CVE-2026-18540: undici vulnerable to downstream response splitting via retry interceptor

Severity: low (CVSS 3.7 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: 2026-09-04 · updated 2026-09-16
Weakness: CWE-444

Description

undici's retry interceptor can append the body of a ranged retry response to bytes already delivered from an earlier partial response while still presenting the original response's status and headers. This happens when an upstream server delivers part of a body without a trustworthy resume checkpoint, for example a non-success response whose headers were already sent or a partial-content response with an unusable content range, then closes the connection and answers the resumed range request with more bytes. As a result the response body can be longer than the Content-Length that the application observes. An application that relays such a response to a downstream HTTP/1.1 peer without normalizing the framing can emit a body that exceeds the forwarded Content-Length, and the excess bytes can be interpreted as the start of a following response, which enables downstream response splitting or desynchronization. Exploitation requires an attacker-controlled upstream server and an application that forwards the response through a framing-sensitive path. This affects undici versions before 6.28.1, from 7.0.0 up to 7.29.1, and from 8.0.0 up to 8.10.2. Users should upgrade to undici 6.28.1, 7.29.1, or 8.10.2.

Affected products

undici: before 6.28.1; from 7.0.0 before 7.29.1; from 8.0.0 before 8.10.2 — fixed in 6.28.1, 7.29.1, 8.10.2
Npm Undici: before 6.28.1; from 7.0.0 before 7.29.1; from 8.0.0 before 8.10.2 — fixed in 6.28.1, 7.29.1, 8.10.2

How to fix

Upgrade to a fixed version: undici 6.28.1, 7.29.1, 8.10.2; Npm Undici 6.28.1, 7.29.1, 8.10.2. Check the vendor advisory for mitigations.

References

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