undici vulnerable to downstream response desynchronization via retry interceptor

Summary

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.

Timeline

1. Jul 29, 2026, 8:59 PM UTCCVE published
2. Jul 30, 2026, 2:20 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: partial

3. Jul 30, 2026, 3:18 PM UTCRecord last updated

Affected versions

• undici undici

  fixed in 6.28.0; From 7.0.0 · fixed in 7.29.0; From 8.0.0 · fixed in 8.9.0

• undicinpm

  fixed in 6.28.0

• undicinpm

  From 7.0.0 · fixed in 7.29.0

• undicinpm

  From 8.0.0 · fixed in 8.9.0

References 2