Executive Summary

CVE-2025-40926 is a security vulnerability in the Perl module Plack::Middleware::Session::Simple (version 0.04) that affects how session IDs are generated.

The module generates session IDs using a SHA-1 hash seeded with weak and predictable inputs such as the built-in rand function, the epoch time, and the process ID (PID). Since the PID comes from a small set of numbers and the epoch time can be guessed or leaked, the randomness is insufficient for cryptographic security.

Because of this predictable session ID generation, an attacker could potentially guess or predict valid session IDs, allowing unauthorized access to user sessions.

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Impact Analysis

[{'type': 'paragraph', 'content': 'This vulnerability can impact you by allowing attackers to predict or guess session IDs, which could lead to unauthorized access to your web application sessions.'}, {'type': 'paragraph', 'content': "If an attacker successfully predicts a session ID, they could hijack a user's session, gaining access to sensitive information or performing actions on behalf of the user."}, {'type': 'paragraph', 'content': 'This compromises the confidentiality and integrity of user sessions and can lead to further security breaches within your system.'}] [2]

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Compliance Impact

I don't know

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Detection Guidance

[{'type': 'paragraph', 'content': 'This vulnerability relates to the insecure generation of session IDs in Plack::Middleware::Session::Simple versions through 0.04. Detection involves identifying if your system is using this vulnerable version of the Perl module and if session IDs are generated using the insecure method.'}, {'type': 'paragraph', 'content': 'You can detect the vulnerability by checking the version of Plack::Middleware::Session::Simple installed on your system.'}, {'type': 'list_item', 'content': 'Run a command to check the installed version of the module, for example: perl -MPlack::Middleware::Session::Simple -e \'print $Plack::Middleware::Session::Simple::VERSION . "\\n";\''}, {'type': 'list_item', 'content': 'Inspect your application code or dependencies to see if Plack::Middleware::Session::Simple version 0.04 or earlier is used.'}, {'type': 'paragraph', 'content': 'Additionally, you can monitor session IDs in HTTP cookies to see if they match the predictable pattern described (40-character hexadecimal strings generated from weak entropy sources). However, no specific commands for network detection are provided in the available resources.'}] [2]

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Mitigation Strategies

The primary mitigation step is to upgrade Plack::Middleware::Session::Simple to a version that includes the security fix for CVE-2025-40926.

The fix replaces the insecure session ID generation method with a cryptographically secure random byte generator using the Crypt::SysRandom module.

• Update Plack::Middleware::Session::Simple to version 0.35 or later, where the session ID generation uses Crypt::SysRandom for secure randomness.
• If upgrading is not immediately possible, consider implementing additional controls such as restricting access to session cookies, monitoring for suspicious session activity, and ensuring that session IDs are not exposed via HTTP headers or logs.

Review your session management and cookie handling configurations to ensure they follow best practices, such as using secure, HttpOnly, and SameSite cookie attributes.

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