Predictable Session ID in CGI::Session::ID::md5

Executive Summary

CVE-2026-56016 affects CGI::Session::ID::md5 versions before 4.49 for Perl, where session IDs are generated using predictable, low-entropy sources. The generate_id method creates session IDs by taking an MD5 digest of the process ID (PID), the epoch time, and Perl's built-in rand() function.

All these sources are predictable: the PID comes from a small range, the epoch time can be guessed or read from the HTTP Date header, and Perl's rand() is predictable and reversible. This predictability allows an attacker to guess or predict valid session IDs.

By predicting a session ID, an attacker can impersonate the corresponding session and bypass authentication mechanisms.

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Detection Guidance

This vulnerability involves predictable session IDs generated by CGI::Session::ID::md5 versions before 4.49 in Perl. Detection involves identifying if your system uses a vulnerable version of CGI::Session::ID::md5 and analyzing session IDs for predictability.

To detect the vulnerability on your system, first check the installed version of the CGI::Session Perl module to see if it is before version 4.49.

• Run the command: perl -MCGI::Session -e 'print $CGI::Session::VERSION, "\n"' to display the installed CGI::Session version.

If the version is before 4.49, the system is potentially vulnerable. Next, you can monitor session IDs generated by the application to check if they are predictable.

• Capture HTTP traffic using tools like tcpdump or Wireshark to collect session IDs from cookies or URL parameters.
• Analyze captured session IDs for patterns or predictability, such as repeated or sequential values, or correlation with timestamps or process IDs.

Because the vulnerability arises from low-entropy sources (process ID, epoch time, and Perl's rand()), you can attempt to reproduce session IDs by scripting the MD5 generation using these inputs and comparing them to observed session IDs.

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Impact Analysis

This vulnerability allows an attacker to predict session IDs, which can lead to session hijacking.

An attacker who successfully predicts a session ID can impersonate a legitimate user, bypass authentication, and gain unauthorized access to the application or system.

This can result in unauthorized data access, manipulation, or other malicious activities depending on the privileges of the compromised session.

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Compliance Impact

This vulnerability allows attackers to predict session IDs due to the use of low-entropy sources in generating them. As a result, attackers can impersonate legitimate sessions and bypass authentication.

Such unauthorized access and session hijacking can lead to exposure or compromise of sensitive personal or health information, which would violate common standards and regulations like GDPR and HIPAA that require strong authentication and protection of user data.

Therefore, systems using vulnerable versions of CGI::Session::ID::md5 may fail to meet compliance requirements related to secure session management and data protection.

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Mitigation Strategies

To mitigate this vulnerability, you should upgrade CGI::Session::ID::md5 to version 4.49 or later. The fixed version uses the Crypt::SysRandom module to generate cryptographically secure random bytes for session IDs, replacing the previous predictable method based on low-entropy sources such as process ID, epoch time, and Perl's rand() function.

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