I am the original author of the software you refer to. Before its complete rewrite (released two months ago), Cryptocat was based on a codebase that I wrote starting in my early undergrad and that was notorious for containing many high-severity vulnerabilities. The most important was "Decryptocat", documented by Steve Thomas [0] but we also had:
* AES-CTR counter-reuse (2012)
* Biased Fortuna CSPRNG implementation [1] (2013)
To be sure, Cryptocat's substandard reputation was well-deserved; but all the same, I think that every vulnerability that did pop up was addressed responsibly and with full disclosure.
Since then, there has been a major rewrite that I am conversely quite happy with.
* AES-CTR counter-reuse (2012)
* Biased Fortuna CSPRNG implementation [1] (2013)
To be sure, Cryptocat's substandard reputation was well-deserved; but all the same, I think that every vulnerability that did pop up was addressed responsibly and with full disclosure.
Since then, there has been a major rewrite that I am conversely quite happy with.
[0] https://tobtu.com/decryptocat.php
[1] https://nakedsecurity.sophos.com/2013/07/09/anatomy-of-a-pse...