CVE-2026-16729: undici vulnerable to cookie attribute injection via unsanitized domain and unparsed setCookie fields

Severity: medium (CVSS 6.5 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: 2026-07-29 · updated 2026-08-05
Weakness: CWE-74

Description

undici's setCookie function does not fully sanitize cookie attributes. 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 domain value is not checked for semicolons and entries in the unparsed array are not sanitized, so attacker-influenced input can inject additional cookie attributes. For example, a domain value containing a semicolon can append attributes such as SameSite, and an unparsed entry can inject attributes such as HttpOnly, without the caller setting them. Applications that pass user-controlled input to these fields, such as multi-tenant or reverse-proxy servers that scope session cookies to a tenant-supplied domain, can have SameSite CSRF protections bypassed, or the Secure, HttpOnly, and SameSite attributes forced, stripped, or overridden. The issue is fixed in undici 6.28.0, 7.29.0, and 8.9.0.

Affected products

undici: before 6.28.0; from 7.0.0 before 7.29.0; from 8.0.0 before 8.9.0 — fixed in 6.28.0, 7.29.0, 8.9.0
Npm Undici: before 6.28.0; from 7.0.0 before 7.29.0; from 8.0.0 before 8.9.0 — fixed in 6.28.0, 7.29.0, 8.9.0

How to fix

Upgrade to a fixed version: undici 6.28.0, 7.29.0, 8.9.0; Npm Undici 6.28.0, 7.29.0, 8.9.0. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2026-16729 or the next vulnerability affects your stack