CVE-2025-40923 identifies a cryptographic weakness in the Plack::Middleware::Session Perl module prior to version 0.35. The vulnerability stems from the use of an insecure session ID generation mechanism. Specifically, the default session ID is created by hashing a combination of the built-in Perl rand() function output, the current epoch time, and the process ID (PID) using SHA-1. The rand() function in Perl is not designed for cryptographic security and can be predicted if the seed or internal state is known or inferred. The PID is drawn from a limited range of values, and the epoch time can often be estimated or leaked via HTTP headers such as the Date header. This combination makes the session IDs predictable, allowing attackers to potentially guess valid session tokens without authentication or user interaction. Exploiting this vulnerability could enable session hijacking, unauthorized access to user accounts, and potential privilege escalation within web applications using this middleware. Although no public exploits are reported yet, the vulnerability’s CVSS score of 7.3 (high) reflects the significant risk posed by the ease of exploitation and the impact on confidentiality, integrity, and availability of affected systems. The vulnerability is cataloged under CWE-340, which concerns the generation of predictable numbers or identifiers, a common cryptographic flaw. The affected versions are limited to 0.01 and earlier, with the issue resolved in version 0.35 by adopting a cryptographically secure random number generator for session ID creation.

For European organizations, this vulnerability poses a substantial risk to web applications that rely on Plack::Middleware::Session for session management, particularly those using affected versions prior to 0.35. Successful exploitation can lead to session hijacking, allowing attackers to impersonate legitimate users, access sensitive data, or perform unauthorized actions. This can compromise confidentiality and integrity of user data and disrupt availability if attackers manipulate sessions to cause denial of service. Sectors such as finance, healthcare, e-commerce, and government services, which often handle sensitive personal and financial information, are particularly vulnerable. The predictability of session IDs increases the attack surface for automated attacks and targeted intrusions. Additionally, the vulnerability could facilitate lateral movement within networks if session tokens grant access to internal systems. Given the widespread use of Perl in legacy and modern web applications across Europe, organizations that have not updated this middleware are at risk. The absence of known exploits in the wild suggests a window of opportunity for proactive mitigation before widespread attacks occur.

The primary mitigation is to upgrade Plack::Middleware::Session to version 0.35 or later, where the session ID generation uses a cryptographically secure random number generator, eliminating predictability. Organizations should audit their Perl web applications to identify usage of the vulnerable middleware version and prioritize patching. If immediate upgrade is not feasible, consider implementing compensating controls such as: enforcing strict session expiration and rotation policies to limit the window of token validity; monitoring session activity for anomalies indicative of hijacking attempts; restricting session access by IP address or device fingerprinting to reduce the risk of token misuse; disabling or filtering HTTP headers that leak epoch time information, such as the Date header, to reduce predictability; and employing web application firewalls (WAFs) to detect and block suspicious session-related requests. Additionally, developers should review session management practices to ensure adherence to cryptographic best practices and avoid reliance on insecure random number generators. Regular security assessments and penetration testing focused on session management can help detect residual weaknesses.