VulnerabilityPublished 6 May 2026

CVE-2026-33079: Mistune Has a ReDoS

Affects mistuneSource GCVE Database

Authoritative record:CVE-2026-33079· High 8.7

Not known to be exploited · CVSS 8.7 High : upgrade to 3.2.1 soon.

Data as of 27 Oct 2026, 02:16 UTC

CVSS 4.0 base score

8.7High

How severe the flaw is on the 0–10 CVSS scale.

Patch: Available – fixed in 3.2.1

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

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: None
Vulnerable system availability: High
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: High: There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the Vulnerable System; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the Vulnerable System (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 6 May 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade mistune to 3.2.1.

Affected software

Product	Affected	Fixed in
PyPImistune	Affected >=3.0.0a1 <3.2.1	Fixed in 3.2.1

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

A patch is available in mistune version 3.2.1 that fixes the ReDoS vulnerability by modifying the LINK_TITLE_RE regex to eliminate overlapping alternatives causing exponential backtracking. Users should upgrade to mistune 3.2.1 or later. No other mitigation is required as the issue is resolved by this official fix.

Description

Mistune versions from 3.0.0a1 up to but not including 3.2.1 contain a Regular Expression Denial of Service (ReDoS) vulnerability in the LINK_TITLE_RE regex used for parsing Markdown link titles. An attacker can craft a small Markdown input (under 100 bytes) with repeated escaped punctuation sequences to cause exponential backtracking in the regex engine, resulting in significant parsing delays (e.g., ~6 seconds for 58 bytes). This affects any application or service that parses user-supplied Markdown using mistune, including web apps, documentation systems, APIs, and Jupyter tooling. The issue is fixed in mistune 3.2.1 by modifying the regex to remove ambiguity in backslash handling, eliminating the exponential backtracking.

Impact

This vulnerability allows an unauthenticated attacker to cause denial of service by submitting a small crafted Markdown input to any application or service that uses mistune for Markdown parsing. The parser becomes unresponsive for several seconds, impacting availability. The attack surface includes web applications with Markdown input fields, documentation systems, API endpoints processing Markdown, and Jupyter tools relying on mistune. There is no indication of privilege escalation or code execution. No known exploits in the wild have been reported.

Summary

The vulnerability is a ReDoS in the LINK_TITLE_RE regex in mistune's helpers.py, where overlapping alternatives in the regex cause exponential backtracking when parsing crafted Markdown link titles containing repeated escaped punctuation sequences (e.g., repeated '\!' pairs without a closing quote). This leads to O(2^N) time complexity in parsing, causing denial of service. The flaw is reachable via inline links and block link reference definitions. Proof-of-concept code demonstrates parsing times increasing exponentially with input size. The fix in version 3.2.1 excludes the backslash character from the catch-all character class in the regex, ensuring a backslash is only matched by the escaped-punctuation branch, thus removing ambiguity and preventing the exponential backtracking.

CWE-1333Inefficient Regular Expression Complexity (opens in a new tab)
Base weaknessLikelihood of exploit: High
The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
Architecture and Design
Use regular expressions that do not support backtracking, e.g. by removing nested quantifiers.
System Configuration
Set backtracking limits in the configuration of the regular expression implementation, such as PHP's pcre.backtrack_limit. Also consider limits on execution time for the process.
Implementation
Do not use regular expressions with untrusted input. If regular expressions must be used, avoid using backtracking in the expression.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
Attack patterns
Regular Expression Exponential Blowup (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 27 Oct 2026, 02:16 UTC

WeaknessesCWE-1333

Technical details

References

Community reviews

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