heap-buffer-overflow (/lib/x86_64-linux-gnu/libasan.so.5+0xc717c) in strncat

[p870613]

hi,
I find bug, heap-buffer-overflow in 499ed89
gcc --version:

➜  1 git:(master) ✗ gcc --version
gcc (Ubuntu 9.3.0-10ubuntu2) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

os & kernel :

➜  1 git:(master) ✗ uname -r
5.4.0-37-generic
➜  1 git:(master) ✗ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 20.04 LTS
Release:	20.04
Codename:	focal

command line: ./pdfalto -f 1 ~/Desktop/pdfalto/small.pdf
poc:
poc.zip

log:

➜  pdfalto git:(master) ✗ ./pdfalto -f 1 ~/Desktop/pdfalto/small.pdf
/home/lin/pdfalto/pdfalto, 18
=================================================================
==2515089==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x603000000052 at pc 0x7ffa86c6917d bp 0x7ffdb915e560 sp 0x7ffdb915dd08
WRITE of size 18 at 0x603000000052 thread T0
    #0 0x7ffa86c6917c in strncat (/lib/x86_64-linux-gnu/libasan.so.5+0xc717c)
    #1 0x40e04e in main /home/lin/pdfalto/src/pdfalto.cc:191
    #2 0x7ffa868820b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #3 0x408bfd in _start (/home/lin/pdfalto/pdfalto+0x408bfd)

0x603000000052 is located 0 bytes to the right of 18-byte region [0x603000000040,0x603000000052)
allocated by thread T0 here:
    #0 0x7ffa86cafbc8 in malloc (/lib/x86_64-linux-gnu/libasan.so.5+0x10dbc8)
    #1 0x40dfd8 in main /home/lin/pdfalto/src/pdfalto.cc:189
    #2 0x7ffa868820b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)

SUMMARY: AddressSanitizer: heap-buffer-overflow (/lib/x86_64-linux-gnu/libasan.so.5+0xc717c) in strncat
Shadow bytes around the buggy address:
  0x0c067fff7fb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c067fff8000: fa fa 00 00 00 02 fa fa 00 00[02]fa fa fa fa fa
  0x0c067fff8010: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8020: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==2515089==ABORTING

Thanks !