CVE-2024-49768: Waitress has request processing race condition in HTTP pipelining with invalid first request

Severity: medium (CVSS 4.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2024-10-29 · updated 2026-06-17
Weakness: CWE-367

Description

Waitress is a Web Server Gateway Interface server for Python 2 and 3. A remote client may send a request that is exactly recv_bytes (defaults to 8192) long, followed by a secondary request using HTTP pipelining. When request lookahead is disabled (default) we won't read any more requests, and when the first request fails due to a parsing error, we simply close the connection. However when request lookahead is enabled, it is possible to process and receive the first request, start sending the error message back to the client while we read the next request and queue it. This will allow the secondary request to be serviced by the worker thread while the connection should be closed. Waitress 3.0.1 fixes the race condition. As a workaround, disable channel_request_lookahead, this is set to 0 by default disabling this feature.

Affected products

Pylons Waitress: from 2.0.0 before 3.0.1 — fixed in 3.0.1

How to fix

Upgrade to a fixed version: Pylons Waitress 3.0.1. Check the vendor advisory for mitigations.

References

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