CVE-2024-56830

Executive Summary

CVE-2024-56830 affects the Perl module Net::EasyTCP versions 0.15 through 0.26. The vulnerability arises from the module’s fallback to Perl’s built-in rand() function when no strong cryptographic random number generator (e.g., Math::Random::Secure) is available. This weakens the randomness of session tokens, nonces, or other security-critical values generated by the module, making them predictable and potentially exploitable for session hijacking or replay attacks.

The CVSS score is 5.4 (Medium), with a low attack complexity and no privileges required. The EPSS score is 0.0%, indicating no active exploitation in the wild at the time of analysis. No public proof-of-concept or exploit code has been released.

Supply Chain & Package Security

This vulnerability is in a Perl CPAN module. Organizations using Net::EasyTCP in their Perl applications should audit their dependency trees. The module is not hosted on NPM, PyPI, Go, or Ruby, but it is distributed via CPAN. The lack of a patch or official advisory from the maintainer increases supply chain risk.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The vulnerability can be exploited remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – The exploit does not affect other components.
• Confidentiality (C): Low (L) – Limited information disclosure possible.
• Integrity (I): Low (L) – Limited ability to modify data.
• Availability (A): None (N) – No impact on availability.

Threat Level: Medium. The vulnerability is easy to exploit but requires the absence of a strong random module. It is not currently exploited in the wild.

Deep Dive & Mechanics

The Net::EasyTCP module is designed to simplify TCP client-server communication in Perl. It optionally uses random numbers for session IDs or nonces. The vulnerable code path checks for the presence of a strong random module (e.g., Math::Random::Secure). If not found, it falls back to Perl’s built-in rand() function, which is a linear congruential generator (LCG) and is not cryptographically secure.

Perl’s rand() is seeded with a predictable value (often based on time and process ID) unless explicitly seeded with a strong source. An attacker who can observe multiple random values can predict future values, potentially allowing session hijacking or replay attacks.

The vulnerability is present in all versions from 0.15 to 0.26. The module does not enforce the use of a strong random module, leaving the decision to the user.

Vulnerable vs. Patched Code

Vulnerable Code (Net::EasyTCP 0.15-0.26):

# In the module's initialization or random generation routine
if (eval { require Math::Random::Secure; 1 }) {
    *random_bytes = sub { Math::Random::Secure::irand() };
} else {
    *random_bytes = sub { int(rand(2**32)) };  # Weak fallback
}

Patched Code (Hypothetical):

# Force strong randomness or die
use Math::Random::Secure qw(irand);
*random_bytes = sub { irand() };
# Or at least warn the user
if (!eval { require Math::Random::Secure; 1 }) {
    die "Net::EasyTCP requires Math::Random::Secure for secure operation";
}

Explanation: The vulnerable code silently falls back to weak randomness. The patched version either requires the strong module or warns the user.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Target system running a Perl application using Net::EasyTCP 0.15-0.26.
• No Math::Random::Secure installed.
• Ability to observe multiple random values (e.g., session IDs).

Exploitation Steps:

1. Identify the vulnerability: Check if the target uses Net::EasyTCP and if Math::Random::Secure is absent.
2. Collect random values: Capture several session IDs or nonces generated by the application.
3. Predict future values: Use the LCG properties of Perl’s rand() to predict the next values. Tools like randcrack can assist.
4. Hijack session: Use the predicted value to forge a valid session ID.

Lab Setup:

# Install vulnerable module
cpan install Net::EasyTCP@0.26
# Create a simple server that generates session IDs
perl -MNet::EasyTCP -e '
    my $server = Net::EasyTCP->new( Port => 8080 );
    $server->on_connect(sub {
        my $session_id = int(rand(2**32));
        print "Session ID: $session_id\n";
    });
    $server->start;
'

Blue Team

Detection:

• Monitor for use of Net::EasyTCP in your Perl applications.
• Check if Math::Random::Secure is installed: perl -MMath::Random::Secure -e 1
• Look for predictable session IDs in logs.

Mitigation:

• Install Math::Random::Secure: cpan install Math::Random::Secure
• Upgrade to a patched version of Net::EasyTCP if available.
• Replace Net::EasyTCP with a more secure alternative.

Indicators of Compromise (IOCs)

• File: Net/EasyTCP.pm (versions 0.15-0.26)
• Behavior: Use of Perl’s rand() for security-sensitive values.
• Logs: Session IDs that follow a predictable pattern (e.g., sequential or low entropy).
• Network: TCP connections to services using Net::EasyTCP without strong randomness.

Detection Engineering

YARA Rule

rule Net_EasyTCP_Vulnerable {
    meta:
        description = "Detects vulnerable Net::EasyTCP versions 0.15-0.26"
        author = "Valters IT"
        date = "2025-04-10"
        hash = "unknown"
    strings:
        $vuln_func = "int(rand(2**32))"
        $module_check = "Math::Random::Secure"
    condition:
        $vuln_func and not $module_check
}

Sigma Rule

title: Net::EasyTCP Weak Randomness
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects use of vulnerable Net::EasyTCP module with weak rand()
author: Valters IT
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/perl'
        CommandLine|contains: 'Net::EasyTCP'
    condition: selection
falsepositives:
    - Legitimate use with strong random module installed
level: medium

Suricata Rule

alert tcp any any -> any any (msg:"Net::EasyTCP Weak Randomness"; flow:to_server; content:"Net::EasyTCP"; nocase; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Install Math::Random::Secure on all systems using Net::EasyTCP.
2. Verify that the module uses strong randomness: perl -MMath::Random::Secure -e 'print "OK\n"'
3. Monitor for any unexpected session IDs or nonces.

Long-term Fix:

• Replace Net::EasyTCP with a more secure alternative (e.g., IO::Socket::IP with manual randomness).
• Contact the module maintainer for an official patch.
• Implement a software composition analysis (SCA) tool to detect vulnerable dependencies.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert