Xlockmore 4.16 Buffer Overflow Vulnerability

xlock is an X11 utility used to lock Xwindows displays until the password of the user running X is entered correctly. Of course, in order to perform the password-check xlock must be setuid root and have access to the shadowed passwd file. In the xlockmore distributions versions prior to 4.16.1, a buffer overflow vulnerability was present in xlock that permitted a user to view parts of the shadowed passwd file. This is achieved by overwriting (with an oversized -mode argument) a global variable storing a pointer to a string printed in the "usage" output. The pointer would be overwritten with an address pointing to the shadowed passwd data. With the long argument, xlock would find and an error in the command syntax and exit, printing the usage information (along with the shadowed passwd text).


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