It appears that there was a fairly significant memory leak in the Crypt_CBC.php module.
I've made the changes, and released the latest version of the class through the regular PEAR channels.
If you have PEAR installed and configured properly, then a simple:
pear upgrade Crypt_CBC
will work. But it probably won't, because PEAR's upgrade feature isn't working too well right now, so grab the file from the website:
and unpack it where appropriate.
NOTE: this fix really requires libmcrypt >= 2.4.16, but I would highly recommend the latest stable version (right now, it's 2.5.0).