CVE-2026-3255: HTTP::Session2 Information Disclosure Flaw

CVE-2026-3255 Overview

HTTP::Session2 versions before 1.12 for Perl may generate weak session IDs using the rand() function. The session ID generator returns a SHA-1 hash seeded with the built-in rand function, the epoch time, and the PID. The PID will come from a small set of numbers, and the epoch time may be guessed if it is not leaked from the HTTP Date header. The built-in rand() function is unsuitable for cryptographic usage.

HTTP::Session2 after version 1.02 will attempt to use the /dev/urandom device to generate a session ID, but if the device is unavailable (for example, under Windows), then it will revert to the insecure method described above.

Critical Impact

Attackers can predict session IDs through weak random number generation, potentially enabling session hijacking and unauthorized access to user accounts.

Affected Products

• HTTP::Session2 versions prior to 1.12 for Perl
• Perl applications using HTTP::Session2 on Windows systems
• Systems where /dev/urandom is unavailable

Discovery Timeline

• 2026-02-27 - CVE-2026-3255 published to NVD
• 2026-03-04 - Last updated in NVD database

Technical Details for CVE-2026-3255

Vulnerability Analysis

This vulnerability stems from the use of Perl's built-in rand() function for cryptographic purposes where it is fundamentally unsuitable. The session ID generation mechanism combines three predictable or guessable values: the rand() output, the epoch time, and the process ID (PID).

The weakness is compounded by the fact that PIDs typically fall within a limited range (commonly 0-32768 on many systems), significantly reducing the entropy of the generated session ID. Additionally, the epoch time can often be determined or closely estimated by examining HTTP response headers that include timestamp information.

While versions after 1.02 attempted to mitigate this by using /dev/urandom, the fallback mechanism to the insecure method creates a vulnerability window, particularly on Windows systems where /dev/urandom is not available.

Root Cause

The root cause is the use of a cryptographically weak pseudo-random number generator (PRNG) for security-critical session ID generation. The rand() function in Perl is designed for general-purpose randomness, not cryptographic security, making its output predictable to attackers who can model the internal state of the generator. This is classified as CWE-338: Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG).

Attack Vector

An attacker can exploit this vulnerability over the network without authentication. By collecting multiple session IDs and analyzing their patterns, an attacker can potentially:

1. Estimate the epoch time from leaked HTTP Date headers
2. Enumerate the small range of possible PID values
3. Brute-force or predict the rand() output based on known seeding weaknesses
4. Generate valid session IDs to hijack user sessions

The following patch shows the fix implemented in version 1.12:

 {{$NEXT}}
 
     - Removed HTTP::Session2::ClientStore and HTTP::Session2::ClientStore2
+    - Use Crypt::SysRandom to generate the session_id
 
 1.11 2026-02-24T22:44:48Z
 

Source: GitHub Commit Patch

The dependency changes in the cpanfile reflect the security improvement:

 requires 'Mouse';
 requires 'parent';
 requires 'Digest::SHA';
+requires 'Crypt::SysRandom';
 requires 'MIME::Base64', '3.11';
-requires 'Time::HiRes';
-requires 'Data::MessagePack';
-requires 'Crypt::CBC';
 
 on 'test' => sub {
     requires 'Test::More', '0.98';
     requires 'Test::WWW::Mechanize::PSGI';
     requires 'Plack::Response';
-    requires 'Crypt::Rijndael';
     requires 'Plack::Request';
 };
 

Source: GitHub Commit Patch

Detection Methods for CVE-2026-3255

Indicators of Compromise

• Unusual patterns in session ID values that may indicate prediction attempts
• Multiple failed authentication attempts followed by successful session access
• Session IDs being used from unexpected IP addresses or user agents
• Anomalous timing patterns in session creation requests

Detection Strategies

• Monitor application logs for session anomalies or concurrent sessions from different geographic locations
• Implement session fingerprinting to detect session ID reuse across different client characteristics
• Audit Perl application dependencies to identify vulnerable HTTP::Session2 versions using cpan -D HTTP::Session2
• Review web server logs for requests probing session management endpoints

Monitoring Recommendations

• Enable verbose logging for session creation and validation events
• Implement real-time alerting for suspicious session activity patterns
• Deploy SentinelOne Singularity to monitor for exploitation attempts targeting session management
• Regularly audit installed Perl modules and their versions across all systems

How to Mitigate CVE-2026-3255

Immediate Actions Required

• Upgrade HTTP::Session2 to version 1.12 or later immediately
• Audit all Perl applications using HTTP::Session2 to identify affected deployments
• Invalidate all existing sessions after upgrading to force regeneration with secure session IDs
• On Windows systems, prioritize the upgrade as the fallback mechanism is more likely to be triggered

Patch Information

The vendor has released version 1.12 which addresses this vulnerability by replacing the weak random number generation with Crypt::SysRandom. The patch is available through the GitHub Commit Patch and can be installed via CPAN. For detailed changes, refer to the MetaCPAN Changes Log.

Workarounds

• If immediate upgrade is not possible, implement additional session validation such as IP binding and user-agent verification
• Consider implementing a custom session ID generator using Crypt::SysRandom or similar cryptographically secure modules
• Reduce session lifetimes to minimize the window of opportunity for session hijacking
• Implement additional authentication factors for sensitive operations

# Upgrade HTTP::Session2 via CPAN
cpan HTTP::Session2

# Verify installed version
perl -MHTTP::Session2 -e 'print $HTTP::Session2::VERSION'

# Install Crypt::SysRandom dependency if needed
cpan Crypt::SysRandom