wdub1 is static compiled ARMv7 binary. For some reason, IDA won't disassemble the binary correctly. We have to manually override the CPU architecture setting to ARMv7-M, then again, manually disassemble the instruction in Thumb mode with IDA's "MakeCode" functionality.





the real main routine is at 0x9210



strace tells us that wdub1 sets ALARM timer, also it can't be run from terminal (Socket operation on non-socket).


so what we need to do in order to debug this binary is 1. disable alarm, 2. redirect I/O to socket.

but the binary is static compiled and stripped. so it is difficult to disable alarm by patching. so we use signal masking to disable SIGALRM.


#include <stdio.h>

#include <signal.h>


int main(){


sigset_t signal_set;

sigaddset(&signal_set, SIGALRM);

sigprocmask(SIG_BLOCK, &signal_set, NULL);

char* args[] = {"./wdub", 0};

execve(args[0], args, 0);

return 0;


}



After disable the alarm and redirect the I/O with xinet.d, we can run the binary and debug it.














at 0xc3b2 ~ 0xc3c2 the HTTP request object (12byte) is created.



HTTP request object is consisted with [size], [????], [pointer to request string]



the request string is parsed and processed according to GET, POST, OPTION, TRACE, PATCH...





While parsing PATCH request, it calculates the size of Content-Length which is 'unsigned int' and checks if its smaller than 2000 bytes., however, while processing X-Offset Header, the value is treated as 'signed int', so we can bypass size limit by passing X-Offset value negative.

this creates integer overflow bug, and lets us overflow the stack buffer.





The PATCH request loads the existing file to stack buffer size of 'Content-Length + X-Offset'.

So, first, we PATCH the index.html into size of N, then we PATCH again with negative X-Offset value in order to read index.html into small size of stack buffer.




With such two PATCH request, we can overflow alloca stack buffer. and we can get PC control



After, this step, we can ROP to execute execve("/bin/sh").

Since, there is no ASLR, and the binary uses deterministic heap memory allocator, we can put any data at known memory address. so we put "/bin/sh" and the address of "/bin/sh" and pass them to R0, R1. and we set R2(environ) to 0 then we jump to SVC 0 gadget with R7=11 (sys_execve).


We could find such gadgets, from code section.


'''

.text:0000D95E 28 46                                   MOV     R0, R5

.text:0000D960 31 46                                   MOV     R1, R6

.text:0000D962 3A 46                                   MOV     R2, R7

.text:0000D964 98 47                                   BLX     R3

'''


GADGET = 0xd95e+1

SVC = 0xd9b4+1          # .text:0000D9B4       SVC     0

R3PC = 0x1b476+1        # .text:0001B476       POP     {R3,PC}

R7PC = 0x8a50+1         # .text:00008A50       POP     {R7,PC}



After ROP, we get shell.



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