The vulnerability stems from improper output encoding in error pages generated by mailman's driver script. The Debian bug report shows the patch adds XML escaping for both environment variables (os.environ items) and traceback output. The original vulnerable code directly printed user-controlled URL parameters in error responses without sanitization, particularly in the sections handling: 1) Traceback output after exceptions 2) Environment variable display in debug output. The patch specifically introduces xml.sax.saxutils.escape() calls to sanitize these outputs, confirming these were the vulnerable code paths.