Research / Advisories

Same certificate, different answers: how TLS libraries enforce X.509 Name Constraints

1SEAL-2026-012 · Published 2026-10-14

Published

Name Constraints restrict the names that a subordinate certificate authority may certify. Oleh Konko (1seal) reported findings on how TLS libraries enforce them; CERT/CC coordinated the disclosure and published three vulnerability notes on 13 and 14 October 2026. Some reports describe implementation defects; others concern behaviour that RFC 5280 does not fully specify. Fixes have been released for several libraries; other reports are merged upstream, in triage, disputed or declined. According to CERT/CC, the findings matter only where an attacker can obtain a trusted signature over chosen certificate fields, an affected validation path is reachable, and the application uses the accepted name for authorization. The notes do not report exploitation.