Predictable session IDs in Perl HTTP::Session enable hijacking

TL;DR — If your Perl app uses HTTP::Session defaults, your session cookies may be guessable due to weak entropy in the default ID generator; switch to a cryptographically secure session ID source.

What happened

HTTP::Session is a Perl library used by web applications to manage session state, typically by issuing a session ID (often stored in a cookie) that authenticates subsequent requests.

An oss-security disclosure for CVE-2026-3256 states that HTTP::Session through 0.53 defaults to HTTP::Session::ID::SHA1, which generates session IDs using a SHA-1 hash seeded with Perl’s built-in rand(), high resolution epoch time, and PID. The advisory notes that the PID comes from a small set of numbers and the epoch time may be guessable (including cases where time may be inferred from an HTTP Date header).

This is a classic appsec failure mode: predictable “security tokens” (session IDs, CSRF tokens, signing/encryption secrets) quietly collapse authentication boundaries, and the weakness often persists because the default looks “random enough” in casual review.

Who is impacted

• Applications using the http-session distribution (HTTP::Session) through version 0.53.
• Deployments relying on default session ID generation (not explicitly configuring a secure ID module).
• Environments where /dev/urandom is unavailable (the advisory explicitly calls out Windows as an example), increasing the chance projects stick to insecure default behavior.

Notable affected components called out by the advisory:

• HTTP::Session::ID::SHA1
• HTTP::Session::ID::MD5 (described as having a similar flaw)

What to do now

• Follow vendor remediation guidance from the advisory’s workaround section:

  • "Users on systems with a /dev/urandom device should configure the module to use HTTP::Session::ID::Urandom."

  • "Users on systems without a /dev/urandom (such as Windows) device will need to create custom ID modules that make use of module such as Crypt::SysRandom or Crypt::URandom."

• Inventory where HTTP::Session is used (including transitive dependencies) and confirm which session ID implementation is actually configured in production.
• If you suspect exposure (e.g., public-facing apps with long-lived sessions), invalidate active sessions and review authentication logs for anomalous session reuse patterns consistent with session guessing.