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By Date By Thread AnchorCMS - PHP Object Injection (CVE-2015-5687) and More From: Scott Arciszewski <scott () paragonie com>

Date: Thu, 27 Aug 2015 09:04:54 -0400

In the near future on an IRC server near you: <user1> Hey, what's the best command line tool for verifying that my copy of AnchorCMS 0.9.x is secure? <user2> rm 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. https://github.com/paragonie/anchor-cms/blob/1207ab1b656bc5687bc2d2d6f57e6c4d0ca7d15f/system/session/drivers/cookie.php#L19-L35 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(). https://github.com/paragonie/anchor-cms/blob/07933dbc7939326bb4973827a0934d1a610851d1/system/helpers.php#L55-L59 https://github.com/paragonie/anchor-cms/blob/66581e5969029e7b6dfddfe3326bb9f15f27b859/anchor/libraries/hash.php#L15 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. https://github.com/anchorcms/anchor-cms/pull/904 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 Chief Development Officer Paragon Initiative Enterprises <https://paragonie.com> _______________________________________________ Sent through the Full Disclosure mailing list https://nmap.org/mailman/listinfo/fulldisclosure Web Archives & RSS: http://seclists.org/fulldisclosure/ By Date By Thread Current thread: AnchorCMS - PHP Object Injection (CVE-2015-5687) and More Scott Arciszewski (Aug 27) Re: AnchorCMS - PHP Object Injection (CVE-2015-5687) and More Scott Arciszewski (Aug 29)

Scott Arciszewski (Aug 27)