CVE-2026-109829

undici vulnerable to cache poisoning and cross-user disclosure via cross-origin redirects

Workarounds

Compose the interceptors as compose(cache(), redirect()), use per-origin dispatchers and cache stores, or disable shared caching on redirect-capable requests.

When an originless Agent composes the redirect and cache interceptors as compose(redirect(), cache()) with a shared cache (type: 'shared'), the cache builds its key from the original request URL before the redirect handler changes the origin. The response produced after a redirect is then stored under the original key, even when the redirect crossed to a different origin.

An attacker who controls the target of a redirect from a trusted URL can have their response cached under that trusted URL. A later caller requesting the trusted URL receives the attacker's body from the shared cache without either origin being contacted, allowing persistent cache poisoning. The same key confusion also discloses private data: when an authenticated request returns a private, no-store redirect to a user-specific resource, the final response is stored under the original URL and can be served to a later unauthenticated caller.

The cache interceptor was introduced in undici 7.0.0, and all 7.x and 8.x releases before the patched versions are affected. Applications are affected only when an originless Agent, a shared MemoryCacheStore or SqliteCacheStore, and the composition compose(redirect(), cache()) are all used. Origin-bound Client and Pool dispatchers, and the reverse composition compose(cache(), redirect()), are not affected.

CVE-2026-109829 has a CVSS score of 7.4 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (7.30.1, 8.12.2); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Upgrade to 7.30.1 or 8.12.2. Before storing a response, undici now verifies that the origin that produced it still matches the cache key, and preserves each redirect hop's cache-control policy, so cross-origin redirect responses and responses behind a no-store or private hop are no longer cached under the original key.

1. What is CVE-2026-109829? CVE-2026-109829 is a high-severity security vulnerability in undici (npm), affecting versions >= 7.0.0, < 7.30.1. It is fixed in 7.30.1, 8.12.2.
2. How severe is CVE-2026-109829? CVE-2026-109829 has a CVSS score of 7.4 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of undici are affected by CVE-2026-109829? undici (npm) versions >= 7.0.0, < 7.30.1 is affected.
4. Is there a fix for CVE-2026-109829? Yes. CVE-2026-109829 is fixed in 7.30.1, 8.12.2. Upgrade to this version or later.
5. Is CVE-2026-109829 exploitable, and should I be worried? Whether CVE-2026-109829 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-109829 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-109829?
   • Upgrade undici to 7.30.1 or later
   • Upgrade undici to 8.12.2 or later

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