CVE-2026-9679 (MEDIUM): detected in Lambda Docker Images.

[the-lambda-watchdog]

CVE Details

CVE ID	Severity	Affected Package	Installed Version	Fixed Version	Date Published	Date of Scan
CVE-2026-9679	MEDIUM	undici	6.25.0	6.27.0, 7.28.0, 8.5.0	2026-06-17T18:18:06.307Z	2026-06-23T10:18:18.216181542Z

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Affected Docker Images

Image Name	SHA
public.ecr.aws/lambda/nodejs:24	public.ecr.aws/lambda/nodejs@sha256:904f7a98db07d37b59bf7edd90ed97bbba1cb574fd86db462205c68cfc55486c

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Description

Impact:
undici's cookie parser in parseSetCookie percent-decodes cookie values via qsUnescape, turning encoded sequences like %0D%0A, %00, %3B, and %3D into their literal byte equivalents. RFC 6265 §5.4 does not specify any decoding and browsers do not decode either.

Applications that parse a Set-Cookie header and then forward the parsed value into a response header (proxies, middleware, SSR frameworks) become vulnerable to HTTP response header injection: an attacker-controlled upstream can inject arbitrary Set-Cookie, Location, or Cache-Control headers into the application's downstream response, enabling session fixation, open redirect, or cache poisoning.

Affected applications are those that use undici's cookie parsing (parseSetCookie, parseCookie, getSetCookies) and forward the parsed cookie value into a response header.

This was introduced in undici 7.0.0 via PR #3789.

Patches:
Upgrade to undici v6.26.0, v7.28.0 or v8.5.0.

Workarounds:
If upgrade is not immediately possible, do not forward values returned by parseSetCookie/parseCookie/getSetCookies directly into response headers; sanitize the value first to strip or reject CR, LF, NUL, ;, and = bytes.

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Remediation Steps

• Update the affected package undici from version 6.25.0 to 6.27.0, 7.28.0, 8.5.0.

About this issue

• This issue may not contain all the information about the CVE nor the images it affects.
• This issue will not be updated with new information and the list of affected images may have changed since the creation of this issue.
• For more, visit Lambda Watchdog.
• This issue was created automatically by Lambda Watchdog.