Chw00t: Breaking unices’ chroot solutions
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The document discusses various techniques for breaking out of chroot jail environments on Unix-like operating systems. It begins with background information on the speaker and a brief history of chroot. It then explains what chroot is and common uses before detailing requirements for a reasonably secure chroot. The bulk of the document summarizes different techniques for escaping chroot restrictions like using classic chroot flaws, file descriptor passing, Unix domain sockets, mount, /proc, and moving directories out of the chrooted environment. It provides examples and demonstrates some techniques. It concludes by discussing future work hardening containers and operating systems against these issues.
Chw00t: Breaking unices’ chroot solutions
- 1. Chw00t: Breaking unices’ chroot solutions Balázs Bucsay - Бaлaж Бучaи OSCE, OSCP, GIAC GPEN, OSWP http://rycon.hu/ @xoreipeip
- 2. Bio / Balazs Bucsay • Hungarian Hacker • Strictly technical certificates: OSCE, OSCP, OSWP and GIAC GPEN • Currently working for a large telecommunication company • Started with ring0 debuggers and disassemblers in 2000 (13 years old) • Major project in 2009: GI John a distributed password cracker • Webpage: http://rycon.hu • Twitter: @xoreipeip • Linkedin: http://www.linkedin.com/in/bucsayb
- 3. Chroot’s brief history • Introduced in Version 7 Unix - 1979 • Implemented in BSD - 1982 • Hardened version was implemented in FreeBSD - 2000 • Virtuozzo (OpenVZ) containers - 2000 • Chroot on Steroids: Solaris container - 2005 • LXC: Linux Containers - 2008
- 4. What is Chroot? • A privileged system call on Unix systems • Changes the dedicated root vnode of a process (all children inherit this) • Some OS stores chroots in linked lists • Prevents access to outside of the new root • Requires root: prevents crafted chroots for privilege escalation
- 5. What’s this used for? • Testing environments • Dependency control • Compatibility • Recovery • Privilege separation??
- 8. Requirements for reasonable chroot • All directories must be root:root owned • Superuser process cannot be run in chroot • Distinct and unique user (uid, gid) has to be used • No sensitive files (or files at all) can be modified or created
- 9. Requirements for reasonable chroot • Close all file descriptors before chrooting • chdir before chroot • /proc should not be mounted • + Use /var/empty for empty environment
- 10. Chroot scenarios Shell access: • SSH access to a chrooted environment • Chrooted Apache running with mod_cgi/mod_php/… • Exploiting a vulnerable chrooted app Only filesystem access: • Chrooted SCP/FTP access
- 11. Breakage techniques mostly summarised • Get root (not all techniques need it) • Get access to a directory’s file descriptor outside of the chroot • Find original root • Chroot into that • Escaped • Only a few OS stores chroots in linked lists, if you can break out of one, you broke out all of them
- 13. Breakage techniques: kernel exploit/module Not going to talk about this #root: MIGHT needed
- 14. Breakage techniques: misconfigurations • Hard to recognise and exploit • Wrong permissions on files or directories • Dynamic loading of shared libraries • Hardlinked suid/sgid binaries using chrooted shared libraries • For example: • /etc/passwd ; /etc/shadow • /lib/libpam.so.0 - used by /bin/su • These can be used to run code as root #root: NOT needed
- 15. Breakage techniques: classic • Oldest and most trivial • mkdir(d); chroot(d); cd ../../../; chroot(.) • chroot syscall does not chdir into the directory, stays outside #root: needed
- 17. Breakage techniques: classic+fd saving • Based on the classic • Saving the file descriptor of CWD before chroot • mkdir(d); n=open(.); chroot(d); fchdir(n); cd ../../../../; chroot(.) • Some OS might changing the CWD to the chrooted one #root: needed
- 19. Breakage techniques: Unix Domain Sockets • UDS are similar to Internet sockets • File descriptors can be passed thru • Creating secondary chroot and passing outside fd thru • Or using outside help (not really realistic) • Abstract UDS does not require filesystem access #root: needed
- 21. Breakage techniques: mount() • Mounting root device into a directory • Chrooting into that directory • Linux is not restrictive on mounting #root: needed
- 22. Breakage techniques: /proc • Mounting procfs into a directory • Looking for a pid that has a different root/cwd entry • for example: /proc/1/root • chroot into that entry #root: needed
- 23. Breakage techniques: move-out-of-chroot • The reason why I started to work on this • Creating chroot and a directory in it • Use the directory for CWD • Move the directory out of the chroot #root: MIGHT needed
- 25. Breakage techniques: ptrace() • System call to observe other processes • Root can attach to any processes • User can attach to same uid processes (when euid=uid) • Change original code and run shellcode #root: NOT needed
- 26. DEMO
- 27. Results Debian 7.8;2.6.32/ Kali 3.12 Ubuntu 14.04.1;3.13.0 -32-generic DragonFlyBS D 4.0.5 x86_64 FreeBSD 10.- RELEASE amd64 NetBSD 6.1.4 amd64 OpenBSD 5.5 amd64 Solaris 5.11 11.1 i386 Mac OS X Classic YES YES DoS NO NO NO YES YES Classic FD YES YES NO NO NO NO YES YES Unix Domain Sockets YES YES DoS PARTIALLY NO PARTIALLY YES YES Mount YES YES NO NO NO NO NO NO /proc YES YES NO NO NO NO YES NO move-out-of-chroot YES YES DoS PARTIALLY NO YES YES YES Ptrace YES PARTIALLY NO? YES NO YES N/A N/A
- 28. Results (FreeBSD jail) FreeBSD 10.- RELEASE amd64 FreeBSD Jail 10.- RELEASE amd64 Classic NO NO Classic FD NO NO Unix Domain Sockets PARTIALLY PARTIALLY Mount NO NO /proc NO NO move-out-of-chroot PARTIALLY PARTIALLY Ptrace YES NO
- 29. If you want me to release this tool
- 30. Filesystem access only • Move-out-of-chroot still works on FTP/SCP • Privilege escalation is possible on misconfigured environment • Shell can be popped by replacing or placing shared libraries/malicious files in chroot
- 31. Linux Containers • Privileged container (no user namespaces) can create nested containers • Host container has access to guest container’s filesystem • Based on the move-out-of-chroot technique, real host’s file system is accessible
- 32. DEMO 2
- 33. Future work • Testing new UNIX operating systems (eg. AIX, HP-UX) • Looking for other techniques • Improving code and Ptrace technique
- 34. Future work
- 35. Greetz to: • My girlfriend and family • Wolphie and Solar Designer for mentoring • Spender and Kristof Feiszt for reviewing
- 36. References • http://www.bpfh.net/simes/computing/chroot-break.html • http://www.unixwiz.net/techtips/chroot-practices.html • http://linux-vserver.org/Secure_chroot_Barrier • http://phrack.org/issues/59/12.html • http://lwn.net/Articles/421933/ • https://securityblog.redhat.com/2013/03/27/is-chroot-a-security- feature/ • http://www.lorien.ch/images/chroot.gif