The xterm terminal emulator in XFree86 4.2.0 and earlier allows attackers to modify the window title via a certain character escape sequence and then insert it back to the command line in the user's terminal, e.g. when the user views a file containing the malicious sequence, which could allow the attacker to execute arbitrary commands.
The DEC UDK processing feature in the xterm terminal emulator in XFree86 4.2.99.4 and earlier allows attackers to cause a denial of service via a certain character escape sequence that causes the terminal to enter a tight loop.