libxml 2.9.2 Stack Overflow

Published
Credit
Risk
2016.05.04
Simon Lees
Medium
CWE
CVE
Local
Remote
CWE-119
CVE-2016-3627
No
Yes

CVSS Base Score
Impact Subscore
Exploitability Subscore
5/10
2.9/10
10/10
Exploit range
Attack complexity
Authentication
Remote
Low
No required
Confidentiality impact
Integrity impact
Availability impact
None
None
Partial

Hi
This is a disclosure of the following issue that was raised a week ago
on the distro's mailing list. Both bugs on the gnome bugtracker are
currently private and should be made public now. The two attached
patches are based off the 2.9.3 libxml2 release.

A couple of weeks back while working on a related bug [CVE-2016-3627] I
discovered a specially created xml file is capable of triggering a stack
overflow before libxml2 can detect its a invalid xml file.

We raised this issue upstream on 2016-04-18 and informed them that we
would place a two week embargo on the issue in case we didn't here back.
As of yet we have had no response so we have posted here.
https://bugzilla.gnome.org/show_bug.cgi?id=765207

We intend to keep the current embargo (ending May 3) unless we get
advise otherwise here. Below is a script to generate the xml file along
with a tested patch to fix the issue. I will also include our
unpublished patch and simplified reproducer for CVE-2016-3627 as again
we have had no response upstream and its likely that you will want to
fix this less severe issue at the same time.
https://bugzilla.gnome.org/show_bug.cgi?id=762100

python3 repoducer.py ; xmllint repo.xml

repoducer.py
-----------------------------------------------------------------------
#!/bin/python3

f = open('repo.xml', 'w')

f.write( "<!DOCTYPE a [ ")

i = 1

while (i < 30000):
f.write ("<!ENTITY a" + str(i) + " \"&a" + str(i+1) + ";\">")
i = i+1

f.write("<!ENTITY a" + str(i+1) + " \"&a1;\">]> <bruces bogans=\"&a1;\">")

f.close()
-----------------------------------------------------------------------

Patch for this issue.
-----------------------------------------------------------------------
From: Peter Simons <psimons@suse.com>
Date: Fri, 15 Apr 2016 11:56:55 +0200
Subject: Add missing increments of recursion depth counter to XML
parser.

The functions xmlParserEntityCheck() and xmlParseAttValueComplex() used
to call
xmlStringDecodeEntities() in a recursive context without incrementing the
'depth' counter in the parser context. Because of that omission, the parser
failed to detect attribute recursions in certain documents before
running out
of stack space.
---
parser.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/parser.c b/parser.c
index 9604a72..4da151f 100644
--- a/parser.c
+++ b/parser.c
@@ -144,8 +144,10 @@ xmlParserEntityCheck(xmlParserCtxtPtr ctxt, size_t
size,

ent->checked = 1;

+ ++ctxt->depth;
rep = xmlStringDecodeEntities(ctxt, ent->content,
XML_SUBSTITUTE_REF, 0, 0, 0);
+ --ctxt->depth;

ent->checked = (ctxt->nbentities - oldnbent + 1) * 2;
if (rep != NULL) {
@@ -3966,8 +3968,10 @@ xmlParseEntityValue(xmlParserCtxtPtr ctxt,
xmlChar **orig) {
* an entity declaration, it is bypassed and left as is.
* so XML_SUBSTITUTE_REF is not set here.
*/
+ ++ctxt->depth;
ret = xmlStringDecodeEntities(ctxt, buf, XML_SUBSTITUTE_PEREF,
0, 0, 0);
+ --ctxt->depth;
if (orig != NULL)
*orig = buf;
else
@@ -4092,9 +4096,11 @@ xmlParseAttValueComplex(xmlParserCtxtPtr ctxt,
int *attlen, int normalize) {
} else if ((ent != NULL) &&
(ctxt->replaceEntities != 0)) {
if (ent->etype != XML_INTERNAL_PREDEFINED_ENTITY) {
+ ++ctxt->depth;
rep = xmlStringDecodeEntities(ctxt, ent->content,
XML_SUBSTITUTE_REF,
0, 0, 0);
+ --ctxt->depth;
if (rep != NULL) {
current = rep;
while (*current != 0) { /* non input consuming */
@@ -4130,8 +4136,10 @@ xmlParseAttValueComplex(xmlParserCtxtPtr ctxt,
int *attlen, int normalize) {
(ent->content != NULL) && (ent->checked == 0)) {
unsigned long oldnbent = ctxt->nbentities;

+ ++ctxt->depth;
rep = xmlStringDecodeEntities(ctxt, ent->content,
XML_SUBSTITUTE_REF, 0, 0, 0);
+ --ctxt->depth;

ent->checked = (ctxt->nbentities - oldnbent + 1) * 2;
if (rep != NULL) {
--
2.7.4

-----------------------------------------------------------------------

CVE-2016-3627 - simplified reproducers
echo '<!DOCTYPE b [ <!ENTITY b "&b;"> ]> <b b="&b;">' | xmllint -recover -
echo '<!DOCTYPE b [ <!ENTITY b "&c;"> <!ENTITY c "&d;"> <!ENTITY d
"&b;">]> <test123="&c;">' | xmllint -recover -
-----------------------------------------------------------------------


CVE-2016-3627 - Patch
-----------------------------------------------------------------------
From: Peter Simons <psimons@suse.com>
Date: Thu, 14 Apr 2016 16:15:13 +0200
Subject: [PATCH] xmlStringGetNodeList: limit the function to 1024 recursions
to avoid CVE-2016-3627

This patch prevents stack overflows like the one reported in
https://bugzilla.gnome.org/show_bug.cgi?id=762100.
---
tree.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/tree.c b/tree.c
index 6a158ce..9c9f0ec 100644
--- a/tree.c
+++ b/tree.c
@@ -1464,6 +1464,8 @@ out:
return(ret);
}

+static xmlNodePtr xmlStringGetNodeListInternal(const xmlDoc *doc, const
xmlChar *value, size_t recursionLevel);
+
/**
* xmlStringGetNodeList:
* @doc: the document
@@ -1475,6 +1477,11 @@ out:
*/
xmlNodePtr
xmlStringGetNodeList(const xmlDoc *doc, const xmlChar *value) {
+ return xmlStringGetNodeListInternal(doc, value, 0);
+}
+
+static xmlNodePtr
+xmlStringGetNodeListInternal(const xmlDoc *doc, const xmlChar *value,
size_t recursionLevel) {
xmlNodePtr ret = NULL, last = NULL;
xmlNodePtr node;
xmlChar *val;
@@ -1483,6 +1490,8 @@ xmlStringGetNodeList(const xmlDoc *doc, const
xmlChar *value) {
xmlEntityPtr ent;
xmlBufPtr buf;

+ if (recursionLevel > 1024) return(NULL);
+
if (value == NULL) return(NULL);

buf = xmlBufCreateSize(0);
@@ -1593,8 +1602,9 @@ xmlStringGetNodeList(const xmlDoc *doc, const
xmlChar *value) {
else if ((ent != NULL) && (ent->children == NULL)) {
xmlNodePtr temp;

- ent->children = xmlStringGetNodeList(doc,
- (const xmlChar*)node->content);
+ ent->children = xmlStringGetNodeListInternal(doc,
+ (const xmlChar*)node->content,
+ recursionLevel+1);
ent->owner = 1;
temp = ent->children;
while (temp) {
--
2.7.4

-----------------------------------------------------------------------

Cheers

--

Simon Lees (Simotek) http://simotek.net

Emergency Update Team keybase.io/simotek
SUSE Linux Adeliade Australia, UTC+9:30
GPG Fingerprint: 5B87 DB9D 88DC F606 E489 CEC5 0922 C246 02F0 014B


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