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POWriting a return-to-libc attack, but libc is loaded at 0x00 in memory
 

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  1. POWriting a return-to-libc attack, but libc is loaded at 0x00 in memory
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    <p>I'm writing a return to libc attack for my systems security class. First, the vulnerable code:</p> <pre><code>//vuln.c #include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; int loadconfig(void){ char buf[1024]; sprintf(buf, "%s/.config", getenv("HOME")); return 0; } int main(int argc, char **argv){ loadconfig(); return 0; } </code></pre> <p>I want to use a return to libc attack. Compiling and debugging the program:</p> <pre><code>$ gcc -g -fno-stack-protector -o vuln vuln.c $ gdb vuln (gdb) break loadconfig (gdb) run Reached breakpoint blah blah blah. (gdb) p $ebp $1 = (void *) 0xbfffefb0 (gdb) p system $2 = {&lt;text variable, no debug info&gt;} 0x0016db20 &lt;system&gt; (gdb) p exit $3 = {&lt;text variable, no debug info&gt;} 0x001639e0 &lt;exit&gt; (gdb) x/2000s $esp ... 0xbffff5af: "SHELL=/bin/bash" </code></pre> <p>To execute the attack, I want to overflow the buffer into <code>loadconfig</code>'s return address (aka <code>$esp+4</code>), replacing it with the return address for <code>system</code>, then the return address for <code>exit</code> (since <code>system</code> expects a real return address), then the command name (the address of <code>SHELL=/bin/bash</code> plus 6, to trim the <code>SHELL=</code> part). This should be possible by crafting a <code>$HOME</code> environment variable of 1024 characters of crap, then the little-endian address of <code>system</code>, <code>exit</code>, and <code>/bin/bash</code>.</p> <p>However, with every computer I've tried, <code>system</code> gets loaded at an address that starts with 0x00, which will null terminate the string that <code>sprintf</code> is reading and stop the attack dead. Is there some way to force <code>libc</code> to load elsewhere in memory, or am I misinterpreting the attack?</p> <p>For reference, I'm running an Ubuntu Server 11.10 virtual machine in VirtualBox (Windows host), with <code>gcc</code> version 4.6.1 and <code>gdb</code> version 7.3-2011.08. edit: ASLR is disabled, and I compiled with <code>-fno-stack-protector</code> to remove the canary. Since I'm not executing anything from the stack, I don't need to <code>execstack</code> it.</p>
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    1. COAre you allowed to put code on the stack? Because you could put a return value to an address later on in the string containing code that executes your exploit (of course carefully crafted to avoid 0 bytes).
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    2. CORight, that's a standard buffer overflow attack. I specifically want to perform a return to libc attack, rather than writing any shell code.
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    3. COhave you considered the problem may be that you've added debugging info?
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      1. USMax DeLiso
 

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