CVE-2026-84933
Description
A flaw was found in undici. The software's caching mechanism, when operating in shared cache mode, improperly stores responses that include 'Set-Cookie' headers. This oversight allows an untrusted server to inject cookies into cached responses. Consequently, sensitive cookie information from one user can be inadvertently disclosed to another user accessing the same cached content.
Statement
This flaw affects applications that use undici's shared-cache interceptor (interceptors.cache(), type: 'shared', which is the default) against untrusted or multi-tenant upstream servers. In that configuration, undici stores and re-serves the Set-Cookie header of cached responses, allowing one user's session cookie to be disclosed to another caller that hits the same cache key, and allowing a malicious upstream to inject cookies into responses served to other users. Applications that only use undici as an HTTP client without enabling the shared cache interceptor, or that only use the private cache mode, are not affected. Red Hat products that bundle undici versions prior to 7.0.0 (5.x/6.x lines) are not affected, as the vulnerable cache interceptor was introduced in undici 7.0.0.
Mitigation
Upgrade to undici 7.29.1 or 8.10.2 (or later), where the shared cache interceptor no longer stores or re-serves responses containing Set-Cookie. Where an immediate upgrade is not possible: use a private cache (type: 'private') for per-user responses instead of the shared cache; avoid caching responses that carry a Set-Cookie header; or, if your service itself acts as a shared cache in front of an untrusted origin, strip Set-Cookie from upstream responses before caching them.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 6.5 | 8.2 | 6.5 |
| Attack Vector | Network | Network | Network |
| Attack Complexity | High | Low | High |
| Privileges Required | None | None | None |
| User Interaction | None | None | None |
| Scope | Unchanged | Unchanged | Unchanged |
| Confidentiality | High | High | High |
| Integrity Impact | Low | Low | Low |
| Availability Impact | None | None | None |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:L/A:N
NVD: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:L/A:N
Understanding the Weakness (CWE)
Confidentiality
Technical Impact: Read Application Data
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