Technical Advisory – Apple iOS / OSX: Foundation NSXMLParser XXE Vulnerability

 Virtual Security Research, LLC.
                     Security Advisory


Advisory Name: Apple Foundation NSXMLParser XML eXternal Entity (XXE) Flaw
 Release Date: 2014-09-17
  Application: Apple iOS Foundation Framework
	       Apple OS X Foundation Framework
     Versions: iOS 7.0, 7.1, OS X 10.9 - 10.9.4 
     Severity: High
       Author: George D. Gal 
Vendor Status: Fix Available
CVE Candidate: CVE-2014-4374


Product Description
From [1]:
"Xcode includes software development kits (SDKs) that enable you to create
  applications that run on specific versions of iOS or OS X—including 
  versions different from the one you are developing on. This technology 
  lets you build a single binary that takes advantage of new features when 
  running on a system that supports them, and gracefully degrades when 
  running on an older system. Some Apple frameworks automatically modify 
  their behavior based on the SDK an application is built against for 
  improved compatibility."

Vulnerability Overview
In May 2014, VSR identified a vulnerability in versions 7.0 and 7.1 of the iOS
SDK whereby the NSXMLParser class, resolves XML External Entities by default
despite documentation which indicates otherwise.  In addition, settings to
change the behavior of XML External Entity resolution appears to be 

This vulnerability, commonly known as XXE (XML eXternal Entities) attacks
could allow for an attacker's ability to use the XML parser to carry out 
attacks ranging from network port scanning, information disclosure, 
denial of service, and potentially to carry out remote file retrieval.

Further review also revealed that the Foundation Framework used in OS X
10.9.x is also vulnerable.

The severity of this vulnerability varies. For example, in situations where
the application does not reflect user influenced XML, retrieval of files
may be limited, however using external HTTP entities could be used to 
conduct port scans. In other scenarios if core iOS applications transmit XML
over plaintext protocols, these protocols could potentially be intercepted
to leak contents of any file on the mobile device. For App Store applications
files which could be accessed may be limited to those under the individual
chrooted application directories, or in the case of jailbroken devices, any
file on the filesystem.

Vulnerability Details

Apple's NSXMLParser documentation [2] indicates that external entity
resolution is disabled in the parser by default. However, inspection of 
multiple applications running on iOS 7.0 and 7.1 now appear to resolve 
external entities by default, and even when attempting to disable entity
resolution explicitly as shown below:

    [nsXmlParser setShouldResolveExternalEntities:NO];

The following source code demonstrates the flaw:

- (void) doParse:(NSData *)data {
    // create and init NSXMLParser object
    NSXMLParser *nsXmlParser = [[NSXMLParser alloc] initWithData:data];
    // Why does the following not even work!?
    [nsXmlParser setShouldResolveExternalEntities:NO];
    // create and init our delegate
    VSRParser *parser = [[VSRParser alloc] initXMLParser];
    // set delegate
    [nsXmlParser setDelegate:parser];
    // parsing...
    BOOL success = [nsXmlParser parse];
    // test the result
    if (success) {
        NSLog(@"No errors");
        NSMutableArray *stuff = [parser tests];
    } else {
        NSLog(@"Error parsing document!");
    [parser release];
    [nsXmlParser release];

When using a vulnerable input XML file as shown below, the XML parser attempts
to perform network name resolution and access the resource defined by &http;



The following DNS and web server log entries demonstrate attempts to resolve

2014-05-19_13:26:28.31088 ...

XX.XX.XX.XX - - [19/May/2014:09:26:28 -0400] "GET /xxe HTTP/1.0" 404 446 "-" "-"

In more serious exploitation scenarios, plaintext XML communications between
a server and iOS mobile application, or OS X client application could be 
intercepted and modified in transit to reference a file present on the client
device. If the device reflects this value in subsequent communications or 
errors the contents of files stored on the device could be leaked to an 

Versions Affected
VSR's analysis revealed that the IOS 7.0, 7.1 SDKs are vulnerable, while 
earlier versions of IOS and the IOS SDK do not appear to be affected. This
vulnerability affects the Mac OSX Foundation, however VSR has not verified 
the earliest version of the Foundation framework for OSX which is affected.

Vendor Response
The following timeline details Apple's response to the reported issue:

2014-05-19    Apple was provided a draft advisory.
2014-07-10    Apple confirms issues to be fixed in iOS 8 and OSX Yosemite
2014-09-17    Apple releases iOS 8 and OS X 10.9.5 which is presumed to 
	      fix this vulnerability.

This vulnerability will require an updated version of iOS and OS X from 
Apple, as even built-in methods for disabling XXE appears to be 
non-functional. Users should update their devices to iOS 8 and OS X Mavericks
10.9.5 with the Software Update [3] mechanism.

Common Vulnerabilities and Exposures (CVE) Information
The Common Vulnerabilities and Exposures (CVE) project has assigned
the number CVE-2014-4374 to this issue.  This is a candidates for
inclusion in the CVE list (, which standardizes
names for security problems.







This advisory is distributed for educational purposes only with the sincere 
hope that it will help promote public safety.  This advisory comes with 
absolutely NO WARRANTY; not even the implied warranty of merchantability or 
fitness for a particular purpose.  Neither Virtual Security Research, LLC nor
the author accepts any liability for any direct, indirect, or consequential
loss or damage arising from use of, or reliance on, this information.

See the VSR disclosure policy for more information on our responsible 
disclosure practices:

     Copyright 2014 Virtual Security Research, LLC.  All rights reserved.

To view the advisory as a txt. click here.

Editor's note: This work was originally published by VSR on their website at VSR is now a part of NCC Group, so we have migrated this content to The advisory text as above has been copy-pasted to this blog for historical reference.

Call us before you need us.

Our experts will help you.

Get in touch