CVE-2025-15604: Amon2 Information Disclosure Vulnerability

CVE-2025-15604 Overview

CVE-2025-15604 is a critical insecure random number generation vulnerability affecting Amon2, a popular Perl web application framework. Versions prior to 6.17 implement a weak random_string function that relies on cryptographically unsuitable methods for generating security-critical values such as session identifiers, cookie signing secrets, and CSRF protection tokens.

The vulnerability stems from multiple implementation flaws across different version ranges. In versions 6.06 through 6.16, when the /dev/urandom device is unavailable, the framework falls back to generating random bytes by concatenating a SHA-1 hash seeded with Perl's built-in rand() function, the process ID (PID), and high-resolution epoch time. Earlier versions had even weaker implementations, with versions before 6.06 having no fallback mechanism and versions before 6.04 using rand() directly for alphanumeric string generation.

Critical Impact

Attackers can predict session tokens, CSRF tokens, and cryptographic secrets, enabling session hijacking, authentication bypass, and cross-site request forgery attacks against applications built with vulnerable Amon2 versions.

Affected Products

• Tokuhirom Amon2 versions prior to 6.17 for Perl
• Amon2 versions 6.06 through 6.16 (weak /dev/urandom fallback)
• Amon2 versions before 6.04 (direct rand() usage)

Discovery Timeline

• 2026-03-28 - CVE-2025-15604 published to NVD
• 2026-04-01 - Last updated in NVD database

Technical Details for CVE-2025-15604

Vulnerability Analysis

This vulnerability represents a fundamental weakness in cryptographic random number generation (CWE-338). The random_string function in Amon2 is used for generating security-sensitive values including session identifiers, secrets for signing or encrypting cookie session data, and tokens for Cross-Site Request Forgery (CSRF) protection.

The core issue is that Perl's built-in rand() function is a pseudorandom number generator (PRNG) designed for general-purpose use, not cryptographic applications. When combined with predictable values like the PID (which typically falls within a small, enumerable range) and epoch time (which may be leaked via HTTP Date headers or guessed based on request timing), an attacker can potentially reconstruct the seed values and predict the output of the random string generation.

Root Cause

The root cause is the use of cryptographically weak random number generation in security-critical contexts. Specifically:

1. Perl's rand() function uses a linear congruential generator or similar PRNG algorithm that is deterministic and predictable given the seed
2. Limited entropy sources when /dev/urandom is unavailable, relying on easily guessable or leakable values (PID, timestamp)
3. Information leakage through HTTP Date headers that can expose the approximate time used in seed generation
4. Small PID space that can be enumerated by attackers to reconstruct potential seeds

Attack Vector

The vulnerability is exploitable over the network without authentication or user interaction. An attacker can exploit this weakness through the following approach:

The attack involves collecting timing information from HTTP response headers to estimate the epoch time used during token generation. Since process IDs typically range from 1 to 32768 (or similar limited ranges depending on the operating system), an attacker can enumerate possible PID values combined with the estimated timestamp to reconstruct potential seeds.

Once the seed is determined, the attacker can predict:

• Session identifiers, enabling session hijacking
• Cookie signing secrets, allowing forged session cookies
• CSRF tokens, bypassing CSRF protection mechanisms

The MetaCPAN Security Guide provides additional context on proper random data generation for security purposes. Technical details of the fix can be reviewed in the MetaCPAN Version Diff.

Detection Methods for CVE-2025-15604

Indicators of Compromise

• Applications running Amon2 versions prior to 6.17 that generate session tokens or CSRF tokens
• Systems where /dev/urandom is unavailable or restricted, forcing the weak fallback mechanism
• Evidence of session hijacking or CSRF bypass attacks in application logs

Detection Strategies

• Audit Perl application dependencies for Amon2 versions prior to 6.17 using CPAN module version checks
• Review application logs for suspicious session token reuse patterns that may indicate prediction attacks
• Monitor for unusual authentication or session-related anomalies that could indicate exploitation
• Implement entropy monitoring on systems to detect when /dev/urandom becomes unavailable

Monitoring Recommendations

• Enable detailed logging for session creation and CSRF token generation events
• Monitor for multiple failed authentication attempts followed by successful access using predicted tokens
• Implement anomaly detection for session token patterns that may indicate predictable generation
• Set up alerts for systems where cryptographic entropy sources become unavailable

How to Mitigate CVE-2025-15604

Immediate Actions Required

• Upgrade Amon2 to version 6.17 or later immediately across all affected applications
• Audit all applications using Amon2 to identify vulnerable deployments
• Ensure /dev/urandom is available and accessible on all systems running Amon2 applications
• Invalidate all existing sessions and regenerate all secrets after upgrading

Patch Information

The vulnerability has been addressed in Amon2 version 6.17. The fix ensures proper cryptographic random number generation is used for all security-sensitive operations. Details of the changes are available in the MetaCPAN Release Changes and the associated GitHub Pull Request.

Administrators should update using CPAN or their preferred Perl module management tool:

# Upgrade Amon2 to the patched version
cpanm Amon2@6.17

# Verify the installed version
perl -MAmon2 -e 'print $Amon2::VERSION'

Workarounds

• Ensure /dev/urandom is always available and accessible to the application process to avoid triggering the weak fallback
• Consider implementing a custom random string generator using Crypt::URandom or similar cryptographically secure modules as an interim measure
• Remove HTTP Date headers from responses to reduce timestamp information leakage if immediate patching is not possible
• Implement additional session validation mechanisms such as IP binding or user agent verification as defense-in-depth measures

# Verify /dev/urandom availability
ls -la /dev/urandom

# Check if Perl can access urandom
perl -e 'open(my $fh, "<", "/dev/urandom") or die "Cannot access urandom: $!"; print "urandom accessible\n";'

# Upgrade Amon2 via CPAN
cpan install Amon2