CVE-2026-59922Public exploitNetwork attack vector

CPU exhaustion via repeated formatting-marker scans

Published Jul 8, 2026 · Updated Jul 8, 2026

Algorithmic-complexity denial of service in Mistune before 3.3.0 allows remote attackers to exhaust CPU with crafted Markdown. The formatting parser in src/mistune/plugins/formatting.py scans forward for a closing marker at every candidate start, producing quadratic work on repeated closed marker pairs. Reachability requires an application to parse attacker-controlled Markdown with the strikethrough, mark, or insert plugin enabled; default plugin-free parsing is unaffected, and sustained requests can starve application workers.

CVSS severity7.5
High
EPSS probability0.64%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Algorithmic-complexity denial of service in Mistune before 3.3.0 allows remote attackers to exhaust CPU with crafted Markdown. The formatting parser in src/mistune/plugins/formatting.py scans forward for a closing marker at every candidate start, producing quadratic work on repeated closed marker pairs. Reachability requires an application to parse attacker-controlled Markdown with the strikethrough, mark, or insert plugin enabled; default plugin-free parsing is unaffected, and sustained requests can starve application workers.

The record

CVE
CVE-2026-59922
Published
Jul 8, 2026
Updated
Jul 8, 2026
Vendor
Lepture
Product
Mistune
Classifications
CWE-1333, CWE-407, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 8, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 8, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 3.3.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Lepture · Mistuneversion=< 3.3.0

Published CVSS scores

7.5GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-1333Inefficient Regular Expression Complexity
CWE-407Inefficient Algorithmic Complexity
T1499.004Application or System Exploitation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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