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February 7, 2023

GHSL-2022-128: Quadratic complexity algorithm in cmark - CVE-2023-22486

Kevin Backhouse

Coordinated Disclosure Timeline


A crafted markdown document can trigger a quadratic complexity algorithm in cmark.



Tested Version

Latest master: cd5b2f6


Issue: Quadratic behavior in handle_close_bracket (is_image logic) (GHSL-2022-128)

A markdown document containing a large number of repetitions of the characters ![[]() can trigger quadratic behavior.

Proof of concept:

python3 -c 'print("![[]()" * 20000)' | cmark

Increasing the number 20000 in the above command causes the running time to increase quadratically.

This is a sample stack trace from the quadratic algorithm:

  1. handle_close_bracket (subj=0x7fffffffbda0) at src/inlines.c:1230
  2. 0x000055555556adcb in parse_inline (subj=0x7fffffffbda0, parent=0x5555555ab790, options=1024) at src/inlines.c:1356
  3. 0x000055555556b09e in cmark_parse_inlines (mem=0x5555555a90e0 <DEFAULT_MEM_ALLOCATOR>, parent=0x5555555ab790, refmap=0x5555555aa570, options=1024) at src/inlines.c:1398
  4. 0x000055555556545d in process_inlines (mem=0x5555555a90e0 <DEFAULT_MEM_ALLOCATOR>, root=0x5555555aa370, refmap=0x5555555aa570, options=1024) at src/blocks.c:408
  5. 0x00005555555658ab in finalize_document (parser=0x5555555aa2c0) at src/blocks.c:530
  6. 0x0000555555567626 in cmark_parser_finish (parser=0x5555555aa2c0) at src/blocks.c:1303
  7. 0x0000555555562fa2 in main (argc=3, argv=0x7fffffffdfc8) at src/main.c:203

The quadratic behavior is caused by the loop at src/inlines.c:1229:

while (opener != NULL) {
  if (!opener->image) {
    if (!opener->active) {
    } else {
      opener->active = false;
  opener = opener->previous;

The malicious input (see above) causes this loop to repeatedly iterate all the way back to the beginning of the list.


This issue could be used in a denial-of-service attack on websites that use cmark to render markdown documents.



This issue was discovered and reported by GHSL team member @kevinbackhouse (Kevin Backhouse).


You can contact the GHSL team at, please include a reference to GHSL-2022-128 in any communication regarding this issue.