CVE-2026-18540: Undici Vulnerability (CVSS 3.7)

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.

Metrics

CVSS 3.1

3.7/10

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

EPSS Probability

0.24%

15.1th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-444

Affected Software

Vendor	Product	Versions
Nodejs	Undici	< 6.28.1
Nodejs	Undici	>= 7.0.0, < 7.29.1
Nodejs	Undici	>= 8.0.0, < 8.10.2

References

https://cna.openjsf.org/security-advisories.htmlVendor Advisory
https://github.com/nodejs/undici/security/advisories/GHSA-r53p-7pc4-xj5rMitigation, Vendor Advisory

Timeline

Published: Sep 4, 2026
Last Modified: Sep 16, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2026-18540?

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.

How severe is CVE-2026-18540?

CVE-2026-18540 has a CVSS score of 3.7/10 (LOW severity). The EPSS model estimates a 0.24% probability of exploitation in the next 30 days.

How do I fix CVE-2026-18540?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

How Strix Helps

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