It used to be Well Known that xterm's way of opening a log file was insecure. Well, that was 5+ years ago so I decided to take a look at the current state of affairs. Things have changed, but mostly to "different" rather than "better". Problem: when log files are enabled, they are created in the following way (checking in XFree86 3.3.6 source; matches Solaris binaries) and are subject to race conditions: 1. File is checked for existance using access. 2. If file does not exist, it is created in a subprocess using user's real uid/gid. [ok] 3. File is checked for existance using access. 4. File is checked for write permission using access. 5. File is opened O_WRONLY | O_APPEND. [plonk] A little symlink magic between 4 and 5 and you have write access to any file if your xterm is setuid/setgid. General attack idea: ls -lL `which xterm` # If not setuid/setgid, you are safe touch dummy symlink-flipflop link dummy /.rhosts xterm -l -lf link -e echo + + Moral: access() is totally useless for security purposes. Use it only as a means of providing better error messages (as it might not be easy to get an error message out from a subprocess). Morten
This archive was generated by hypermail 2b30 : Fri Apr 13 2001 - 15:38:21 PDT