- A reflected XSS caused by unsafe handling of the $PHP_SELF variable
- A reflected XSS caused by unsafe handling of user supplied encrypted data
- A remote command execution vulnerability (only in very specific configurations)
The second XSS is rather interresting. SquirrelMail comes with a test script which (given ciphertext and a key) decrypts user supplied data. The decrypted data was then displayed unfiltered (resulting in an obvious XSS). Pretty stealthy and should circumvent any IDS or WAF. Having this kind of script in your webroot is obviously a bad idea anyway.
The last bug occurs only in very specific configurations. If you have more than one imap server, there is a way to configure that in SquirrelMail. If you configure this and use the example map_yp_alias function for this, unauthenticated attackers could execute shell commands on your server. If you built your own function for this, you should probably check if it handles input correctly. It is unlikely that you're running this kind of configuration, but there are probably a couple of installations out there using this.