CVE-2026-5081: Perl Apache::Session Info Disclosure Flaw

CVE-2026-5081 Overview

CVE-2026-5081 affects the Perl module Apache::Session::Generate::ModUniqueId in versions 1.54 through 1.94. The module derives session identifiers from the UNIQUE_ID environment variable produced by the Apache mod_unique_id plugin. That value is constructed from the server IPv4 address, process ID, epoch timestamp, a 16-bit counter, and a thread index, with no obfuscation or cryptographic randomness. Attackers who observe prior session IDs can enumerate the input components and predict valid session tokens. The flaw maps to [CWE-340: Generation of Predictable Numbers or Identifiers] and enables session hijacking against any web application that relies on the affected generator.

Critical Impact

Predictable session identifiers allow remote attackers to forge or guess valid sessions and impersonate authenticated users without credentials.

Affected Products

• Apache::Session::Generate::ModUniqueId version 1.54 through 1.94
• Perl applications using Apache::Session with the ModUniqueId generator
• Apache HTTP Server deployments exposing the UNIQUE_ID environment variable to session logic

Discovery Timeline

• 2026-05-06 - CVE-2026-5081 published to NVD
• 2026-05-06 - Last updated in NVD database

Technical Details for CVE-2026-5081

Vulnerability Analysis

The Apache::Session::Generate::ModUniqueId module was introduced in version 1.54 as an alternative session ID generator for the Apache::Session Perl framework. Instead of using a cryptographically secure random source, it copies the UNIQUE_ID value set by Apache's mod_unique_id module into the session identifier. The Apache documentation explicitly states that mod_unique_id is intended for log correlation, not security. The session token therefore inherits all of the structural weaknesses of the underlying request identifier.

Root Cause

The identifier concatenates five low-entropy fields: the 32-bit server IPv4 address, the process ID, the Unix epoch timestamp, a 16-bit per-process counter, and a thread index. None of these fields are hashed, salted, or combined with random data. An attacker who collects a few legitimate session IDs can reverse the encoding and recover each component. The server IP is often public, the timestamp leaks through the HTTP Date response header, and process IDs and counters change predictably between requests.

Attack Vector

An unauthenticated remote attacker requests one or more sessions to harvest sample identifiers. They decode the structure to determine the server IP, current process IDs, and counter range. Using the Date header to anchor the timestamp, the attacker generates candidate session IDs spanning the predicted PID and counter values. Submitting those candidates as session cookies allows the attacker to hijack active sessions belonging to other users. No user interaction or privileges are required.

No public exploit code is currently available. Refer to the OpenWall OSS-Security Discussion for technical details and the MetaCPAN Module Documentation for the recommended replacement generator.

Detection Methods for CVE-2026-5081

Indicators of Compromise

• Multiple successful authentications to the same account from different IP addresses within a short window.
• Session cookies whose decoded structure matches the mod_unique_id format: IP, PID, timestamp, counter, thread index.
• High volumes of HTTP requests carrying sequential or near-sequential session identifiers from a single source.
• Application logs showing valid session reuse without a corresponding login event.

Detection Strategies

• Inventory Perl applications and identify any use of Apache::Session::Generate::ModUniqueId in installed CPAN modules between versions 1.54 and 1.94.
• Inspect issued session IDs and verify they are not derived from the UNIQUE_ID environment variable.
• Correlate session creation events with authentication events to flag sessions that appear without a login.

Monitoring Recommendations

• Alert on session reuse across geographically distinct IP addresses for the same authenticated user.
• Monitor for brute-force patterns where a client submits many session cookies with small structural differences.
• Log and review the Date response header alongside session creation timestamps to detect timing-based enumeration.

How to Mitigate CVE-2026-5081

Immediate Actions Required

• Switch the session ID generator to Apache::Session::Generate::Random or another cryptographically secure generator.
• Invalidate all existing sessions issued by Apache::Session::Generate::ModUniqueId and require users to re-authenticate.
• Audit deployed Perl applications for direct or transitive dependencies on the affected generator.

Patch Information

The maintainers recommend using Apache::Session::Generate::Random, which produces session IDs from a secure random source. Review the MetaCPAN Module Documentation for configuration guidance and update application code to specify the secure generator explicitly.

Workarounds

• Configure Apache::Session to use the Random generator instead of ModUniqueId until applications can be patched.
• Place affected applications behind a reverse proxy that rewrites or supplements session cookies with a server-side secure token.
• Reduce session lifetime and bind sessions to client attributes such as IP address or TLS fingerprint to limit the window for hijacking.

# Configuration example: replace ModUniqueId with the secure Random generator
# In the Apache::Session tie configuration, change:
#   Generate => 'ModUniqueId'
# to:
#   Generate => 'Random'
tie %session, 'Apache::Session::File', undef, {
    Directory     => '/var/lib/sessions',
    LockDirectory => '/var/lock/sessions',
    Generate      => 'Random',
};