Hi,
I am working on a project in which I use the afl fuzzer to fuzz
different open-source software. In doing so, I discovered a
heap buffer overflow in sed/execute.c, line 992. Following is a
detailed backtrace:

=================================================================
==18674==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000000fb8 at pc 0x55d4f61c3909 bp 0x7ffc4fa90580 sp 0x7ffc4fa90578
READ of size 8 at 0x602000000fb8 thread T0
    #0 0x55d4f61c3908 in append_replacement sed/execute.c:992
    #1 0x55d4f61c3908 in do_subst sed/execute.c:1071
    #2 0x55d4f61c3908 in execute_program sed/execute.c:1507
    #3 0x55d4f61c516a in process_files sed/execute.c:1677
    #4 0x55d4f619c649 in main sed/sed.c:377
    #5 0x7f15904d2a86 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21a86)
    #6 0x55d4f619d119 in _start (/home/jefeus/sed/sed/sed+0xc119)

Address 0x602000000fb8 is a wild pointer.
SUMMARY: AddressSanitizer: heap-buffer-overflow sed/execute.c:992 in append_replacement
Shadow bytes around the buggy address:
  0x0c047fff81a0: fa fa 00 00 fa fa 00 00 fa fa 01 fa fa fa fd fd
  0x0c047fff81b0: fa fa 00 00 fa fa 00 00 fa fa 01 fa fa fa 00 00
  0x0c047fff81c0: fa fa 00 00 fa fa 00 00 fa fa 00 00 fa fa 00 00
  0x0c047fff81d0: fa fa 00 00 fa fa 00 00 fa fa fd fd fa fa fd fa
  0x0c047fff81e0: fa fa 00 00 fa fa 00 00 fa fa 00 fa fa fa 00 fa
=>0x0c047fff81f0: fa fa fa fa fa fa fa[fa]fa fa fa fa fa fa fa fa
  0x0c047fff8200: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8210: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8220: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8230: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8240: 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
==18674==ABORTING

To reproduce this bug run sed with:
sed -f <script-file> <input-file>
Where <script-file> and <input-file> are the two files attached.
These files demonstrate how this bug can be used to print undefined
memory. By replacing the first line in <input-file> with a long string
of "A"s, a segmentation fault can be produced.

I was able to reproduce this bug with the current git version, the
debian version and on arch linux.

cheers,
project-repo