CVE-2025-40931: Apache::Session::Generate::MD5 Auth Bypass

CVE-2025-40931 Overview

CVE-2025-40931 is an Insecure Random Number Generation vulnerability affecting Apache::Session::Generate::MD5 versions through 1.94 for Perl. The module generates session IDs using cryptographically weak methods, combining the built-in rand() function with predictable seed values including epoch time and process ID (PID). This weakness could allow attackers to predict session identifiers and hijack user sessions, potentially gaining unauthorized access to protected systems.

Critical Impact

Attackers can predict session IDs by leveraging the weak randomness of Perl's built-in rand() function combined with guessable seed values (epoch time from HTTP Date headers and limited PID space), enabling session hijacking and unauthorized system access.

Affected Products

• Apache::Session::Generate::MD5 versions through 1.94
• Perl applications using Apache::Session with MD5 session generation
• Web applications relying on Apache::Session for session management

Discovery Timeline

• 2026-03-05 - CVE-2025-40931 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2025-40931

Vulnerability Analysis

The vulnerability resides in how Apache::Session::Generate::MD5 constructs session identifiers. The module uses MD5 hashing with a seed composed of three weak entropy sources: the output of Perl's built-in rand() function, the current epoch timestamp, and the process ID (PID). Each of these components introduces predictability that undermines the security of the generated session IDs.

The PID value comes from a constrained numeric space (typically 0-65535 on most Unix-like systems), making it feasible to enumerate. The epoch time can often be obtained or closely estimated from the HTTP Date response header that many web servers include. Most critically, Perl's rand() function is not designed for cryptographic purposes—it uses a deterministic pseudo-random number generator (PRNG) that can be predicted if the seed state is known or can be inferred.

Root Cause

The root cause is the use of cryptographically unsuitable random number generation (CWE-338). The Apache::Session::Generate::MD5 module relies on Perl's rand() function, which implements a non-cryptographic PRNG. When combined with guessable or enumerable seed inputs (epoch time and PID), the resulting MD5 hash becomes predictable. This design flaw violates fundamental session security principles that require session identifiers to be generated using cryptographically secure random sources.

Attack Vector

This vulnerability is exploitable over the network without requiring authentication or user interaction. An attacker can mount a session prediction attack using the following methodology:

1. Reconnaissance: Extract the server timestamp from HTTP Date headers to narrow the epoch time window
2. PID Enumeration: Brute-force the limited PID space (typically under 65,536 values)
3. PRNG State Recovery: Analyze multiple session IDs to potentially recover the rand() seed state
4. Session Prediction: Generate candidate session IDs by combining recovered/guessed values through MD5 hashing
5. Session Hijacking: Test predicted session IDs against the target application to hijack valid user sessions

The vulnerable session generation logic can be reviewed in the MetaCPAN Apache Session Source. The session ID is constructed by concatenating the output of rand(), the epoch time (time()), and the PID ($$), then hashing the result with MD5.

Detection Methods for CVE-2025-40931

Indicators of Compromise

• Unusual patterns of session ID enumeration attempts in web server logs
• Multiple failed session validation attempts from single IP addresses with incrementing or pattern-based session IDs
• Anomalous session hijacking where valid sessions appear to be used from unexpected IP addresses or user agents
• High volumes of requests with invalid but structurally valid MD5-formatted session identifiers

Detection Strategies

• Audit Perl application dependencies to identify usage of Apache::Session version 1.94 or earlier with MD5 session generation
• Implement session anomaly detection to flag sessions accessed from multiple distinct client fingerprints
• Monitor for brute-force patterns against session validation endpoints
• Deploy web application firewall rules to detect and rate-limit session enumeration attempts

Monitoring Recommendations

• Enable verbose logging for session creation and validation events including timestamps and client metadata
• Implement real-time alerting on session validation failure rate spikes
• Track session ID reuse and flag any session accessed from dramatically different network locations within short timeframes
• Review HTTP response headers to ensure Date header exposure is minimized where not operationally required

How to Mitigate CVE-2025-40931

Immediate Actions Required

• Identify all applications using Apache::Session::Generate::MD5 through dependency audits
• Replace the MD5 session generator with a cryptographically secure alternative such as Apache::Session::Generate::UUID or a custom generator using Crypt::URandom
• Invalidate all existing sessions to prevent exploitation of previously generated predictable session IDs
• Implement session binding to client attributes (IP address, user agent) as a defense-in-depth measure

Patch Information

No official patch has been identified in the available CVE data. Organizations should migrate to secure session generation mechanisms as outlined in the MetaCPAN Security Guide. Monitor the OSS-Security mailing list discussion for updates on vendor remediation.

Workarounds

• Switch to Apache::Session::Generate::UUID which uses stronger randomness sources
• Implement a custom session generator using Crypt::URandom or /dev/urandom for cryptographically secure random data
• Add session validation layers that bind sessions to client characteristics (IP subnet, TLS session, user agent hash)
• Reduce session lifetime to limit the attack window for session prediction attempts
• Remove or obfuscate HTTP Date headers to reduce timing information leakage

# Example: Replace session generator in Apache::Session configuration
# In your Perl application configuration, change from:
# Generate => 'MD5'
# To a more secure alternative:
# Generate => 'UUID'

# Or implement cryptographically secure session generation:
# use Crypt::URandom qw(urandom);
# my $session_id = unpack('H*', urandom(32));