VulnerabilityPublished 13 Aug 2026Updated 29 Oct 2026

CVE-2026-59928: Mistune block_parser · quadratic-time parsing on long lists of repeated reference-link definitions

Affects jupyterlabSource GCVE Database

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Authoritative record:CVE-2026-59928· 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 fixes the quadratic-time parsing issue by optimizing the reference-link definition handling to linear time. Users should upgrade to version 4.6.1 or later to remediate this vulnerability. No additional mitigation actions are required once patched.

Description

CVE-2026-59928 is a denial-of-service vulnerability in the Mistune markdown parser used by JupyterLab. It arises from quadratic-time parsing behavior when processing long lists of repeated reference-link definitions, causing CPU usage to increase dramatically with input size. This can be exploited by submitting markdown documents with many repeated reference-link definitions, leading to high CPU consumption and potential service disruption. The issue affects Mistune versions used in JupyterLab from 1.2.0 up to but not including 4.6.1. A fix is available in version 4.6.1 that optimizes parsing to linear time.

Impact

An attacker can cause a denial-of-service condition by submitting specially crafted markdown documents with many repeated reference-link definitions, causing the parser to consume excessive CPU resources due to quadratic-time parsing. This can degrade or disrupt service availability for applications using Mistune with default configurations on attacker-controlled input. There is no impact on confidentiality or integrity. The vulnerability is exploitable without authentication and requires only attacker-supplied markdown input.

Summary

The vulnerability in Mistune's block_parser.py involves an algorithmic complexity flaw in handling reference-link definitions. The parser performs a quadratic-time scan when processing N repeated or distinct reference-link definitions, due to repeated forward scans and linear lookups in the ref_links environment dictionary. This causes parse time to grow as O(N²), with empirical tests showing parse times increasing from ~1.1 seconds for 5000 repeated definitions to ~4.5 seconds for 10000. An attacker can exploit this by submitting markdown with many repeated reference-link definitions, causing excessive CPU usage and potential denial of service. The root cause is the lack of amortization in scanning the def-list, which can be resolved by a single forward pass classification and insertion into the dictionary, reducing complexity to O(N). The vulnerability affects Mistune versions bundled with JupyterLab >=1.2.0 and <4.6.1, with a patch available in 4.6.1.

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Description

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

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