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I read this excellent article about vulnerability research into a DLINK wireless router. The researcher explains clearly the coding mistake that leads to the vulnerability. He obviously reverse-engineered a binary running on the device but skips a step in how he acquired the code:

The login action is performed via the URI /apply_sec.cgi. A quick search reveals that the apply_sec.cgi code is located at function do_ssc (0x40a210) in the /www/cgi/ssi binary.

What are some methods for acquiring these binaries (that is, the firmware)? I've been learning how to use Ghidra. Looking at vulnerable binaries from routers would be a good learning exercise.

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    The firmware? Often the manufacturer has the firmware updates available on their website for download. What device specifically are you asking about? – Daisetsu Oct 9 at 21:22
  • Yes, the firmware. I edited the question to make that clearer. The device in question is a ZyXEL FR1000Z – mcgyver5 Oct 9 at 22:25
  • The CenturyLink site has firmware updates for this router but I'm more interested in the old, vulnerable firmware version that exists on the device. – mcgyver5 Oct 9 at 22:31
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Aside from the already-mentioned option of pulling and extracting firmware updates, most routers offer a command-line admin interface, available over SSH or even Telnet. Connect to this interface, and you'll be dropped into what is supposed to be a secure, restricted shell. In practice, the shell will almost certainly have some way to do command injection (for example, on some routers, running any allowed command followed by $(command) or even ; command will work).

Once you've got command injection, verify that you're root (quite likely). At that point, you can use common Linux commands (the device will have a limited command selection but you should be able to find what you need) to get a way to copy binary files. Extract individual binaries and/or entire volume images to your PC through that, and analyze them to your heart's content.

If you aren't root, you'll need to find a local EoP vulnerability on the router; these are common enough in Linux that old firmware will almost certainly have some, though you might need to compile the exploit for your specific platform. I've yet to see a SOHO router where I needed to do this; though.

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