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.
The product uses a function that accepts a format string as an argument, but the format string originates from an external source.
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Mod_auth_openidc | Openidc | * | 2.4.9 (excluding) |
| Libapache2-mod-auth-openidc | Ubuntu | bionic | * |
| Libapache2-mod-auth-openidc | Ubuntu | esm-apps/bionic | * |
| Libapache2-mod-auth-openidc | Ubuntu | esm-apps/focal | * |
| Libapache2-mod-auth-openidc | Ubuntu | focal | * |
| Libapache2-mod-auth-openidc | Ubuntu | hirsute | * |
| Libapache2-mod-auth-openidc | Ubuntu | trusty | * |
| Libapache2-mod-auth-openidc | Ubuntu | upstream | * |
| Libapache2-mod-auth-openidc | Ubuntu | xenial | * |
When an attacker can modify an externally-controlled format string, this can lead to buffer overflows, denial of service, or data representation problems. It should be noted that in some circumstances, such as internationalization, the set of format strings is externally controlled by design. If the source of these format strings is trusted (e.g. only contained in library files that are only modifiable by the system administrator), then the external control might not itself pose a vulnerability.