CVE-2025-40926: Plack Session::Simple Auth Bypass Flaw

CVE-2025-40926 Overview

CVE-2025-40926 is a critical insecure random number generation vulnerability affecting Plack::Middleware::Session::Simple versions through 0.04 for Perl. The vulnerability stems from the module's default session ID generator, which uses a SHA-1 hash seeded with cryptographically weak sources: Perl's built-in rand function, the epoch time, and the process ID (PID).

This flawed session ID generation mechanism creates a predictable pattern that attackers can exploit. The PID originates from a small set of numbers, the epoch time may be guessed or directly leaked from the HTTP Date header, and Perl's rand function is fundamentally unsuitable for cryptographic purposes. This combination makes session IDs predictable, potentially allowing attackers to hijack user sessions and gain unauthorized access to systems.

Critical Impact
Predictable session IDs could allow attackers to forge valid session tokens, hijack authenticated user sessions, and gain unauthorized access to protected systems and sensitive data.

Affected Products

Plack::Middleware::Session::Simple versions through 0.04
Perl-based web applications using the affected session middleware
Applications relying on Plack::Middleware::Session (related vulnerability CVE-2025-40923)

Discovery Timeline

March 5, 2026 - CVE-2025-40926 published to NVD
March 5, 2026 - Last updated in NVD database

Technical Details for CVE-2025-40926

Vulnerability Analysis

This vulnerability is classified under CWE-338 (Use of Cryptographically Weak Pseudo-Random Number Generator). The core issue lies in the session ID generation mechanism within the Plack::Middleware::Session::Simple module.

Session management is a critical security component in web applications, and session IDs must be generated using cryptographically secure random number generators (CSPRNGs) to prevent prediction attacks. The vulnerable implementation combines three predictable or easily guessed values to create session identifiers:

Perl's rand function: This function uses a linear congruential generator (LCG), which is deterministic and unsuitable for security-sensitive applications
Epoch time: The current timestamp, which can often be deduced or leaked through HTTP response headers
Process ID (PID): Operating systems typically assign PIDs from a limited range, making them trivially guessable

Even hashing these values with SHA-1 does not provide meaningful security, as the underlying entropy pool is insufficient. An attacker who can determine or enumerate these seed values can recreate the same session IDs generated by the server.

Root Cause

The root cause is the use of non-cryptographic random sources in security-critical session ID generation. The module's session ID generator at line 43 of Session/Simple.pm relies on Perl's built-in rand function combined with predictable system values (time and PID), violating fundamental cryptographic principles for session token generation.

The MetaCPAN Security Guide explicitly warns against using rand for security purposes and recommends proper cryptographic alternatives.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability through the following approach:

The vulnerability allows session prediction attacks where an adversary enumerates possible session IDs by:

Observing the HTTP Date header to determine server time
Enumerating the small PID space (typically 0-65535 on most systems)
Predicting or brute-forcing the rand output given knowledge of seed timing

Once a valid session ID is predicted, the attacker can inject it into their requests to impersonate legitimate authenticated users, bypassing authentication entirely. This attack can be conducted remotely without any prior access to the target system.

Detection Methods for CVE-2025-40926

Indicators of Compromise

Unusual session activity patterns with multiple session IDs being tested against the application
Authentication events from unexpected geographic locations or IP addresses using valid session tokens
Multiple concurrent sessions for the same user account without corresponding login events
HTTP requests containing sequentially similar or predictable session ID patterns

Detection Strategies

Monitor authentication logs for session reuse from different IP addresses or user agents
Implement session binding to client characteristics (IP, User-Agent fingerprint) to detect hijacking attempts
Deploy web application firewalls (WAF) with rules to detect session enumeration attacks
Analyze session creation patterns for statistical anomalies indicating predictable generation

Monitoring Recommendations

Enable verbose logging for session creation and destruction events in Plack applications
Implement real-time alerting for failed session validation attempts exceeding normal thresholds
Correlate session activity with authentication events to identify sessions that bypass login flows
Review server logs for evidence of sequential or patterned session ID probing

How to Mitigate CVE-2025-40926

Immediate Actions Required

Upgrade Plack::Middleware::Session::Simple to a patched version that uses cryptographically secure random number generation
Review and terminate all existing sessions to invalidate potentially compromised session tokens
Implement additional session validation controls such as binding sessions to client IP addresses
Consider migrating to Plack::Middleware::Session with proper secure random generation if an immediate patch is unavailable

Patch Information

A patch has been committed to address this vulnerability. The fix replaces the insecure rand-based session ID generation with a cryptographically secure alternative. Review the GitHub patch commit for implementation details and the GitHub Pull Request for the complete discussion.

Organizations should upgrade to the patched version as soon as it becomes available on CPAN. In the interim, implement the workarounds described below.

Workarounds

Configure a custom session ID generator using cryptographically secure modules such as Crypt::URandom or Bytes::Random::Secure
Implement a middleware wrapper that overrides the default session generation with secure random bytes
Add server-side session validation checks including timestamp verification and client fingerprinting
Deploy network-level controls to rate-limit session establishment and detect enumeration attempts

bash

# Install cryptographically secure random module for Perl
cpanm Crypt::URandom

# Verify installed version of affected module
perl -MPlack::Middleware::Session::Simple -e 'print $Plack::Middleware::Session::Simple::VERSION'

# Apply the security patch (if available)
cpanm --upgrade Plack::Middleware::Session::Simple