CVE-2025-40923 identifies a security vulnerability in the Plack::Middleware::Session module for Perl, specifically versions before 0.35. This module is responsible for managing session IDs in web applications using the Plack framework. The vulnerability arises from the use of an insecure method to generate session identifiers. The default session ID generator uses a SHA-1 hash seeded with three components: the built-in Perl rand() function, the epoch time, and the process ID (PID). Each of these components contributes to the predictability of the session ID. The rand() function in Perl is not cryptographically secure and can be predicted if the internal state is known or inferred. The epoch time, representing the current time in seconds since Unix epoch, can often be guessed or derived from HTTP headers such as the Date header. The PID is drawn from a limited range of possible values, further reducing entropy. Combining these predictable inputs results in session IDs that can be feasibly guessed or reproduced by an attacker. This predictability violates secure session management principles and can allow attackers to hijack sessions by guessing valid session IDs, leading to unauthorized access to user accounts or sensitive data. The vulnerability is classified under CWE-340 (Generation of Predictable Numbers or Identifiers) and CWE-338 (Use of Cryptographically Weak Pseudo-Random Number Generator). No known exploits are currently reported in the wild, and no CVSS score has been assigned yet. However, the vulnerability is publicly disclosed and should be addressed promptly.

For European organizations using Perl-based web applications that rely on Plack::Middleware::Session versions prior to 0.35, this vulnerability poses a significant risk. Exploitation could allow attackers to predict session IDs, enabling session hijacking attacks. This compromises confidentiality by exposing user sessions and potentially sensitive data. Integrity may also be affected if attackers perform unauthorized actions within hijacked sessions. Availability impact is generally limited but could arise if attackers disrupt sessions or perform denial-of-service via session manipulation. Given the widespread use of Perl in legacy and some modern web applications across Europe, especially in sectors like government, finance, and healthcare, the risk is non-trivial. Attackers could leverage this vulnerability to impersonate legitimate users, escalate privileges, or access restricted resources. The lack of cryptographically secure session IDs undermines trust in affected applications and could lead to regulatory non-compliance under GDPR if personal data is exposed. Although no active exploits are reported, the vulnerability's nature makes it a likely target for attackers once widely known.

European organizations should immediately audit their use of Plack::Middleware::Session in Perl web applications to identify affected versions (before 0.35). The primary mitigation is to upgrade to version 0.35 or later, where the session ID generation method is expected to be improved with cryptographically secure random number generation. If upgrading is not immediately feasible, organizations should implement custom session ID generators using secure sources of entropy such as /dev/urandom or cryptographically secure random number generators available in Perl (e.g., Crypt::PRNG or Crypt::Random modules). Additionally, HTTP headers that leak timing information, such as the Date header, should be minimized or removed to reduce predictability of the epoch time component. Implementing additional session security controls like short session lifetimes, IP address binding, and multi-factor authentication can help mitigate the impact of session hijacking. Regular security testing and code reviews focusing on session management practices are recommended. Monitoring for unusual session activity and implementing anomaly detection can provide early warning of exploitation attempts.