[Expat-bugs] [ expat-Bugs-1990430 ] Parser crash with specially formatted UTF-8 sequences
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Fri Jun 13 18:41:51 CEST 2008
Bugs item #1990430, was opened at 2008-06-10 22:45
Message generated for change (Comment added) made by petervalchev
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Category: www.libexpat.org
Group: None
Status: Open
Resolution: Fixed
Priority: 5
Private: Yes
Submitted By: Peter Valchev (petervalchev)
Assigned to: Karl Waclawek (kwaclaw)
Summary: Parser crash with specially formatted UTF-8 sequences
Initial Comment:
I have discovered a way to crash libexpat's xml parser with certain specially formatted UTF-8 sequences. All applications that link w/ expat and use it to render user-provided XML files are affected. As far as I see, the issue is not exploitable, just denial of service.
This is the patch that I have come up with, also attached to this email:
+++ lib/xmltok_impl.c 2007-12-21 11:11:42.054417000 -0800
@@ -1745,6 +1745,9 @@
switch (BYTE_TYPE(enc, ptr)) {
#define LEAD_CASE(n) \
case BT_LEAD ## n: \
+ if (end - ptr < n) { \
+ return; \
+ } \
ptr += n; \
break;
LEAD_CASE(2) LEAD_CASE(3) LEAD_CASE(4)
The parser's updatePosition function which keeps track of the current position pointer increments the ptr by {2, 3, 4} to skip past multibyte character ombinations, and this causes ptr in the "while (ptr != end)" loop to jump past the terminating condition, causing the loop to continue reading past 'end' and into out of bounds memory until a crash.
In general this parser does not appear the most robust and could be the source of some security issues.
A fault file is attached. To reproduce, compile examples/outline.c and run against it. This patch may not be 100% complete...
Contact:
Peter Valchev <pvalchev at google.com>
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>Comment By: Peter Valchev (petervalchev)
Date: 2008-06-13 10:41
Message:
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Yes, I originally had the same patch (while ptr < end) but chose the
latter to keep in line with the rest of the file, sigh.
Now the next question is, there are many while (ptr != end) loops still
left in that file if you search.. some of the look like they could have the
same problem. Should all of them be changed, to be on the paranoid side?
The original construct is just evil and waiting for bugs like this to
happen...
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Comment By: Karl Waclawek (kwaclaw)
Date: 2008-06-13 07:24
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I applied the original fix I had in mind, which I didn't use because I
thought yours might perform better. It is quite simple: replace "while (ptr
!= end)" with "while (ptr < end)".
Seems to work so far. Fixed in xmltok_impl.c rev. 1.15.
Karl
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Comment By: Peter Valchev (petervalchev)
Date: 2008-06-12 23:15
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Ugh the padding I inserted broke the example. New attached tarball with
expat-fault3.xml crashes both outline and xmlwf
(gdb) r < /tmp/expat-fault3.
Starting program: /usr/src/lib/libexpat/examples/outline <
/tmp/expat-fault3.
/bin/ksh: cannot open /tmp/expat-fault3.: No such file or directory
Program exited with code 01.
You can't do that without a process to debug.
(gdb) r < /tmp/expat-fault3.xml
Starting program: /usr/src/lib/libexpat/examples/outline <
/tmp/expat-fault3.xml
Program received signal SIGSEGV, Segmentation fault.
little2_updatePosition (enc=0x2676dec0, ptr=0x7f4ba000 <Address 0x7f4ba000
out of bounds>,
end=0x7f4b9003 "", pos=0x7de87198) at xmltok_impl.c:1748
1748 switch (BYTE_TYPE(enc, ptr)) {
(gdb) bt
#0 little2_updatePosition (enc=0x2676dec0, ptr=0x7f4ba000 <Address
0x7f4ba000 out of bounds>,
end=0x7f4b9003 "", pos=0x7de87198) at xmltok_impl.c:1748
#1 0x0676b8a3 in XML_GetCurrentLineNumber (parser=0x7de87000)
at /usr/src/lib/libexpat/lib/xmlparse.c:1799
#2 0x1c000a24 in main ()
File Added: expat-fault3.tar.gz
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Comment By: Peter Valchev (petervalchev)
Date: 2008-06-12 23:14
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Ugh the padding I inserted broke the example. New attached tarball with
expat-fault3.xml crashes both outline and xmlwf
(gdb) r < /tmp/expat-fault3.
Starting program: /usr/src/lib/libexpat/examples/outline <
/tmp/expat-fault3.
/bin/ksh: cannot open /tmp/expat-fault3.: No such file or directory
Program exited with code 01.
You can't do that without a process to debug.
(gdb) r < /tmp/expat-fault3.xml
Starting program: /usr/src/lib/libexpat/examples/outline <
/tmp/expat-fault3.xml
Program received signal SIGSEGV, Segmentation fault.
little2_updatePosition (enc=0x2676dec0, ptr=0x7f4ba000 <Address 0x7f4ba000
out of bounds>,
end=0x7f4b9003 "", pos=0x7de87198) at xmltok_impl.c:1748
1748 switch (BYTE_TYPE(enc, ptr)) {
(gdb) bt
#0 little2_updatePosition (enc=0x2676dec0, ptr=0x7f4ba000 <Address
0x7f4ba000 out of bounds>,
end=0x7f4b9003 "", pos=0x7de87198) at xmltok_impl.c:1748
#1 0x0676b8a3 in XML_GetCurrentLineNumber (parser=0x7de87000)
at /usr/src/lib/libexpat/lib/xmlparse.c:1799
#2 0x1c000a24 in main ()
File Added: expat-fault3.tar.gz
----------------------------------------------------------------------
Comment By: Karl Waclawek (kwaclaw)
Date: 2008-06-12 20:40
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What is supposed to happen with the second test case?
It reports an error, but does not crash with xmlwf or outline.
IE reports the same error, so that is probably OK.
----------------------------------------------------------------------
Comment By: Peter Valchev (petervalchev)
Date: 2008-06-12 16:08
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Thanks.
Actually rechecking in this area again, I think I found another issue,
which seems to not be covered with the patch I provided :( I am attaching
the new test case.
I haven't found many other issues at this point... I stumbled upon this
one fairly quickly and my above observations were more general than
anything. If I do find more I'll be sure to tell you.
File Added: expat-fault2.xml
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Comment By: Karl Waclawek (kwaclaw)
Date: 2008-06-11 08:46
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Can reproduce. The problem is that this code can be called *after* an
error has been found (to report line and column number). Therefore it
should not rely on correct byte counts for multibyte characters.
Patch applied in xmltok_impl.c rev. 1.14.
Would you please also report all the other issues you have found?
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