions.info/catastrophic.html
For more information
If you have any questions or comments about this advisory:
- open an issue at https://github.com/Pylons/waitress/issues (if not sensitive or security related)
- email the Pylons Security mailing list: pylons-project-security@googlegroups.com (if security related)
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker needs basic access or low-level privileges. The attacker needs the user to perform some action, like clicking a link. The impact is confined to the system where the vulnerability exists. There is a high impact on the availability of the system.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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