Vulnerability in Node.js Undici
CVE-2026-84961
undici's BalancedPool constructor passes its entire options object through an internal deep-clone that serializes and reparses the value as JSON. Because JSON cannot represent functions, any function-valued TLS option, such as a caller-supplied checkServerIdentity callback or a custom connector inside the connect option, is silently discarded before it reaches the TLS layer. As a result a peer whose certificate the application's custom checkServerIdentity was written to reject, but which still passes Node's default hostname and chain checks, is accepted when reached through BalancedPool. The Client, Pool, and Agent dispatchers are not affected because they extract the connect and tls options before cloning. This affects undici versions from 7.24.1 up to 7.29.1 and from 8.0.0 up to 8.10.2, and only when the application supplies a function-valued connect or tls option to BalancedPool. Users should upgrade to undici 7.29.1 or 8.10.2.
Vulnerability class: Improper Certificate Validation
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.4 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N.
EPSS exploit prediction
EPSS: 0.001 (3.5th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 3rd percentile — 3.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.001 · newest: 0.001 · change: 0.000
Affected products
Weakness classification (CWE)
References
- ce714d77-add3-4f53-aff5-83d477b104bb (Third Party Advisory)
- ce714d77-add3-4f53-aff5-83d477b104bb (Mitigation, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-84961?
- CVE-2026-84961 is a high-severity vulnerability in Node.js Undici, classified under Improper Certificate Validation. CVSS score: 7.4/10. Published 2026-09-04.
- How severe is CVE-2026-84961?
- High severity. CVSS v3 base score is 7.4 out of 10.