The HTTPD binary in multiple ZTE routers has a stack-based buffer overflow vulnerability in check_data_integrity function. This function is responsible for validating the checksum of data in post request. The checksum is sent encrypted in the request, the function decrypts it and stores the checksum on the stack without validating it. An unauthenticated attacker can get RCE as root by exploiting this vulnerability.
A stack-based buffer overflow condition is a condition where the buffer being overwritten is allocated on the stack (i.e., is a local variable or, rarely, a parameter to a function).