SektionEins GmbH
www.sektioneins.de
-= Security Advisory =-
Advisory: PHP openssl_x509_parse() Memory Corruption Vulnerability
Release Date: 2013/12/13
Last Modified: 2013/12/13
Author: Stefan Esser [stefan.esser[at]sektioneins.de]
Application: PHP 4.0.6 - PHP 4.4.9
PHP 5.0.x
PHP 5.1.x
PHP 5.2.x
PHP 5.3.0 - PHP 5.3.27
PHP 5.4.0 - PHP 5.4.22
PHP 5.5.0 - PHP 5.5.6
Severity: PHP applications using openssl_x509_parse() to parse a
malicious x509 certificate might trigger a memory
corruption that might result in arbitrary code execution
Risk: Critical
Vendor Status: Vendor has released PHP 5.5.7, PHP 5.4.23 and PHP 5.3.28
that contain a fix for this vulnerability
Reference:
http://www.sektioneins.de/advisories/advisory-012013-php-openssl_x509_parse-memory-corruption-vulnerability.html
Overview:
Quote from http://www.php.net
"PHP is a widely-used general-purpose scripting language that
is especially suited for Web development and can be embedded
into HTML."
The PHP function openssl_x509_parse() uses a helper function
called asn1_time_to_time_t() to convert timestamps from ASN1
string format into integer timestamp values. The parser within
this helper function is not binary safe and can therefore be
tricked to write up to five NUL bytes outside of an allocated
buffer.
This problem can be triggered by x509 certificates that contain
NUL bytes in their notBefore and notAfter timestamp fields and
leads to a memory corruption that might result in arbitrary
code execution.
Depending on how openssl_x509_parse() is used within a PHP
application the attack requires either a malicious cert signed
by a compromised/malicious CA or can be carried out with a
self-signed cert.
Details:
The PHP function openssl_x509_parse() is used by PHP applications
to parse additional information out of x509 certificates, usually
to harden SSL encrypted communication channels against MITM
attacks. In the wild we have seen the following use cases for this
function:
* output certificate debugging information
(e.g. cacert.org/analyse.php)
* webmail application with SMIME support
* client certificate handling
* certificate pinning
* verification of other certificate properties
(e.g. a default Wordpress install if ext/curl is not loaded)
When we backported security fixes for some previous security
vulnerabilities in PHP's openssl to PHP 4.4.9 as part of our
PHP security backport services that we provide to customers,
we performed a quick audit of openssl_x509_parse() and all the
functions it calls, which led to the discovery of a memory
corruption vulnerability.
Within the function openssl_x509_parse() the helper function
asn1_time_to_time_t() is called two times to parse the
notBefore and notAfter ASN1 string timestamps from the cert
into integer time_t values as you can see below:
add_assoc_long(return_value, "validFrom_time_t",
asn1_time_to_time_t(X509_get_notBefore(cert) TSRMLS_CC));
add_assoc_long(return_value, "validTo_time_t",
asn1_time_to_time_t(X509_get_notAfter(cert) TSRMLS_CC));
When you take a look into this helper function you will see
that it only contains a quickly hacked parser that was never
really improved since its introduction in PHP 4.0.6. The author
of this parser was even aware of its hackishness as you can see
from the error message contained in the code:
static time_t asn1_time_to_time_t(ASN1_UTCTIME * timestr TSRMLS_DC) /*
{{{ */
{
/*
This is how the time string is formatted:
snprintf(p, sizeof(p), "%02d%02d%02d%02d%02d%02dZ",ts->tm_year%100,
ts->tm_mon+1,ts->tm_mday,ts->tm_hour,ts->tm_min,ts->tm_sec);
*/
time_t ret;
struct tm thetime;
char * strbuf;
char * thestr;
long gmadjust = 0;
if (timestr->length < 13) {
php_error_docref(NULL TSRMLS_CC, E_WARNING, "extension author
too lazy to parse %s correctly", timestr->data);
return (time_t)-1;
}
However the actual problem of the code should become obvious when
you read the rest of the parsing code that attempts to first
duplicate the timestamp string and then parses the timestamp by
going through the copy in reverse order and writing five NUL bytes
into the duplicated string.
strbuf = estrdup((char *)timestr->data);
memset(&thetime, 0, sizeof(thetime));
/* we work backwards so that we can use atoi more easily */
thestr = strbuf + timestr->length - 3;
thetime.tm_sec = atoi(thestr);
*thestr = '\0';
thestr -= 2;
thetime.tm_min = atoi(thestr);
*thestr = '\0';
thestr -= 2;
thetime.tm_hour = atoi(thestr);
*thestr = '\0';
thestr -= 2;
thetime.tm_mday = atoi(thestr);
*thestr = '\0';
thestr -= 2;
thetime.tm_mon = atoi(thestr)-1;
*thestr = '\0';
thestr -= 2;
thetime.tm_year = atoi(thestr);
The problem with this code is that ASN1 strings can contain NUL
bytes, while the parser is not binary safe. This means if a
timestamp string inside a x509 certificate contains a NUL byte
at e.g. position 13 the estrdup() will only allocate 14 bytes
for a copy of the string, but the parser will attempt to write
five NUL bytes to memory addressed by the ASN1 length of the
string. If the real string length is longer than 16 bytes this
will result in writes of NUL bytes outside of the allocated
buffer.
Because of PHP's deterministic heap memory layout that can be
controlled a lot by sending e.g. POST variables and using
duplicate variable names to poke memory holes this vulnerability
must be considered exploitable. However the actual exploit will
depend a lot on how the PHP application uses openssl_x509_parse()
and a lot of other factors.
Depending on which of the actual use cases the function is used
for by an application, an attacker can trigger the memory
corruption with a self-signed certificate. An example for this
is the public analyse.php x509 cert debugging script provided
by CACert on their webserver.
Other applications like Wordpress use openssl_x509_parse() to
further verify SSL certificates whenever Wordpress connects to
a HTTPS URL (in case ext/curl is not loaded which is the default
for several linux distributions). Because the parsing only
happens after the initial SSL connection is established this
can only be abused by attackers controlling a malicious trusted
cert. However recent disclosures of alleged NSA capabilities,
the French incident and disclosures about fully compromised
trusted CAs in the past years have shown that this capability
might be in the reach of malicious attackers.
Proof of Concept:
The following x509 certificate demonstrates the out of bounds write:
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
Disclosure Timeline:
01. December 2013 - Notified security@php.net
Provided description, POC cert, demo
valgrind output and patch
02. December 2013 - security@php.net acknowledges and
says thank you for report and patch
02. December 2013 - security@php.net announces that planned
release date is 12th December
03. December 2013 - Notification from RedHat Security that
CVE-2013-6420 was assigned to this issue
09. December 2013 - RedHat Security tells php.net that they
should commit the fix silently and add
info about it only after release
They further tell php.net to tell us to
not discuss the vulnerability in public
prior to patches being available
10. December 2013 - security@php.net fixes the vulnerability
openly and does not attempt to hide that
the commit is a security fix as RedHat
Security suggested
11. December 2013 - RedHat Security Announces that they now
consider this vulnerability public and
sends out their own patches with big
announcement one day before php.net is
ready to release their own fixes
12. December 2013 - security@php.net pushes PHP updates to
the PHP 5.3, PHP 5.3 and PHP 5.5 branches
to the mirros as was previously agreed upon
13. December 2013 - New PHP releases are announce on php.net
13. December 2013 - Public Disclosure of this advisory
Recommendation:
It is recommended to upgrade to the latest version of PHP
which also fixes additional non security problems reported
by third parties.
Grab your copy at:
http://www.php.net/get/php-5.5.7.tar.bz2/from/a/mirror
CVE Information:
The Common Vulnerabilities and Exposures project (cve.mitre.org) has
assigned the name CVE-2013-6420 to this vulnerability.
GPG-Key:
pub 4096R/D6A3FE46 2013-11-06 Stefan Esser
Key fingerprint = 0A04 AB88 90D2 E67C 3D3D 86E1 AA39 B97F D6A3 FE46
Copyright 2013 SektionEins GmbH. All rights reserved.
