Inconsistent interpretation of HTTP requests in undici - CVE-2026-18540
Published: September 6, 2026
Vulnerability identifier: #VU147181
CSH Severity: Low
CVSS v4: 6.3 [CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-18540
CWE-ID: CWE-444
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote attacker to cause downstream response splitting or desynchronization.
The vulnerability exists due to inconsistent interpretation of HTTP response framing in the retry interceptor when resuming a partial response from an attacker-controlled upstream. A remote attacker can cause resumed response bytes to be appended to a partially delivered response body while its original status and headers are retained to cause downstream response splitting or desynchronization.
Exploitation requires interceptors.retry() to be enabled and a downstream forwarder that does not recalculate Content-Length.
Affected software
undici
How to mitigate CVE-2026-18540
Install security update from vendor's website.
undici - addressed in versions 6.28.1, 7.29.1, 8.10.2