Skip to content

Catastrophic backtracking in regex allows Denial of Service

critical severity CVE-2020-5236 published Feb 4, 2020 • updated Feb 5, 2020
@Pylons Pylons/waitress
Packages Affected versions Patched versions
waitress (pip) = 1.4.2 1.4.3


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.


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.


If you have deployed a reverse proxy in front of Waitress it may already be rejecting requests that include invalid headers.


The Pylons Project would like to thank Fil Zembowicz for reaching out and disclosing this vulnerability!


Catastrophic backtracking explained:

For more information

If you have any questions or comments about this advisory:

@bertjwregeer bertjwregeer published the maintainer security advisory Feb 3, 2020
You can’t perform that action at this time.