CVE-2026-49851: DoS (CVSS 7.5)

Description

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, Mistune is vulnerable to a CPU exhaustion DoS due to superlinear (approximately O(n²)) behavior in parse_link_text. When parsing Markdown containing many consecutive [ characters, parse_link_text repeatedly scans the input using a regex search inside a loop. Each iteration re-scans a large portion of the remaining string, resulting in quadratic-time behavior. An attacker-controlled Markdown input can therefore trigger excessive CPU usage with a very small payload. This vulnerability is fixed in 3.3.0.

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CVSS v3.1 Score

7.5

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS — Exploit Prediction

0.0035

Probability of exploitation

0.27%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-400 Uncontrolled Resource Consumption

CWE-407 CWE-407

CWE-770 CWE-770

CWE-1333 CWE-1333

References

Other References

https://github.com/lepture/mistune/security/advisories/GHSA-qcq2-496w-v96p https://access.redhat.com/security/cve/CVE-2026-49851 https://bugzilla.redhat.com/show_bug.cgi?id=2492304 https://github.com/lepture/mistune/security/advisories/GHSA-qcq2-496w-v96p https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-49851.json

Frequently Asked Questions

What is CVE-2026-49851? +

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, Mistune is vulnerable to a CPU exhaustion DoS due to superlinear (approximately O(n²)) behavior in parse_link_text. When parsing Markdown containing many consecutive [ characters, parse_link_text repeatedly scans the input using a regex search inside a loop. Each iteration re-scans a large portion of the remaining string, resulting in quadratic-time behavior. An attacker-controlled Markdown input can therefore trigger excessive CPU usage with a very small payload. This vulnerability is fixed in 3.3.0. It has a CVSS v3.1 base score of 7.5 (HIGH).

How severe is CVE-2026-49851? +

CVE-2026-49851 has a CVSS v3.1 score of 7.5 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0035, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-49851? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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