Description
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.
Published: 2026-09-04
Score: 7.4 High
EPSS: n/a
KEV: No
Impact: n/a
Action: n/a
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Remediation

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History

Fri, 04 Sep 2026 17:00:00 +0000

Type Values Removed Values Added
Description 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.
Title undici vulnerable to TLS certificate validation bypass via dropped connect options in BalancedPool
Weaknesses CWE-295
References
Metrics cvssV3_1

{'score': 7.4, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N'}


Subscriptions

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cve-icon MITRE

Status: PUBLISHED

Assigner: openjs

Published:

Updated: 2026-09-04T18:37:44.620Z

Reserved: 2026-09-02T17:45:44.775Z

Link: CVE-2026-84961

cve-icon Vulnrichment

No data.

cve-icon NVD

Status : Received

Published: 2026-09-04T17:17:02.227

Modified: 2026-09-04T17:17:02.227

Link: CVE-2026-84961

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

No data.

Weaknesses