[personal profile] mjg59
Reverse engineering protocols is a great deal easier when they're not encrypted. Thankfully most apps I've dealt with have been doing something convenient like using AES with a key embedded in the app, but others use remote protocols over HTTPS and that makes things much less straightforward. MITMProxy will solve this, as long as you're able to get the app to trust its certificate, but if there's a built-in pinned certificate that's going to be a pain. So, given an app written in C running on an embedded device, and without an easy way to inject new certificates into that device, what do you do?

First: The app is probably using libcurl, because it's free, works and is under a license that allows you to link it into proprietary apps. This is also bad news, because libcurl defaults to having sensible security settings. In the worst case we've got a statically linked binary with all the symbols stripped out, so we're left with the problem of (a) finding the relevant code and (b) replacing it with modified code. Fortuntely, this is much less difficult than you might imagine.

First, let's find where curl sets up its defaults. Curl_init_userdefined() in curl/lib/url.c has the following code:
set->ssl.primary.verifypeer = TRUE;
set->ssl.primary.verifyhost = TRUE;
#ifdef USE_TLS_SRP
set->ssl.authtype = CURL_TLSAUTH_NONE;
#endif
set->ssh_auth_types = CURLSSH_AUTH_DEFAULT; /* defaults to any auth
type */
set->general_ssl.sessionid = TRUE; /* session ID caching enabled by
default */
set->proxy_ssl = set->ssl;

set->new_file_perms = 0644; /* Default permissions */
set->new_directory_perms = 0755; /* Default permissions */

TRUE is defined as 1, so we want to change the code that currently sets verifypeer and verifyhost to 1 to instead set them to 0. How to find it? Look further down - new_file_perms is set to 0644 and new_directory_perms is set to 0755. The leading 0 indicates octal, so these correspond to decimal 420 and 493. Passing the file to objdump -d (assuming a build of objdump that supports this architecture) will give us a disassembled version of the code, so time to fix our problems with grep:
objdump -d target | grep --after=20 ,420 | grep ,493

This gives us the disassembly of target, searches for any occurrence of ",420" (indicating that 420 is being used as an argument in an instruction), prints the following 20 lines and then searches for a reference to 493. It spits out a single hit:
43e864: 240301ed li v1,493
Which is promising. Looking at the surrounding code gives:
43e820: 24030001 li v1,1
43e824: a0430138 sb v1,312(v0)
43e828: 8fc20018 lw v0,24(s8)
43e82c: 24030001 li v1,1
43e830: a0430139 sb v1,313(v0)
43e834: 8fc20018 lw v0,24(s8)
43e838: ac400170 sw zero,368(v0)
43e83c: 8fc20018 lw v0,24(s8)
43e840: 2403ffff li v1,-1
43e844: ac4301dc sw v1,476(v0)
43e848: 8fc20018 lw v0,24(s8)
43e84c: 24030001 li v1,1
43e850: a0430164 sb v1,356(v0)
43e854: 8fc20018 lw v0,24(s8)
43e858: 240301a4 li v1,420
43e85c: ac4301e4 sw v1,484(v0)
43e860: 8fc20018 lw v0,24(s8)
43e864: 240301ed li v1,493
43e868: ac4301e8 sw v1,488(v0)

Towards the end we can see 493 being loaded into v1, and v1 then being copied into an offset from v0. This looks like a structure member being set to 493, which is what we expected. Above that we see the same thing being done to 420. Further up we have some more stuff being set, including a -1 - that corresponds to CURLSSH_AUTH_DEFAULT, so we seem to be in the right place. There's a zero above that, which corresponds to CURL_TLSAUTH_NONE. That means that the two 1 operations above the -1 are the code we want, and simply changing 43e820 and 43e82c to 24030000 instead of 24030001 means that our targets will be set to 0 (ie, FALSE) rather than 1 (ie, TRUE). Copy the modified binary back to the device, run it and now it happily talks to MITMProxy. Huge success.

(If the app calls Curl_setopt() to reconfigure the state of these values, you'll need to stub those out as well - thankfully, recent versions of curl include a convenient string "CURLOPT_SSL_VERIFYHOST no longer supports 1 as value!" in this function, so if the code in question is using semi-recent curl it's easy to find. Then it's just a matter of looking for the constants that CURLOPT_SSL_VERIFYHOST and CURLOPT_SSL_VERIFYPEER are set to, following the jumps and hacking the code to always set them to 0 regardless of the argument)

Unoptimised code helps

Date: 2017-04-11 11:25 pm (UTC)
bwh: (Default)
From: [personal profile] bwh
I notice that the disassembled code keeps reloading v0, so apparently it's unoptimised. Patching the code might have been harder with optimisations enabled.

Thanks a bunch

Date: 2017-04-12 07:07 am (UTC)
From: (Anonymous)
Thanks a bunch for this, which is extremely instructive in many respects. Technically, of course, but also on why GPL (or why not, depending on who you are).

Anyway, and as BWH said, thanks $DEITY the compiler doesn't optimize more aggressively. Just imagine it discovering that this whole struct is constant anyway and inlining (and constant-propagating) the whole thing :-(

Date: 2017-04-13 09:10 am (UTC)
From: (Anonymous)
Another way to locate the code is to search the binary for 0xc0dedbad (CURLEASY_MAGIC_NUMBER in curl/urldata.h) The constant is only used in Curl_open (curl/url.c) and that function also contains a call to Curl_init_userdefined. It's also a nice libcurl fingerprint in general.

-gerd

Profile

Matthew Garrett

About Matthew

Power management, mobile and firmware developer on Linux. Security developer at Google. Ex-biologist. @mjg59 on Twitter. Content here should not be interpreted as the opinion of my employer.

Page Summary

Expand Cut Tags

No cut tags