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Showing content from https://github.com/Python-Markdown/markdown/issues/1534 below:

Uncaught exceptions/crashes when parsing malformed <![ marked sections in Python-Markdown 3.8 with extra extension · Issue #1534 · Python-Markdown/markdown · GitHub

While fuzz-testing the Python-Markdown library (version 3.8) with the extra extension enabled, I found that certain malformed inputs containing <![ sequences cause the parser to throw uncaught exceptions and crash. These inputs appear to break the parser's handling of XML-style marked sections, leading to errors like "expected name token" or "unknown status keyword."

1. Run the following test script (using Python-Markdown 3.8 with extra extension):

import markdown

print(markdown.__version__)

crash_inputs = [
    "<![",
    "<![>og))/uw_     f{tv+pAr$Ss+[6;^{=<:>g2oV|.pdTMu(Q-E#",
    "<![ g'\"7z5r7cojSO;2LAo0(1Vv5G>,-P",
]

for i, crash_input in enumerate(crash_inputs, 1):
    print(f"\nTesting crash input #{i}:\n{repr(crash_input)}")
    try:
        output = markdown.markdown(crash_input, extensions=["extra"])
        print("No crash, output:")
        print(output)
    except Exception as e:
        print(f"Crash confirmed! Exception:\n{e}")

1. Observe that the first three inputs cause exceptions, crashing the parser.

The parser should gracefully handle or sanitize malformed <![ marked sections without raising uncaught exceptions.

Uncaught exceptions are raised, such as:

3.8

Testing crash input #1:
'<!['
Crash confirmed! Exception:
expected name token at '<![\n\n'

Testing crash input #2:
'<![>og))/uw_     f{tv+pAr$Ss+[6;^{=<:>g2oV|.pdTMu(Q-E#'
Crash confirmed! Exception:
expected name token at '<![>og))/uw_     f{t'

Testing crash input #3:
'<![ g\'"7z5r7cojSO;2LAo0(1Vv5G>,-P'
Crash confirmed! Exception:
expected name token at '<![ g\'"7z5r7cojSO;2L'

• expected name token at '<![\n\n'
• expected name token at '<![>og))/uw_ f{t'
• expected name token at '<![ g\'"7z5r7cojSO;2L'

• Potential Denial of Service by crashing apps parsing untrusted Markdown.
• Stability risk in applications relying on Python-Markdown.
• Possible information leakage via stack traces if errors are exposed.

• Python-Markdown version: 3.8
• Python version: 3.11
• OS: Linux and Windows 11

I performed fuzz testing and isolated these inputs causing crashes. I'm happy to provide more test cases or logs if needed.

Thank you for your attention to this issue!

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