CVE-2021-4045 is a Command Injection vulnerability that allows Remote Code Execution in the TP-Link Tapo c200 IP camera. It affects all firmware versions prior to 1.1.16 Build 211209 Rel. 37726N due to insufficient checks on user input in uhttpd, which is one of the main binaries of the device. Full write up can be found here.
Proof of Concept (pwnTapo.py)
This is an extended version of the PoC made to show the vulnerability at the CryptoParty Madrid 2022. It has 2 modes:
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shell: Spawns a root shell on the target
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rtsp: Gives access to the live video stream over RTSP
Function exec_and_read_json uses popen to execute commands:
exec_and_read_json is used by 2 unnamed functions, which I named set_language and wifi_connect. They respectively deal with language and wifi configuration (obviously). wifi_connect seems to parse single quotes ('), however, set_language doesn't. This means that if we can control the input for set_language function, we can successfully inject our own commands.
Function set_language is used by uh_slp_proto_request, the function I mentioned before, which passes as input some parsed data received from the user.
To parse the user data, uh_slp_proto_request checks if it is a valid JSON object. Then, it gets a string value identified by key "method" and a dictionary value identified by "params" (at least that is what I think, since function call could not be resolved by Ghidra but seemed to work this way). Depending on the selected method, uh_slp_proto_request selects the function which will be called.
So, by sending {"method": "setLanguage", "params":{}} we successfully call set_language function and pass {} as language_json parameter. Then, inside set_language, language_json object is converted to string and inserted directly into "ubus call system_state_audio set_language \'%s\'" to be executed.
By submitting {"method": "setLanguage", "params": {"payload": "'; touch poc;'"}}, ubus call system_state_audio set_language '{"payload": "'; touch poc;'"}' will be executed, which actually contains 3 commands: ubus call system_state_audio set_language '{"payload": "', touch poc, and '"}'. The second one gives us full code execution.
Now, uh_slp_proto_request is used by another unnamed function managing all requests, which I named main_server_function. If a request is valid (does not exceed maximum length, uses http or https depending on the server config, etc.), main_server_function checks if URL contains /cgi-bin/luci or /web-static. If it doesn't, uh_slp_proto_request is called.
By guessing and sending a couple of requests to the camera, we can check that data used by uh_slp_proto_request is regular POST data. So, if we send a POST request to / with the previous payload, uh_slp_proto_request will process this data, call set_language and our payload will be injected in the command executed by exec_and_get_result.
As you can see, I didn't mention anything about authentication, since setLanguage method can be called without login. This allows any user to take full control of the camera with just one unanthenticated request.