OpenSSL 4.0 caches decoded X.509v3 extension values inside the X509 certificate object the first time they are needed. The caching process involves two phases: computing extension values under a read lock and installing them under a write lock. Because multiple threads can hold read locks simultaneously, concurrent caching attempts on the same certificate can lead to multiple threads installing their own cached data and freeing previously installed data still in use by other threads. This results in a use-after-free condition that can cause process crashes (denial of service) when multiple TLS connections concurrently build certificate chains to the same trusted CA certificate. Certificates sent by peers are not affected as they are decoded per connection and not shared. The vulnerability does not affect OpenSSL versions 3.6, 3.5, 3.4, 3.0, 1.1.1, or 1.0.2. The issue was reported in August 2026 and fixed in OpenSSL 4.0.3.

A remote unauthenticated attacker can cause a denial of service by triggering a use-after-free read in multi-threaded TLS clients or servers that verify client certificates. This occurs when multiple connections simultaneously build certificate chains to the same trusted CA certificate, leading to process crashes. There is no confidentiality or integrity impact reported. The FIPS module is not affected.

Users of OpenSSL 4.0 should upgrade to OpenSSL 4.0.3 where this issue is fixed. Earlier OpenSSL versions are not vulnerable. No other mitigation is required as the vulnerability is addressed by the official fix.