undici vulnerable to downstream response splitting via retry interceptor

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.

Scores

EPSS (exploitation in 30 days)

0.31% · top 76% of all CVEs (6 Nov 2026)

CISA SSVC

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

The list uses the assigner's score when there is one, then CISA's, then NVD's: Low 3.7.

Weakness

• CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') (opens in a new tab)All about CWE-444 and its CVEs

Affected products

• undici · undici

  • affected < 6.28.1
  • unaffected 6.28.1
  • affected 7.0.0 – < 7.29.1
  • unaffected 7.29.1
  • affected 8.0.0 – < 8.10.2
  • unaffected 8.10.2

Affected packages

From reviewed advisories of the GitHub Advisory Database.

• npmundiciGHSA-r53p-7pc4-xj5r (opens in a new tab)

References (2)

• https://github.com/nodejs/undici/security/advisories/GHSA-r53p-7pc4-xj5r (opens in a new tab)
• https://cna.openjsf.org/security-advisories.html (opens in a new tab)

Sources

The same CVE in the primary databases and aggregators, to check against.

• CVE.org record (opens in a new tab)
• CVE List JSON (GitHub) (opens in a new tab)
• NVD (opens in a new tab)
• FIRST EPSS (opens in a new tab)
• OSV.dev (opens in a new tab)
• GitHub Advisory GHSA-r53p-7pc4-xj5r (opens in a new tab)
• CIRCL Vulnerability-Lookup (opens in a new tab)

Badge for a README or a ticket

[![CVE-2026-18540](https://trackcve.app/badge/CVE-2026-18540.svg)](https://trackcve.app/cve/CVE-2026-18540)

Record last updated 4 Sept 2026, 17:50 UTC.