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TUCoPS :: Unix :: General :: a6056.htm

man arbitrary code execution



12th Mar 2003 [SBWID-6056]
COMMAND

	man arbitrary code execution

SYSTEMS AFFECTED

	versions prior to 1.5l. 1.51 is immune.

PROBLEM

	Jack Lloyd reported following bug on the *widely used* man page  reader:
	man.
	
	man 1.5l was released today, fixing a bug  which  results  in  arbitrary
	code execution upon reading a specially formatted man  file.  The  basic
	problem is, upon finding a string with a quoting problem,  the  function
	my_xsprintf in util.c will return  "unsafe"  (rather  than  returning  a
	string which could be interpreted by the shell). This  return  value  is
	passed directly to system(3) - meaning if there  is  any  program  named
	`unsafe`, it will execute with the privs of the user.
	
	 Example
	 =======
	
	
	$ cat innocent.1
	.so "".1
	$ cat '"".1' # the outer '' quotes are for the shell the user will never see this
	$ cat `which unsafe`
	#!/bin/sh
	
	echo "oops"
	id -a
	$ man ./innocent.1
	oops
	uid=528(lloyd) gid=100(users) groups=100(users)
	$
	
	
	The location of the man pages and the binary are  basically  irrelevent,
	as long as:
	
	1) man can find the man pages somewhere; both man pages have  to  be  in
	the
	   same subtree due to how man handles .so directives. /usr/share/man/man*
	   works fine, as does the local directory (./manpage.1) case
	2) the shell can find `unsafe` somewhere in $PATH
	
	The severity of this depends on lot on your systems,  but  is  generally
	not very high. People running systems with  publicly  writeable  contrib
	directories should probably do a quick `find . -name unsafe` just to  be
	sure. Average home users probably don't have much to  worry  about,  nor
	do most corporate environments.
	
	

SOLUTION

	A simple workaround is to symlink /bin/unsafe to /bin/true. man 1.5l  is
	not vulnerable to this problem.
	
	I would like to thank Andries Brouwer,  the  current  `man`  maintainer,
	for his fast response. Sources for the new version can be found at
	
	ftp://ftp.win.tue.nl/pub/linux-local/utils/man/
	


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