Smash The Stack: IO Level 4 Walkthrough
In Level 4 of the Smash The Stack IO wargame, executing the binary prints a static welcome message regardless of input arguments: level4@io:/levels$ ./level04 Welcome level5 level4@io:/levels$ ./level04 $(python -c "print 'A' * 1024") Welcome level5 Analyzing the Source Code Inspecting level04.c: #include <stdio.h> #include <stdlib.h> int main() { char username[1024]; FILE* f = popen("whoami", "r"); fgets(username, sizeof(username), f); printf("Welcome %s", username); return 0; } Vulnerability Analysis: Insecure PATH Resolution Notice the popen call: FILE* f = popen("whoami", "r"); The author called whoami as a bare command name rather than specifying an absolute binary path (such as /usr/bin/whoami). When popen() spawns /bin/sh -c whoami, the shell searches for the whoami executable in the directories listed in the current user’s PATH environment variable from left to right. ...