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Multiple security fixes in justhtml

Low severity GitHub Reviewed Published Apr 12, 2026 in EmilStenstrom/justhtml • Updated Apr 14, 2026

Package

pip justhtml (pip)

Affected versions

<= 1.15.0

Patched versions

1.16.0

Description

Summary

justhtml 1.16.0 fixes multiple security issues in sanitization, serialization, and programmatic DOM handling.

Most of these issues affected one of these advanced paths rather than ordinary parsed HTML with the default safe settings:

  • programmatic DOM input to sanitize() or sanitize_dom()
  • reused or mutated sanitization policy objects
  • custom policies that preserve foreign namespaces such as SVG or MathML

Affected versions

  • justhtml <= 1.15.0

Fixed version

  • justhtml 1.16.0 released on April 12, 2026

Impact

Policy reuse and mutation

Nested mutation of sanitization policy internals could weaken later sanitization by leaving stale compiled sanitizers active, or by mutating exported default policy internals process-wide.

In-memory sanitization gaps

Programmatic DOM sanitization could miss dangerous mixed-case tag names such as ScRiPt or StYlE, and custom drop_content_tags values such as {"SCRIPT"} could silently fail to drop dangerous subtrees.

Serialization injection

Crafted programmatic doctype names could serialize into active markup before the document body.

Foreign-namespace policy bypasses

Custom policies that preserve SVG or MathML could allow active SVG features to survive sanitization, including:

  • animation elements such as <set> and <animate> that mutate already-sanitized attributes after sanitization
  • presentation attributes such as fill, clip-path, mask, marker-start, and cursor containing external url(...) references
  • programmatic DOM trees that claim namespace="html" but serialize as <svg> or <math>, bypassing foreign-content checks

Rawtext hardening gap

Mixed-case programmatic style or script nodes could bypass rawtext hardening and preserve active stylesheet content such as remote @import rules.

Default configuration

Most of these issues did not affect the normal JustHTML(..., sanitize=True) path for ordinary parsed HTML.

The main exceptions were policy-mutation issues, which could weaken later sanitization if code mutated nested state on reused policy objects or exported defaults.

Recommended action

Upgrade to justhtml 1.16.0.

If you cannot upgrade immediately:

  • do not mutate DEFAULT_POLICY, DEFAULT_DOCUMENT_POLICY, or nested policy internals
  • avoid reusing policy objects after mutating nested state
  • avoid preserving SVG or MathML for untrusted input
  • avoid preserving style or script in custom policies for untrusted input
  • avoid serializing untrusted programmatic doctypes or DOM trees

Credit

Discovered during an internal security review of justhtml.

References

@EmilStenstrom EmilStenstrom published to EmilStenstrom/justhtml Apr 12, 2026
Published to the GitHub Advisory Database Apr 14, 2026
Reviewed Apr 14, 2026
Last updated Apr 14, 2026

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction Passive
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality Low
Integrity Low
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N/E:U

EPSS score

Weaknesses

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. Learn more on MITRE.

Improper Handling of Case Sensitivity

The product does not properly account for differences in case sensitivity when accessing or determining the properties of a resource, leading to inconsistent results. Learn more on MITRE.

Interpretation Conflict

Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state. Learn more on MITRE.

Modification of Assumed-Immutable Data (MAID)

The product does not properly protect an assumed-immutable element from being modified by an attacker. Learn more on MITRE.

CVE ID

No known CVE

GHSA ID

GHSA-4p64-v8f5-r2gx

Source code

Credits

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