CVE-2026-16728: undici vulnerable to downstream response desynchronization via retry interceptor

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-07-29 · updated 2026-08-04
Weakness: CWE-444

Description

undici's retry interceptor can deliver a response whose body length does not match the Content-Length header exposed to the application after a retry or resume of a partial response. In undici before 6.28.0, from 7.0.0 up to before 7.29.0, and from 8.0.0 up to before 8.9.0, a malicious or faulty upstream can return a partial response with a mismatched framing header, close the socket early, and have the retry interceptor assemble a body of a different length while the original Content-Length stays attached. Applications that use the retry interceptor and forward upstream headers and bodies downstream, such as proxies or gateways, may then emit an invalid HTTP response with a stale Content-Length, leading to downstream response desynchronization, connection hangs, or response corruption. Exploitation requires the retry interceptor enabled, an upstream returning a mismatched partial response, and a downstream forwarder that does not remove or recalculate Content-Length. The issue is fixed in undici 6.28.0, 7.29.0, and 8.9.0.

Affected products

undici: before 6.28.0; from 7.0.0 before 7.29.0; from 8.0.0 before 8.9.0 — fixed in 6.28.0, 7.29.0, 8.9.0
Npm Undici: before 6.28.0; from 7.0.0 before 7.29.0; from 8.0.0 before 8.9.0 — fixed in 6.28.0, 7.29.0, 8.9.0

How to fix

Upgrade to a fixed version: undici 6.28.0, 7.29.0, 8.9.0; Npm Undici 6.28.0, 7.29.0, 8.9.0. Check the vendor advisory for mitigations.

References

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