Product: Acrobat Reader version "x86 linux 5.0.5 Apr 25 2002 11:55:36" (Other UNIX versions probably also affected, see Comments.) Problem and exploit: Acroread creates or overwrites the file /tmp/AdobeFnt06.lst.UID, and changes its permissions to wide open (mode 666); it also follows symlinks. The attack is obvious: ln -s ~victim/.bashrc /tmp/AdobeFnt06.lst.VUID and wait for victim to use acroread; then we can write his .bashrc. Comments: A similar problem has been reported in acroread 4.05 in August 2001: http://online.securityfocus.com/bid/3225 (apparently reported to Adobe in March 2001 and even in October 1999). The problem is worse in acroread 5.05 than was in 4.05: the file is written in /tmp, not the home directory. (The securityfocus description has since been updated to say that also 5.05 has a problem.) The file /tmp/AdobeFnt06.lst.UID is created on exit. Acroread seems to respect the setting of TMPDIR in the environment: then creates the file in that directory, and sets its permission to a sensible 600. Could we mess with the data in /tmp/AdobeFnt06.lst.UID, to substitute fonts so all PDFs look gibberish; or with enough creativity, to show something else? Could we cause a buffer overflow? Running strace on acroread 5.05, I find: lstat64("/tmp/fileBfoZHm", 0xbfffe07c) = -1 ENOENT (No such file or directory) Whatever for? open("/dev/null", O_RDONLY|O_NONBLOCK|O_DIRECTORY) = -1 ENOTDIR (Not a directory) Does not Adobe know that? open("/users/psz/.acrobat/prefs", O_RDWR|O_CREAT|O_TRUNC, 0666) = 4 open("/tmp/Acro9vBGma", O_RDWR|O_CREAT|O_TRUNC|O_EXCL, 0666) = 11 Thankfully my umask is sensible. open("/usr/share/Acrobat/505/Resource/Font/AdobeFnt06.lst.padua", O_WRONLY|O_CREAT|O_TRUNC, 01037355510) = -1 EROFS (Read-only file system) open("/tmp/AdobeFnt06.lst.1001", O_WRONLY|O_CREAT|O_TRUNC, 01037355510) = 4 Semi-random modes, as if Adobe used open() with two arguments only. (Often zero when there is a filename on the acroread command line.) BEWARE, even more if running as root! fchmod(4, 0666) = 0 Applied to "/tmp/AdobeFnt06.lst.1001" above. The mode would be 600 when there is a TMPDIR; I do not know what mode would be applied to "/usr/share/.../AdobeFnt06.lst.padua". Workaround: I use the following wrapper around acroread (move original script or binary to acroread.real, put this in its place). Use TMPDIR, but also ensure file in /tmp is safe (in case writing in TMPDIR fails for some reason: diskquota?). With file in /tmp, leaves no race with the open() in acroread, just a window of opportunity to mess with the data. #!/usr/bin/perl -- $PROG = '/usr/share/Acrobat/505/bin/acroread.real'; $TMPF = "/tmp/AdobeFnt06.lst.$<"; $MYTD = "$ENV{'HOME'}/.acrobat"; $MYTF = "$MYTD/AdobeFnt06.lst.$<"; $ENV{'TMPDIR'} = $MYTD; use Fcntl; sub checkfix { my ($nam, $msg) = @_; ($dev,$ino,$mode,$nlink,$uid,$gid,@rest) = lstat( $nam ); ( -f _ and ! -l _ and ! -d _ ) or die "$msg: $nam is not a file\n"; # BEWARE: on some systems, $gid comes from directory ( $uid == $< and $gid == $( ) or die "$msg: $nam is not your own\n"; ( $nlink == 1 ) or die "$msg: $nam has hardlinks\n"; chmod( 0600, $nam ) or die "$msg: cannot chmod $nam\n"; } $< > 99 or die "No daemons\n"; sysopen( F, $TMPF, O_RDWR|O_CREAT|O_EXCL, 0600 ) and close( F ) #and print "Pre-created $TMPF\n" ; mkdir( $MYTD, 0700 ) #and print "Pre-created $MYTD\n" ; sysopen( F, $MYTF, O_RDWR|O_CREAT|O_EXCL, 0600 ) and close( F ) #and print "Pre-created $MYTF\n" ; &checkfix( $TMPF, "Tricked" ); &checkfix( $MYTF, "Tricked" ); system( $PROG, @ARGV ); &checkfix( $TMPF, "After acroread" ); &checkfix( $MYTF, "After acroread" ); #!# Vendor status: Reported to Adobe upon discovery, on 29 May 2002. After some initial difficulties, they seem to understand that there is a problem and that it needs to be fixed; they say this will take several weeks at least. Acknowledgements: Thanks to a user of my system, for noticing the wide-open permissions on the /tmp files. Thanks to Jarno Huuskonen, for tips and discussion following his coincidental post http://www.securityfocus.com/archive/1/277932 . Paul Szabo - pszat_private http://www.maths.usyd.edu.au:8000/u/psz/ School of Mathematics and Statistics University of Sydney 2006 Australia
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