CVE-2021-32785

mod_auth_openidc is an authentication/authorization module for the Apache
2.x HTTP server that functions as an OpenID Connect Relying Party,
authenticating users against an OpenID Connect Provider. When
mod_auth_openidc versions prior to 2.4.9 are configured to use an
unencrypted Redis cache (OIDCCacheEncrypt off, OIDCSessionType server-cache, OIDCCacheType redis), mod_auth_openidc wrongly performed
argument interpolation before passing Redis requests to hiredis, which
would perform it again and lead to an uncontrolled format string bug.
Initial assessment shows that this bug does not appear to allow gaining
arbitrary code execution, but can reliably provoke a denial of service by
repeatedly crashing the Apache workers. This bug has been corrected in
version 2.4.9 by performing argument interpolation only once, using the
hiredis API. As a workaround, this vulnerability can be mitigated by
setting OIDCCacheEncrypt to on, as cache keys are cryptographically
hashed before use when this option is enabled.