CVE-2026-5087 identifies a cryptographic weakness in the Perl module PAGI::Middleware::Session::Store::Cookie, specifically in versions through 0.001003. The vulnerability arises from the module's method of generating random bytes for the initialization vector (IV) used in cookie encryption. Ideally, the module reads random bytes from the system's /dev/urandom device, which provides cryptographically secure randomness on Unix-like systems. However, on systems lacking /dev/urandom, such as Windows, the module emits a warning suggesting installation of the Crypt::URandom Perl module but does not actually use it. Instead, it falls back to the built-in rand function, which is not suitable for cryptographic purposes due to its predictability and poor entropy. This results in predictable IVs, which can significantly weaken the encryption of session cookies. An attacker who can predict or determine the IV may decrypt the cookie contents or manipulate session data, leading to potential session hijacking or privilege escalation. The module does not currently have a patch or fix, and no known exploits have been reported in the wild. The vulnerability is classified under CWE-338 (Use of Cryptographically Weak PRNG) and CWE-1204 (Use of a Broken or Risky Cryptographic Algorithm). The lack of a CVSS score indicates the need for an expert severity assessment.

The primary impact of this vulnerability is on the confidentiality and integrity of session data stored in cookies encrypted by the affected module. Predictable IVs can allow attackers to decrypt session cookies, exposing sensitive user information such as authentication tokens or personal data. Furthermore, attackers may tamper with session data to escalate privileges or impersonate users, leading to unauthorized access. This can result in account compromise, data breaches, and loss of trust. Since the vulnerability affects a session management component, it can undermine the security of web applications relying on this module. Organizations using this Perl module in their web infrastructure, especially on Windows or other systems without /dev/urandom, are at higher risk. The scope is limited to applications using this specific module for session cookie encryption, but given the widespread use of Perl in legacy and specialized systems, the impact can be significant in affected environments. No known exploits in the wild reduce immediate risk but do not eliminate the potential for future attacks.

To mitigate this vulnerability, organizations should avoid using the affected versions of PAGI::Middleware::Session::Store::Cookie for session encryption until a secure patch or update is released. Immediate steps include: 1) Replace the module with an alternative session management solution that uses a cryptographically secure random number generator for IV generation. 2) If replacement is not immediately possible, modify the module or application code to explicitly use a secure source of randomness, such as the Crypt::URandom module or system cryptographic APIs, ensuring they are properly integrated and tested. 3) On Windows systems, ensure that a secure cryptographic random source is available and used, for example via CryptGenRandom or equivalent. 4) Conduct a thorough review of session management and encryption practices to confirm no other weak randomness sources are used. 5) Monitor for updates or patches from the vendor and apply them promptly once available. 6) Implement additional security controls such as HTTP-only and Secure flags on cookies, and consider encrypting sensitive session data at the application layer as a defense-in-depth measure.