Dancer::Session::Abstract versions through 1.3522 for Perl generates session ids insecurely.
The session id is generated from summing the character codepoints of the absolute pathname with the process id, the epoch time and calls to the built-in rand() function to return a number between 0 and 999-billion, and concatenating that result three times.
The path name might be known or guessed by an attacker, especially for applications known to be written using Dancer with standard installation locations.
The epoch time can be guessed by an attacker, and may be leaked in the HTTP header.
The process id comes from a small set of numbers, and workers may have sequential process ids.
The built-in rand() function is seeded with 32-bits and is considered unsuitable for security applications.
Predictable session ids could allow an attacker to gain access to systems.
CRLF injection vulnerability in the cookie method allows remote attackers to inject arbitrary HTTP headers and conduct HTTP response splitting attacks via a cookie name.
Directory traversal vulnerability (Mojolicious report, but Dancer was vulnerable as well).