Fix cubic ReDoS in fenced code and references #1130
Merged
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Two regular expressions were vulnerable to Regular Expression Denial of Service (ReDoS).
Crafted strings containing a long sequence of spaces could cause Denial of Service by making markdown take a long time to process.
This represents a vulnerability when untrusted user input is processed with the markdown package.
ReferencesProcessor:
markdown/markdown/blockprocessors.py
Lines 559 to 563 in 4acb949
e.g.:
FencedBlockPreprocessor (requires fenced_code extension):
markdown/markdown/extensions/fenced_code.py
Lines 43 to 54 in a114315
e.g.:
Both regular expressions had cubic worst-case complexity, so doubling the number of spaces made processing take 8 times as long.
The cubic behaviour can be seen as follows:
Both regexes had three
[ ]*
groups separated by optional groups, in effect making the regex[ ]*[ ]*[ ]*
.Discovered using doyensec/regexploit.