CVE-2026-13577: Dancer2 versions before 2.2.0 for Perl generate insecure session ids when required CSPRNG modules are unavailable

Dancer2 versions before 2.2.0 for Perl generate insecure session ids when required CSPRNG modules are unavailable.

Dancer2::Core::Role::SessionFactory::generateid silently falls back to a built-in rand-derived session id unless both Math::Random::ISAAC::XS and Crypt::URandom are available.

The fallback session id is generated from a SHA-1 hash of a call to the built-in rand function, the absolute path of the Dancer2::Core::Role::SessionFactory module, an internal counter, the process id, the module instance memory address, and a shuffled string of characters (using the List::Util::shuffle function, which also uses the built-in rand function).

These are all low-entropy and easily guessed sources.

The built-in rand() function is seeded with 32-bits and considered unsuitable for security applications.

Predictable session ids could allow an attacker to gain access to systems.