Vulnerability in OpenSSL - X.509 Policy Constraints Double Locking

If an X.509 certificate contains a malformed policy constraint and policy processing is enabled, then a write lock will be taken twice recursively. On some operating systems (most widely: Windows) this results in a denial of service when the affected process hangs. Policy processing being enabled on a publicly facing server is not considered to be a common setup.

Policy processing is enabled by passing the -policy argument to the command line utilities or by calling the X509_VERIFY_PARAM_set1_policies() function.

Update (31 March 2023): The description of the policy processing enablement was corrected based on CVE-2023-0466.

Found by Polar Bear.
Fix developed by Paul Dale.