CVE-2025-40926

Critical

Published: 05 March 2026

Published

05 March 2026

Modified

12 March 2026

KEV Added

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Patch

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CVSS Score 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS Score 0.0007 21.9th percentile

Risk Priority 20 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2025-40926 is a critical-severity PRNG (CWE-338) vulnerability in Kazeburo Plack\. Its CVSS base score is 9.8 (Critical).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 21.9th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified are NIST 800-53 SC-23 (Session Authenticity) and SI-2 (Flaw Remediation).

Threat & Defense at a Glance

What attackers do: exploitation maps to Exploit Public-Facing Application (T1190) and 1 other technique. What defenders deploy: see the NIST 800-53 controls recommended below.

Threat & Defense Details

Mitigating Controls (NIST 800-53 r5)AI

SC-23 Session Authenticity good match

prevent

Requires generation and management of session identifiers with strength and randomness requirements, directly preventing predictable session IDs from weak generators like Perl's rand, epoch time, and PID.

SI-2 Flaw Remediation good match

prevent

Mandates identification, reporting, and correction of flaws like the insecure session ID generator in Plack::Middleware::Session::Simple, with testing of patches such as upgrading to version 0.05.

CM-6 Configuration Settings partial match

prevent

Enforces secure configuration settings for session management components to use cryptographically strong randomness, mitigating reliance on vulnerable default generators.

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access

Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.

attack.mitre.org →

T1550.004 Web Session Cookie Lateral Movement

Adversaries can use stolen session cookies to authenticate to web applications and services.

attack.mitre.org →

Why these techniques?

Predictable session IDs directly enable remote session hijacking via forged web session cookies (T1550.004) on public-facing Perl web apps (T1190).

Confidence: HIGH · MITRE ATT&CK Enterprise v18.1

NVD Description

Plack::Middleware::Session::Simple versions before 0.05 for Perl generates session ids insecurely. The default session id generator returns a SHA-1 hash seeded with the built-in rand function, the epoch time, and the PID. The PID will come from a small set of…

numbers, and the epoch time may be guessed, if it is not leaked from the HTTP Date header. The built-in rand function is unsuitable for cryptographic usage. Predictable session ids could allow an attacker to gain access to systems. Plack::Middleware::Session::Simple is intended to be compatible with Plack::Middleware::Session, which had a similar security issue CVE-2025-40923.

Deeper analysisAI

CVE-2025-40926 is a vulnerability in Plack::Middleware::Session::Simple versions before 0.05 for Perl, where the default session ID generator produces insecurely predictable identifiers. It computes a SHA-1 hash seeded with Perl's built-in rand function, the epoch time, and the process ID (PID). The PID draws from a limited range of values, epoch time can often be guessed or leaked via the HTTP Date header, and the rand function lacks cryptographic strength, violating CWE-338 (use of weak pseudo-random number generator) and CWE-340 (generation of predictable identifiers). This issue mirrors a similar flaw in the compatible Plack::Middleware::Session (CVE-2025-40923), earning a CVSS v3.1 score of 9.8 for its critical impact.

Remote attackers can exploit this over the network with low complexity and no privileges or user interaction required (AV:N/AC:L/PR:N/UI:N). By predicting session IDs, they can hijack active user sessions, impersonating legitimate users to access protected resources and systems, potentially leading to high confidentiality, integrity, and availability impacts (C:H/I:H/A:H).

Mitigation involves upgrading to version 0.05 or later, as detailed in the release changes and the fixing commit (760bb358b8f53e52cf415888a4ac858fd99bb24e). The patch addresses the weak generator, and the metacpan security guide emphasizes using cryptographically secure random data sources for such purposes. Source code diffs confirm the insecure implementation in prior versions.