CVE-2026-54789 in mod_auth_openidc
Summary
by MITRE • 08/21/2026
mod_auth_openidc is an OpenID Certified authentication and authorization module for the Apache 2.x HTTP server that implements the OpenID Connect Relying Party functionality. Prior to 2.4.19.4, an out-of-bounds read and a one-byte out-of-bounds write exist in the state-cookie parser of `mod_auth_openidc`. The issue is fixed in version 2.4.19.4 by stopping the scan at the string terminator so a value-less token is rejected. No in-product workarounds are available. As a stop-gap, an upstream reverse proxy or WAF that rejects or normalizes malformed `Cookie` headers (tokens lacking `=`) can reduce exposure, but upgrading is the recommended remediation.
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Analysis
by VulDB Data Team • 08/21/2026
The vulnerability identified in mod_auth_openidc prior to version 2.4.19.4 represents a critical memory safety flaw within the authentication module for Apache HTTP servers. This module serves as an OpenID Certified implementation of the OpenID Connect Relying Party functionality, handling sensitive user identity and authorization data. The core technical deficiency lies in the state-cookie parser, which failed to properly validate input boundaries when processing cookie headers. Specifically, the code exhibited both an out-of-bounds read and a one-byte out-of-bounds write condition. These memory corruption issues arise because the parsing logic did not adequately check for string terminators or malformed token structures before accessing memory locations associated with the parsed values.
From a technical perspective, this flaw is classified under CWE-125 Out-of-bounds Read and CWE-787 Out-of-bounds Write in the Common Weakness Enumeration standard. The out-of-bounds read allows an attacker to potentially leak sensitive information from adjacent memory regions by triggering the parser with crafted input that extends beyond expected buffer limits. More critically, the one-byte out-of-bounds write poses a severe risk as it can corrupt heap metadata or overwrite critical application data structures. In the context of web server modules running with elevated privileges, such memory corruption can lead to arbitrary code execution if an attacker can control the content written outside the allocated bounds. This aligns with ATT&CK technique T1059 Command and Scripting Interpreter through potential exploitation chains that leverage buffer overflows for remote code execution.
The operational impact of this vulnerability is significant due to its location within a core authentication component. An unauthenticated attacker can exploit these flaws by sending specially crafted HTTP requests containing malformed Cookie headers, specifically those with tokens lacking the expected equals sign delimiter or otherwise violating standard formatting expectations. Successful exploitation could result in denial of service through application crashes caused by memory corruption, information disclosure via leaked heap contents from the read vulnerability, or full system compromise if the write operation is leveraged to hijack control flow. Since mod_auth_openidc handles session states and authentication tokens, compromising this module effectively undermines the integrity of the entire identity management infrastructure protected by Apache.
The official remediation for this issue involves upgrading mod_auth_openidc to version 2.4.19.4 or later. The fix implements a strict validation mechanism that stops scanning at the string terminator, thereby rejecting value-less tokens and preventing the parser from accessing invalid memory regions. There are no in-product workarounds available within the module itself for older versions. However, as an interim mitigation strategy before upgrading is possible, organizations should deploy upstream reverse proxies or Web Application Firewalls configured to reject or normalize malformed Cookie headers. Specifically, these security controls should be set to block tokens that lack the equals sign separator, which effectively neutralizes the specific input pattern required to trigger the vulnerability. This network-level defense reduces exposure by filtering out malicious requests before they reach the vulnerable Apache module, providing a temporary layer of protection until the software update is applied.