Hiding for Persistance - Backdooring Linux Systems
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In this presentation we take a look at a Linux kernel backdoor to see how it works, and discuss reasons and mitigations. Since this presentation revolves around the (non-public) source code of the backdoor, it is mostly intended for people who attended the session.
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Hiding for Persistance - Backdooring Linux Systems
- 1. Hiding for Persistence - Backdooring Linux Systems
- 2. Key messages • They’re out to get you • Kernel backdoors are hard to find • However, most attackers are not so motivated or well equipped • There is a lot you can do
- 3. Hi Christiaan Ottow • Developer, Sysop, Hacker • Security Coach @ Computest / Pine Digital Security • cottow@computest.nl • @cottow
- 6. Reasons you get pwned • Spam • DDoS • Ransomware • To pwn others • To do you damage • Lulz • Espionage
- 7. Your adversary • Crime groups • State-sponsored attackers • People you’ve pissed off • Bored teenagers
- 10. The bad news
- 11. The good news
- 12. How?
- 13. How to do persistence as uid > 0 • “hidden” directories • “.. “ (note the space) • innocuous filenames • libglsconv.so • process renaming • write to argv[0]
- 14. How to do persistence as uid == 0 • hide inside existing executables • patch /bin/su • patch processes in memory • attach to sshd, patch, detach • kernel module • loadable backdoors! • firmware backdoor • hardware has own microcontroller and “OS”
- 15. <prayer to the demo gods>
- 16. #include <stdio.h> int main() { printf(“Hello, world!n”); return 0; }
- 17. int main() { … printf(“hi”); … } Standard C library kernel printf() write()
- 18. main() sys_write() 0x00000000 0xc0000000 0xffffffff user space memory kernel memory printf() call 0x804031d mov eax, 0x4 int 0x80
- 19. libc libc libcuser kernel disk input devices app app interrupt interrupt syscall syscall applibc
- 20. Let’s look at the code
- 23. 0xc00a3400 0xc0990d00 0xc09912a4 0xc0993600 0xc099fe0a 0xc00a3404 0xc00a3408 0xc00a340c 0xc099…. 0xc099…. 0xc00a….. 0xc00a….. push r15 mov r15d,edi push r14 mov r14,rsi push r13 mov r13,rdx push r12 lea r12,[rip+0x207f78] push rbp lea rbp,[rip+0x207f78]
- 24. 0xc00a3400 0xc0990d00 0xc09912a4 0xc0993600 0xcfe89a40 0xc00a3404 0xc00a3408 0xc00a340c 0xc099…. 0xc099…. 0xc00a….. 0xc00a….. push r15 mov r15d,edi push r14 mov r14,rsi push r13 mov r13,rdx push r12 lea r12,[rip+0x207f78] push rbp lea rbp,[rip+0x207f78] call 0xc099fe0a <filter results>
- 25. Let’s look at the code
- 26. Detection • syscall table should be predictable and boring • server’s external behaviour doesn’t lie - the hacker has a business case • cat and mouse game between detection and hiding • volatility framework for memory inspection
- 27. Prevention - kernel level • grsecurity / selinux • disallow anomalous behaviour • limit what root kan do • disable module loading
- 28. Prevention - hardening • remove unnecessary tools like compilers • isolate services (chroot / containers / cgroups / apparmor) • see CIS and Certified Secure guidelines
- 29. Prevention - HIDS • tripwire / OSSEC • trigger on anomalous events
- 30. Conclusions • They’re out to get you • Most of them aren’t that well resourced • A good backdoor is next to impossible to find • There are excellent mitigations to take • Spend your time and money wisely
- 31. Dan is het ook niet leuk www.werkenbijcomputest.nl Als het niet kapot kan..
- 33. Image credits • Why girl: http://www.cellmaxxindo.com • Lulz: Image courtesy of http://knowyourmeme.com • Trump: http://www.northcountrypublicradio.org/ • The good news: http://theverybesttop10.com • The bad news: http://stuffpoint.com • Questions: http://www.slideshare.net/linaroorg/sfo15tr6-server-ecosystem-day- part-6a