stack-buffer-overflow in static int doctype_scan(const xchar *data)

[Uconquer]

Hi there!

I fuzzed the tool and found some problems

Crash Out Phase

Run:

CFLAGS="-g" LDFLAGS="-g" CC=afl-clang-fast ./configure --disable-shared
AFL_USE_ASAN=1 make -j12
make install
./html2xhtml poc.html

Receive the output:

=================================================================
==209704==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7fd4c5900760 at pc 0x55e5ebaa1d80 bp 0x7ffee77f5d00 sp 0x7ffee77f54c0
READ of size 513 at 0x7fd4c5900760 thread T0
    #0 0x55e5ebaa1d7f in StrstrCheck(void*, char*, char const*, char const*) asan_interceptors.cpp.o
    #1 0x55e5ebaa1a1c in strstr (/root/Desktop/workdir/html2xhtml-1.3/fuzz/bin/html2xhtml+0x55a1c) (BuildId: aae9894f09b69ea8bef28f68d73e1991760dd031)
    #2 0x55e5ebb89242 in doctype_scan /root/Desktop/workdir/html2xhtml-1.3/src/procesador.c:718:9
    #3 0x55e5ebb89242 in saxDoctype /root/Desktop/workdir/html2xhtml-1.3/src/procesador.c:564:14
    #4 0x55e5ebb67a40 in yyparse /root/Desktop/workdir/html2xhtml-1.3/src/htmlgr.y:72:3
    #5 0x55e5ebb7f915 in main /root/Desktop/workdir/html2xhtml-1.3/src/html2xhtml.c:110:20
    #6 0x7fd4c7d00082 in __libc_start_main /build/glibc-LcI20x/glibc-2.31/csu/../csu/libc-start.c:308:16
    #7 0x55e5eba8848d in _start (/root/Desktop/workdir/html2xhtml-1.3/fuzz/bin/html2xhtml+0x3c48d) (BuildId: aae9894f09b69ea8bef28f68d73e1991760dd031)

Address 0x7fd4c5900760 is located in stack of thread T0 at offset 864 in frame
    #0 0x55e5ebb8908f in saxDoctype /root/Desktop/workdir/html2xhtml-1.3/src/procesador.c:550

  This frame has 2 object(s):
    [32, 288) 'msg.i' (line 745)
    [352, 864) 'buffer.i' (line 710) <== Memory access at offset 864 overflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow asan_interceptors.cpp.o in StrstrCheck(void*, char*, char const*, char const*)
Shadow bytes around the buggy address:
  0x7fd4c5900480: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
  0x7fd4c5900500: f8 f8 f8 f8 f2 f2 f2 f2 f2 f2 f2 f2 00 00 00 00
  0x7fd4c5900580: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7fd4c5900600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7fd4c5900680: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x7fd4c5900700: 00 00 00 00 00 00 00 00 00 00 00 00[f3]f3 f3 f3
  0x7fd4c5900780: f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
  0x7fd4c5900800: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7fd4c5900880: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7fd4c5900900: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7fd4c5900980: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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
==209704==ABORTING

Root Cause Analysis

When the strstr function is used, the argument string does not end with a null byte, resulting in a buffer overflow. The buffer variable should be checked for null bytes.

I have attached crash files to help reproduce the issue.
poc.zip