What’s Vulnerable › Vulnerabilities

CVE-2021-32785: Format string bug in the Redis cache implementation

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 2.7% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-07-22 · updated 2026-06-17
• Weakness: CWE-134

Description

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.

Affected products

• Debian Linux: 10.0
• NetApp Cloud Backup: all versions
• OpenIDC mod_auth_openidc: before 2.4.9 — fixed in 2.4.9

How to fix

Upgrade to a fixed version: OpenIDC mod_auth_openidc 2.4.9. Check the vendor advisory for mitigations.

References

• https://github.com/zmartzone/mod_auth_openidc/commit/dc672688dc1f2db7df8ad4abebc367116017a449
• https://github.com/zmartzone/mod_auth_openidc/releases/tag/v2.4.9
• https://github.com/zmartzone/mod_auth_openidc/security/advisories/GHSA-55r8-6w97-xxr4
• https://lists.debian.org/debian-lts-announce/2023/04/msg00034.html
• https://security.netapp.com/advisory/ntap-20210902-0001/
• https://www.oracle.com/security-alerts/cpuapr2022.html
• https://github.com/zmartzone/mod_auth_openidc/commit/dc672688dc1f2db7df8ad4abebc367116017a449
• https://github.com/zmartzone/mod_auth_openidc/releases/tag/v2.4.9
• https://github.com/zmartzone/mod_auth_openidc/security/advisories/GHSA-55r8-6w97-xxr4
• https://lists.debian.org/debian-lts-announce/2023/04/msg00034.html
• https://security.netapp.com/advisory/ntap-20210902-0001/
• https://www.oracle.com/security-alerts/cpuapr2022.html

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