CVE-2026-33079Public exploitNetwork attack vector

Parser hang via ambiguous link-title regex

Published May 6, 2026 · Updated May 6, 2026

Regular expression denial of service in Lepture Mistune 3.0.0a1 through 3.2.0 allows remote attackers to hang parsers via crafted Markdown. LINK_TITLE_RE lets a backslash-punctuation pair match either an escape or two ordinary characters inside a repeated group, causing exponential backtracking when the closing quote is absent. Applications must parse attacker-supplied inline-link titles or reference definitions; a sub-100-byte document can monopolize CPU and make the parsing service unresponsive.

CVSS severity8.7
High
EPSS probability0.53%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Regular expression denial of service in Lepture Mistune 3.0.0a1 through 3.2.0 allows remote attackers to hang parsers via crafted Markdown. LINK_TITLE_RE lets a backslash-punctuation pair match either an escape or two ordinary characters inside a repeated group, causing exponential backtracking when the closing quote is absent. Applications must parse attacker-supplied inline-link titles or reference definitions; a sub-100-byte document can monopolize CPU and make the parsing service unresponsive.

The record

CVE
CVE-2026-33079
Published
May 6, 2026
Updated
May 6, 2026
Vendor
Lepture
Product
Mistune
Classifications
CWE-1333, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 6, 2026CVE publishedPublication date reported by the CVE source.
  2. May 6, 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=>=3.0.0a1, <= 3.2.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Lepture · Mistuneversion=>=3.0.0a1, <= 3.2.0

Published CVSS scores

7.5LeptureCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
7.5Red HatCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
8.7GitHub, Inc.CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 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
T1499.004Endpoint Denial of Service: Application 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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