Date: Thu, 10 Apr 2014 12:03:45 -0500 From: David Noel <david.i.noel@gmail.com> To: freebsd-bugs@freebsd.org, freebsd-hackers <freebsd-hackers@freebsd.org>, freebsd-security@freebsd.org, FreeBSD Questions Mailing List <freebsd-questions@freebsd.org>, secteam <secteam@freebsd.org>, Colin Percival <cperciva@freebsd.org> Subject: MITM attacks against portsnap and freebsd-update Message-ID: <CAHAXwYCGkP-o0VvMXj5S8-KNA45aTvy%2BsrjDL_=8-x9Dza5z5Q@mail.gmail.com>
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I found a few bugs in portsnap and freebsd-update that I'd like to bring to the community's attention and hopefully recruit people to help fix. I mentioned them to Colin (their author) a few years ago and he agreed that they're issues that need to be addressed, but in the time since neither he nor I have been able to get around to fixing them. I'm hoping that someone reading this is able and willing to pitch in. I've also taken this to secteam@, but no one there's made any progress on them in the past 6 months so I figured it was time to approach the broader community. Surely there's a benevolent sh-savvy hacker out there who has time to take these on. They're pretty simple fixes -- the functionality that's needed exists in the scripts already, it just needs to be reused in a few key places. I also think it would be an appropriate time to discuss retiring portsnap. Problem Summary 1. Both portsnap and freebsd-update extract fetched data prior to its SHA256 verification. The extraction libraries used have a long history of bugs so it's reasonable to assume there might be more. Both freebsd-update and portsnap are run as root. Using a vulnerability in the decompression libraries an attacker who was MITM-capable could compromise any FreeBSD system running portsnap or freebsd-update. 2. The portsnap mirroring script (pmirror.sh) lacks of any sort of mechanism to verify the data prior to processing and mirroring it. Without this, mirrors are open to compromise via methods similar to those found in the client-side scripts (decompression library exploitation). It also means an attacker could feed a mirror a corrupt archive, opening users of that mirror to compromise. 3. Both portsnap and freebsd-update are vulnerable to freeze attacks. 4. The addition of subversion in base makes portsnap redundant. Solution Summary 1. A re-working of the snapshot hashing and hash verification process. 2. The addition of hashing and hash verification code to pmirror.sh. 3. The server-side inclusion of date-stamps, and strict client-side enforcement of expiration policies. 4. Retire portsnap. Details The functions of concern in portsnap.sh are fetch_snapshot(), fetch_update(), and fetch_snapshot_verify(). The lines of concern in pmirror.sh are 99-103, 121-125, 138-149, and 153-157 (using revision 257073). The functions of concern in freebsd-update.sh are fetch_metadata(), fetch_files_premerge(), and fetch_files(). Retiring Portsnap With the inclusion of svnlite in 10 I think the valid question comes up as to whether we really need the portsnap system or whether it could be safely retired. Obviously if the conclusion of that discussion is that we don't need it then these bug fixes would be unnecessary. The reason I see for it to be retired is that subversion allows us to easily and securely check out the ports tree. It's a one-line command: `svn co https://...`. Keeping it up-to-date it is another one-liner: `cd /usr/ports; svn update`. With the inclusion of svnlite in base, the portsnap code and servers acting as mirrors become redundant and seem like a waste of resources. PR's I've avoided filing PR's to give myself, Colin, or secteam@ the chance to fix these bugs first. Since none of us have had the time free to do so and because I'm now sharing these bugs publicly with the list I figure it would be an appropriate time to file PR's for them. MITM attacks against freebsd-update: http://www.freebsd.org/cgi/query-pr.cgi?pr=188429 MITM attacks against portsnap: http://www.freebsd.org/cgi/query-pr.cgi?pr=188428 Freeze attacks against freebsd-update: http://www.freebsd.org/cgi/query-pr.cgi?pr=188434 Freeze attacks against portsnap: http://www.freebsd.org/cgi/query-pr.cgi?pr=188430 MITM attacks against portsnap mirrors (pmirror.sh): http://www.freebsd.org/cgi/query-pr.cgi?pr=188432 Retiring portsnap: http://www.freebsd.org/cgi/query-pr.cgi?pr=188433 David Noel
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