VulnerabilityPublished 13 Aug 2026Updated 29 Oct 2026

CVE-2026-44897: Jupyterlab · Mistune Heading ID Attribute has Injection XSS

Affects jupyterlabSource GCVE Database

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Authoritative record:CVE-2026-44897· Medium 6.1

Not known to be exploited · CVSS 6.1 Medium : upgrade to 4.5.7_1 when convenient.

Data as of 30 Oct 2026, 09:14 UTC

CVSS 3.1 base score

6.1Medium

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

Patch: Available – fixed in 4.5.7_1

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

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

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: Required: Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 26 May 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 4.5.7_1.

Affected software

Product	Affected	Fixed in
Homebrewjupyterlab`pkg:brew/jupyterlab`Other threats	Affected >=1.2.0 <4.5.7_1	Fixed in 4.5.7_1

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

AI-suggested

Upgrade JupyterLab to version 4.5.7_1 or later, where this issue is fixed. Avoid using custom heading_id callbacks that return unescaped user-controlled text. Use the default TOC hook or ensure any custom heading_id callback properly escapes or sanitizes the ID values before they are inserted into HTML.

Description

JupyterLab's Mistune HTMLRenderer.heading() method improperly handles the id attribute by inserting it directly into the HTML without escaping. When a custom heading_id callback returns user-controlled heading text containing double quotes, this allows injection of arbitrary HTML attributes, enabling cross-site scripting (XSS). The default TOC hook uses safe auto-generated IDs, but common usage involves deriving IDs from heading text, which is vulnerable. This flaw is fixed in JupyterLab version 4.5.7_1.

Impact

An attacker who can control heading text in Markdown processed by JupyterLab with a custom heading_id callback can inject arbitrary HTML attributes into heading elements. This enables cross-site scripting (XSS) attacks, potentially allowing execution of malicious JavaScript in the context of the user's browser, leading to information disclosure or session hijacking. The default configuration is safe, but common customizations that derive IDs from heading text are vulnerable.

Summary

The vulnerability arises because the HTMLRenderer.heading() function in Mistune concatenates the id attribute value directly into the opening <hN> tag without escaping special characters. If a custom heading_id callback returns raw heading text containing double quotes, an attacker can inject additional attributes such as event handlers into the heading element, resulting in XSS. The default TOC hook generates safe numeric IDs, but typical real-world usage involves slugifying heading text, which can be exploited if the text is attacker-controlled. A proof-of-concept demonstrates injecting an onmouseover event that triggers JavaScript execution. The issue is patched in JupyterLab version 4.5.7_1.

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What to do

Affected software

Description

AI analysisMachine-generated, updated 30 Oct 2026, 09:14 UTC

Technical details

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