Anchor CMS PHP Object Injection Vulnerability

ID 1337DAY-ID-24160
Type zdt
Reporter Scott Arciszewski
Modified 2015-08-28T00:00:00


Exploit for php platform in category web applications

                                            CVE-2015-5687 (PHP Object Injection in AnchorCMS)

Out of the box, AnchorCMS defaults to store all session state in a
cookie (contrast this with only storing a unique identifier in a
cookie which references a server-side storage mechanism, such as a
temporary file or a database row).

Aside: If you have paid attention to my past work with Laravel,
CodeIgniter, and Kohana, you probably already know how this story
ends. This finding is so much worse than anything I've found in any of
those three.

Generally, when a framework or CMS decides to go down the "cookie
session driver" route, they do so knowing that end users have complete
control over the contents of the cookie. Fortunately, there's a
cryptographic tool that allows them to do this almost safely: a
Message Authentication Code.

The first mistake the AnchorCMS cookie session driver made was,
instead of using a MAC (e.g. hash_hmac('sha256', $cookie_data,
$some_key)), they just used an MD5 hash.

The second mistake the AnchorCMS cookie session driver made was the
same one almost every other cookie session driver ever written in PHP
makes: It uses unserialize() on user-provided data. Normally this
isn't exploitable, because a MAC cannot be forged without the
server-side authentication key or a timing side-channel on the MAC
verification. But as we just said, they just prepend an MD5 hash and
use that as an authentication mechanism.

Both mistakes, combined, make this a remotely exploitable PHP object
injection vulnerability. Combine that with some of the recent CVEs for
memory corruption in unserialize() and the fact that AnchorCMS
officially supports EOL'd versions of PHP which do not have these
memory corruptions fixed, we potentially have a wormable remote
exploit on any server running AnchorCMS on the Internet. Wonderful.

Weak PRNG used in security contexts

During the course of writing a patch to migrate the cookie session
driver to HMAC-SHA-256 with constant-time MAC comparison, we
discovered that they did not at any point expose or use a CSPRNG (not
even in the salt generation of their bcrypt implementation).

Instead, they relied on str_shuffle().

We have not explored the full potential of the exploitability of this
issue (why brute force a 32-bit keyspace when you already have the
potential to get a remote shell?), but needless to say we did not use
this PRNG for generating a session authentication key in our patch.


Both of the issues reported above would be patched if this pull
request was reviewed by their team and merged.

It has been 30 days since I wrote the patch and sent the above pull
request. The timeline (sans the Github notifications that the
AnchorCMS team would have received every time I or someone else
commented on this pull request) looks like this.

* 2015-07-28
Initial discovery, contacted team on Gitter, instructed to send a PR,
and we did so.

* 2015-08-11
After radio silence from the AnchorCMS devs, I reached out to Craig
Childs who responded the same day. "Thanks for pointing this out to
me, I understand it’s serious and I’ll look into it as soon as I can."

* 2015-08-27
We've reached the 30 day mark without the security issues being resolved.
Furthermore, Paragon Initiative Enterprises has not received any request
for more time for their team to review the patch. (Note: For most issues,
we won't delay past 30 days without remediation.)

Looking at the above issues, it may surprise you to hear that
AnchorCMS is not an obscure relic of a forgotten age; it has almost
2500 stars and over 400 forks on Github.

The best advice I can offer anyone is to uninstall AnchorCMS from
their servers. These are just the issues that I could find; there is
undoubtedly many more that are just as bad.

In publishing this, we hope that the AnchorCMS development team is
able to wake up and reconnect with the community, and more importantly
that the community is willing to help them fix the myriad of security
vulnerabilities that probably lurk beneath the surface.

Scott Arciszewski

# [2018-01-09]  #