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justhtml has sanitization bypass in custom policies and programmatic DOM

Moderate severity GitHub Reviewed Published Apr 19, 2026 in EmilStenstrom/justhtml • Updated Apr 22, 2026

Package

pip justhtml (pip)

Affected versions

< 1.17.0

Patched versions

1.17.0

Description

Summary

justhtml 1.17.0 fixes multiple security issues in sanitization, serialization, and programmatic DOM handling.

Most of these issues affected advanced or custom configurations rather than the default safe path.

Affected versions

  • justhtml <= 1.16.0

Fixed version

  • justhtml 1.17.0 released on April 19, 2026

Impact

Custom SVG / MathML sanitization policies

Custom policies that preserved foreign namespaces could allow dangerous content to survive sanitization, including:

  • active HTML integration points such as SVG <foreignObject>, MathML <annotation-xml encoding="text/html">, SVG <title> / <desc>, and MathML text integration points
  • mutation-XSS parser-differential payloads that looked inert in memory but became active HTML after reparse
  • SVG filter="url(...)" attributes that could trigger external fetches

These issues affected:

  • JustHTML(..., sanitize=True) with custom foreign-namespace policies
  • sanitize() / sanitize_dom()
  • low-level terminal Sanitize(...) transform execution

Preserved <style> handling

Constructor-time sanitization and explicit Sanitize(...) transforms did not fully match sanitize() / sanitize_dom() when custom policies preserved <style>.

That could leave resource-loading CSS such as @import or background-image:url(...) in sanitized output from HTML string input.

Programmatic DOM serialization

Programmatic script, style, and Comment(...) nodes could still serialize into active markup in some edge cases.

This could affect applications that build or mutate DOM trees directly before calling to_html() or to_markdown(html_passthrough=True).

Cache mutation and DOM cycle handling

Two lower-severity hardening fixes were included:

  • compiled sanitize-pipeline caches could be mutated after warming and weaken later sanitization
  • parent/child cycles in programmatic DOM trees could cause infinite loops in operations such as to_html() and sanitize_dom()

Default configuration

Most of the issues above did not affect ordinary parsed HTML with the default JustHTML(..., sanitize=True) configuration.

The main risk areas were:

  • custom policies that preserve SVG or MathML
  • custom policies that preserve <style>
  • programmatic DOM construction or mutation
  • low-level direct sanitizer/transform APIs

Recommended action

Upgrade to justhtml 1.17.0.

If users cannot upgrade immediately:

  • avoid preserving SVG or MathML for untrusted input
  • avoid preserving <style> for untrusted input
  • avoid mutating programmatic DOM trees with untrusted script, style, or comment content
  • avoid mutating warmed policy internals or sanitizer caches

Credit

Discovered during an internal security review of justhtml.

References

@EmilStenstrom EmilStenstrom published to EmilStenstrom/justhtml Apr 19, 2026
Published to the GitHub Advisory Database Apr 22, 2026
Reviewed Apr 22, 2026
Last updated Apr 22, 2026

Severity

Moderate

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 Present
Privileges Required None
User interaction Passive
Vulnerable System Impact Metrics
Confidentiality None
Integrity High
Availability None
Subsequent System Impact Metrics
Confidentiality None
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:P/PR:N/UI:P/VC:N/VI:H/VA:N/SC:N/SI:L/SA:N

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.

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.

Loop with Unreachable Exit Condition ('Infinite Loop')

The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop. Learn more on MITRE.

CVE ID

No known CVE

GHSA ID

GHSA-vrx2-77f2-ww34

Source code

Credits

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