Heap use after free in the free_element_list function

[danielpyon]

Problem Description

Note: I am aware that this project is unmaintained. However, I am still opening this issue to follow CVE's guidelines for EOL software.

There is a heap use after free in the free_element_list function when the parser handles a specially crafted Markdown file. Here is a minimized proof-of-concept Markdown file that triggers the bug: min_heap_uaf.md. The output is as follows:

=================================================================
==400541==ERROR: AddressSanitizer: heap-use-after-free on address 0x6030000001a8 at pc 0x0000005de092 bp 0x7fffffffe100 sp 0x7fffffffe0f8
READ of size 8 at 0x6030000001a8 thread T0
    #0 0x5de091 in free_element_list /home/sanic/peg-markdown/parsing_functions.c:25:21
    #1 0x5ddfcb in free_element_list /home/sanic/peg-markdown/parsing_functions.c:28:13
    #2 0x5ddfcb in free_element_list /home/sanic/peg-markdown/parsing_functions.c:28:13
    #3 0x5dca5b in markdown_to_g_string /home/sanic/peg-markdown/markdown_lib.c:165:5
    #4 0x5dce84 in markdown_to_string /home/sanic/peg-markdown/markdown_lib.c:175:11
    #5 0x4c4d06 in main /home/sanic/peg-markdown/markdown.c:180:11
    #6 0x7ffff7b25082 in __libc_start_main /build/glibc-SzIz7B/glibc-2.31/csu/../csu/libc-start.c:308:16
    #7 0x41c50d in _start (/home/sanic/peg-markdown/markdown+0x41c50d)

0x6030000001a8 is located 24 bytes inside of 32-byte region [0x603000000190,0x6030000001b0)
freed by thread T0 here:
    #0 0x4949cd in free (/home/sanic/peg-markdown/markdown+0x4949cd)
    #1 0x5ddfd3 in free_element_list /home/sanic/peg-markdown/parsing_functions.c:31:9

previously allocated by thread T0 here:
    #0 0x494c4d in malloc (/home/sanic/peg-markdown/markdown+0x494c4d)
    #1 0x5dd447 in mk_element /home/sanic/peg-markdown/utility_functions.c:75:23
    #2 0x5dd447 in mk_str_from_list /home/sanic/peg-markdown/utility_functions.c:99:14

SUMMARY: AddressSanitizer: heap-use-after-free /home/sanic/peg-markdown/parsing_functions.c:25:21 in free_element_list
Shadow bytes around the buggy address:
  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 00 fa fa 00 00 00 00 fa fa fd fd
  0x0c067fff8010: fd fa fa fa fd fd fd fd fa fa fd fd fd fd fa fa
  0x0c067fff8020: 00 00 00 00 fa fa fd fd fd fd fa fa fd fd fd fd
=>0x0c067fff8030: fa fa fd fd fd[fd]fa fa 00 00 00 00 fa fa 00 00
  0x0c067fff8040: 00 00 fa fa fd fd fd fd fa fa 00 00 00 00 fa fa
  0x0c067fff8050: 00 00 00 00 fa fa 00 00 00 00 fa fa 00 00 00 00
  0x0c067fff8060: fa fa fd fd fd fd fa fa fd fd fd fd fa fa 00 00
  0x0c067fff8070: 00 00 fa fa 00 00 00 00 fa fa fd fd fd fd fa fa
  0x0c067fff8080: fd fd fd fd fa fa 00 00 00 00 fa fa 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
  Shadow gap:              cc
==400541==ABORTING

Reproduction Steps

1. Compile the project using ASAN (Address Sanitizer). For example, CC=afl-clang-fast AFL_USE_ASAN=1 make.
2. Run ./markdown -x min_heap_uaf.md (use the proof-of-concept file attached to this report).
3. Observe the heap use after free in the output.

[attritionorg]

did you request a CVE ID for this @danielpyon

[danielpyon]

@attritionorg Yes, I filed for an EOL CVE last week. Unfortunately, there has been no response so far.

[attritionorg]

For third-party tracking purposes, can you please share that ID? Thanks!

[danielpyon]

@attritionorg I don't think I've been assigned an ID yet, but I will share it once I get one.

[danielpyon]

@attritionorg

• CVE-2023-41458 (use after free)

• CVE-2023-41459 (stack overflow)

• CVE-2023-41460 (heap buffer overflow)

• CVE-2023-41461 (null pointer dereference)

[attritionorg]

Excellent, thank you @danielpyon!