Vulnerability-Lookup

CVE-2026-25543 (GCVE-0-2026-25543)

Vulnerability from cvelistv5 – Published: 2026-02-04 21:45 – Updated: 2026-02-05 18:24

Title

HtmlSanitizer has a bypass via template tag

Summary

HtmlSanitizer is a .NET library for cleaning HTML fragments and documents from constructs that can lead to XSS attacks. Prior to versions 9.0.892 and 9.1.893-beta, if the template tag is allowed, its contents are not sanitized. The template tag is a special tag that does not usually render its contents, unless the shadowrootmode attribute is set to open or closed. This issue has been patched in versions 9.0.892 and 9.1.893-beta.

Severity

6.3 (Medium)


                        
                          CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

SSVC

Exploitation: none Automatable: yes Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-02-05 18:24 UTC

CWE

CWE-116 - Improper Encoding or Escaping of Output

Assigner

GitHub_M CNA under the mitre root

CNA scorecard C 72/100 over 7631 records in the last 180 days details

References

4 references

URL	Tags
https://github.com/mganss/HtmlSanitizer/security/…	x_refsource_CONFIRM
https://github.com/mganss/HtmlSanitizer/commit/0a…	x_refsource_MISC
https://www.nuget.org/packages/HtmlSanitizer/9.0.892	x_refsource_MISC
https://www.nuget.org/packages/HtmlSanitizer/9.1.…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
mganss	HtmlSanitizer	Affected: < 9.0.892 Affected: < 9.1.893-beta	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-25543

Vulnerability from fkie_nvd - Published: 2026-02-04 22:16 - Updated: 2026-06-17 10:24

Severity

6.1 (Medium) - CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Summary

References

URL	Tags
security-advisories@github.com	https://github.com/mganss/HtmlSanitizer/commit/0ac53dca30ddad963f2b243669a5066933d82b81	Patch
security-advisories@github.com	https://github.com/mganss/HtmlSanitizer/security/advisories/GHSA-j92c-7v7g-gj3f	Mitigation, Patch, Vendor Advisory
security-advisories@github.com	https://www.nuget.org/packages/HtmlSanitizer/9.0.892	Product, Release Notes
security-advisories@github.com	https://www.nuget.org/packages/HtmlSanitizer/9.1.893-beta	Product, Release Notes

Impacted products

	Vendor	Product	Version
	htmlsanitizer_project	htmlsanitizer	*
	htmlsanitizer_project	htmlsanitizer	*

JSON

To clipboard

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    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "NONE",
          "baseScore": 6.1,
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          "confidentialityImpact": "LOW",
          "integrityImpact": "LOW",
          "privilegesRequired": "NONE",
          "scope": "CHANGED",
          "userInteraction": "REQUIRED",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
          "version": "3.1"
        },
        "exploitabilityScore": 2.8,
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        "source": "nvd@nist.gov",
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    ],
    "cvssMetricV40": [
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        "cvssData": {
          "Automatable": "NOT_DEFINED",
          "Recovery": "NOT_DEFINED",
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          "exploitMaturity": "NOT_DEFINED",
          "integrityRequirement": "NOT_DEFINED",
          "modifiedAttackComplexity": "NOT_DEFINED",
          "modifiedAttackRequirements": "NOT_DEFINED",
          "modifiedAttackVector": "NOT_DEFINED",
          "modifiedPrivilegesRequired": "NOT_DEFINED",
          "modifiedSubAvailabilityImpact": "NOT_DEFINED",
          "modifiedSubConfidentialityImpact": "NOT_DEFINED",
          "modifiedSubIntegrityImpact": "NOT_DEFINED",
          "modifiedUserInteraction": "NOT_DEFINED",
          "modifiedVulnAvailabilityImpact": "NOT_DEFINED",
          "modifiedVulnConfidentialityImpact": "NOT_DEFINED",
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          "privilegesRequired": "NONE",
          "providerUrgency": "NOT_DEFINED",
          "subAvailabilityImpact": "NONE",
          "subConfidentialityImpact": "LOW",
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          "userInteraction": "NONE",
          "valueDensity": "NOT_DEFINED",
          "vectorString": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X",
          "version": "4.0",
          "vulnAvailabilityImpact": "NONE",
          "vulnConfidentialityImpact": "NONE",
          "vulnIntegrityImpact": "NONE",
          "vulnerabilityResponseEffort": "NOT_DEFINED"
        },
        "source": "security-advisories@github.com",
        "type": "Secondary"
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    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-25543",
          "options": [
            {
              "exploitation": "none"
            },
            {
              "automatable": "yes"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-02-05T18:24:02.525697Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-02-04T22:16:00.523",
  "references": [
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Patch"
      ],
      "url": "https://github.com/mganss/HtmlSanitizer/commit/0ac53dca30ddad963f2b243669a5066933d82b81"
    },
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Mitigation",
        "Patch",
        "Vendor Advisory"
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      "url": "https://github.com/mganss/HtmlSanitizer/security/advisories/GHSA-j92c-7v7g-gj3f"
    },
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Product",
        "Release Notes"
      ],
      "url": "https://www.nuget.org/packages/HtmlSanitizer/9.0.892"
    },
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Product",
        "Release Notes"
      ],
      "url": "https://www.nuget.org/packages/HtmlSanitizer/9.1.893-beta"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Analyzed",
  "weaknesses": [
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          "value": "CWE-116"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Secondary"
    },
    {
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          "lang": "en",
          "value": "CWE-79"
        }
      ],
      "source": "nvd@nist.gov",
      "type": "Primary"
    }
  ]
}

GHSA-J92C-7V7G-GJ3F

Vulnerability from github – Published: 2026-02-03 19:22 – Updated: 2026-02-05 00:36

Summary

HtmlSanitizer has a bypass via template tag

Details

Impact

If the template tag is allowed, its contents are not sanitized. The template tag is a special tag that does not usually render its contents, unless the shadowrootmode attribute is set to open or closed.

The lack of sanitization of the template tag brings up two bypasses:

it is still possible to forcibly render the contents of a <template> tag through mutation XSS. The DOM parsers in browsers such as Chromium have a node depth limit of 512 and tags which are beyond that depth are flattened. This in turn allows elements within <template> (which are not sanitized) to be effectively 'popped out'. An example would look like this: <div>[...]<template><script>alert('xss')</script> where [...] denotes at least another 509 opening <div> tags.
If in addition to the template tag, the shadowrootmode attribute is allowed through sanitizer.AllowedAttributes.Add("shadowrootmode");, the simple payload of <div><template shadowrootmode="open"><script>alert('xss')</script> would bypass the sanitizer. This is because such usage of <template> attaches a shadow root to its parent: <div>, and its contents will be rendered.

Note that the default configuration is not affected because the template tag is disallowed by default.

Patches

The problem has been patched in versions 9.0.892 and 9.1.893-beta.

Workarounds

Disallow the template tag. It is disallowed by default.

Resources

https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/template

Severity

6.3 (Medium)


                  
                    CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

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{
  "affected": [
    {
      "package": {
        "ecosystem": "NuGet",
        "name": "HtmlSanitizer"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "9.0.892"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "NuGet",
        "name": "HtmlSanitizer"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "9.1.878-beta"
            },
            {
              "fixed": "9.1.893-beta"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-25543"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-116",
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-02-03T19:22:06Z",
    "nvd_published_at": "2026-02-04T22:16:00Z",
    "severity": "MODERATE"
  },
  "details": "### Impact\n\nIf the `template` tag is allowed, its contents are not sanitized. The `template` tag is a special tag that does not usually render its contents, unless the `shadowrootmode` attribute is set to `open` or `closed`. \n\nThe lack of sanitization of the template tag brings up two bypasses:\n\n1. it is still possible to forcibly render the contents of a `\u003ctemplate\u003e` tag through mutation XSS. The DOM parsers in browsers such as Chromium have a node depth limit of 512 and tags which are beyond that depth are flattened. This in turn allows elements within `\u003ctemplate\u003e` (which are not sanitized) to be effectively \u0027popped out\u0027. An example would look like this: `\u003cdiv\u003e[...]\u003ctemplate\u003e\u003cscript\u003ealert(\u0027xss\u0027)\u003c/script\u003e` where `[...]` denotes at least another 509 opening `\u003cdiv\u003e` tags.\n2. If in addition to the template tag, the `shadowrootmode` attribute is allowed through `sanitizer.AllowedAttributes.Add(\"shadowrootmode\");`, the simple payload of `\u003cdiv\u003e\u003ctemplate shadowrootmode=\"open\"\u003e\u003cscript\u003ealert(\u0027xss\u0027)\u003c/script\u003e` would bypass the sanitizer. This is because such usage of `\u003ctemplate\u003e` attaches a shadow root to its parent: `\u003cdiv\u003e`, and its contents will be rendered. \n\nNote that the default configuration is not affected because the `template` tag is disallowed by default.\n\n### Patches\n\nThe problem has been patched in versions [9.0.892](https://www.nuget.org/packages/HtmlSanitizer/9.0.892) and [9.1.893-beta](https://www.nuget.org/packages/HtmlSanitizer/9.1.893-beta).\n\n### Workarounds\n\nDisallow the `template` tag. It is disallowed by default.\n\n### Resources\n\nhttps://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/template",
  "id": "GHSA-j92c-7v7g-gj3f",
  "modified": "2026-02-05T00:36:50Z",
  "published": "2026-02-03T19:22:06Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/mganss/HtmlSanitizer/security/advisories/GHSA-j92c-7v7g-gj3f"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-25543"
    },
    {
      "type": "WEB",
      "url": "https://github.com/mganss/HtmlSanitizer/commit/0ac53dca30ddad963f2b243669a5066933d82b81"
    },
    {
      "type": "WEB",
      "url": "https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/template"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/mganss/HtmlSanitizer"
    },
    {
      "type": "WEB",
      "url": "https://github.com/mganss/HtmlSanitizer/releases/tag/v9.0.892"
    },
    {
      "type": "WEB",
      "url": "https://www.nuget.org/packages/HtmlSanitizer/9.0.892"
    },
    {
      "type": "WEB",
      "url": "https://www.nuget.org/packages/HtmlSanitizer/9.1.893-beta"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "HtmlSanitizer has a bypass via template tag"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
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CVE-2026-25543 (GCVE-0-2026-25543)

FKIE_CVE-2026-25543

GHSA-J92C-7V7G-GJ3F

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