VulnerabilityPublished 13 Aug 2026Updated 29 Oct 2026

CVE-2026-59925: Mistune inline_parser · quadratic-time parsing on long runs of `x` and `x` emphasis pairs

Affects jupyterlabSource GCVE Database

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Authoritative record:CVE-2026-59925· High 7.5

Not known to be exploited · CVSS 7.5 High : upgrade to 4.6.1 soon.

Data as of 30 Oct 2026, 09:01 UTC

CVSS 3.1 base score

7.5High

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

Patch: Available – fixed in 4.6.1

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

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

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, 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: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; 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 impacted component (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).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 8 Jul 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 patch is available in Mistune version 4.6.1 that addresses this vulnerability. Users should upgrade to version 4.6.1 or later to mitigate the issue. The fix involves limiting the number of simultaneously tracked unmatched emphasis markers or rewriting the emphasis parsing logic to a delimiter-stack approach to ensure linear-time parsing. No additional mitigation steps are required if the patched version is deployed.

Description

CVE-2026-59925 is a high-severity algorithmic-complexity denial-of-service vulnerability in the Mistune markdown parser's core emphasis parsing. The parser performs quadratic-time work when processing long sequences of well-formed '**x**' or '***x***' emphasis pairs, causing excessive CPU usage. This affects the default Mistune configuration without requiring any plugins. The vulnerability has been confirmed in Mistune versions used by JupyterLab from 1.2.0 up to but not including 4.6.1. A fix is available in version 4.6.1.

Impact

An attacker can cause a denial of service by submitting markdown content with long sequences of '**x**' or '***x***' emphasis pairs, causing the Mistune parser to consume CPU resources quadratically relative to input size. This can lead to server CPU pegging and service disruption, especially when combined with concurrent requests. The vulnerability is network-reachable, requires no authentication, and affects default Mistune configurations without plugins. The CVSS v3.1 score is 7.5 (high), reflecting the denial of service impact on availability.

Summary

The vulnerability arises from the way Mistune's inline emphasis parser scans for matching emphasis delimiters. For each opening emphasis marker ('*', '**', or '***'), the parser scans forward to find the corresponding closing marker using regular expressions. Although each scan is bounded, the parser invokes this scan from every potential start position, resulting in O(N²) parsing time for inputs with repeated emphasis pairs like '**x**' repeated many times. This quadratic complexity can be exploited by an attacker submitting crafted markdown input to cause high CPU usage and potential denial of service. The issue is reproducible on default Mistune without plugins and has been verified against Mistune 3.2.1. The recommended fix involves capping the number of unmatched opening emphasis markers tracked simultaneously or rewriting the parser to use a delimiter-stack algorithm similar to the CommonMark reference implementations, which handle emphasis parsing in linear time.

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Description

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

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