VulnerabilityPublished 13 Aug 2026Updated 29 Oct 2026

CVE-2026-49851: Mistune · Potential DoS via quadratic-time parsing in parse_link_text

Affects jupyterlabSource GCVE Database

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Authoritative record:CVE-2026-49851· High 8.7

Not known to be exploited · CVSS 8.7 High : upgrade to 4.6.1 soon.

Data as of 30 Oct 2026, 09:01 UTC

CVSS 4.0 base score

8.7High

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

Patch: Available – fixed in 4.6.1

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 24 Jun 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: 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 29 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.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), 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 jupyterlab to 4.6.1.

Affected software

Product	Affected	Fixed in
Homebrewjupyterlab`pkg:brew/jupyterlab`Other threats	Affected >=1.2.0 <4.6.1	Fixed in 4.6.1

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

AI-suggested

A fix for this vulnerability is available in Mistune version 4.6.1. Users and administrators should upgrade to this version to remediate the issue. The vendor advisory does not list alternative mitigations, and no action other than applying the official fix is recommended.

Description

Mistune, a Python Markdown parser used in jupyterlab versions >=1.2.0 and <4.6.1, contains a denial-of-service vulnerability due to quadratic-time parsing behavior in the parse_link_text function. An attacker can craft Markdown input with many consecutive '[' characters to cause excessive CPU usage, leading to service disruption. The issue arises from inefficient repeated scanning inside a loop, resulting in O(n²) complexity. A fix is available in version 4.6.1.

Impact

This vulnerability can be exploited by an attacker providing specially crafted Markdown input containing many consecutive '[' characters to cause excessive CPU consumption, resulting in denial of service. Applications parsing user-supplied Markdown with Mistune, such as web applications, API services, and documentation systems, are at risk. The impact is high availability degradation due to CPU exhaustion, with no impact on confidentiality or integrity.

Summary

The Mistune Markdown parser suffers from a CPU exhaustion denial-of-service vulnerability (CVE-2026-49851) caused by superlinear (approximately O(n²)) parsing time in the parse_link_text function. When parsing Markdown with many consecutive '[' characters, parse_link_text repeatedly performs regex searches inside a loop, rescanning large portions of the input string. This results from an outer loop in InlineParser.parse() advancing only one character at a time when parse_link() returns None, combined with parse_link_text performing an O(n) scan for a closing ']', leading to O(n) × O(n) = O(n²) total work. A small attacker-controlled payload (~6 KB) can cause significant CPU load and delay. The vulnerability affects jupyterlab versions >=1.2.0 and <4.6.1 and is fixed in 4.6.1. The CVSS v3.1 score is 7.5 (high severity) with no confidentiality or integrity impact but high availability impact.

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Description

AI analysisMachine-generated, updated 30 Oct 2026, 09:01 UTC

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