Vulnerability-Lookup

CVE-2026-5389 (GCVE-0-2026-5389)

Vulnerability from cvelistv5 – Published: 2026-08-23 13:34 – Updated: 2026-08-24 18:15

Title

justhtml before 1.13.0 XSS via code fence breakout

Summary

justhtml versions before 1.13.0 contain a cross-site scripting vulnerability in the to_markdown() function when serializing attacker-controlled pre content. Attackers can place backticks inside sanitized pre elements to break out of fixed-length code fences, allowing raw HTML to execute when the generated Markdown is rendered by CommonMark or GFM-style renderers.

Severity

5.1 (Medium)


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

6.1 (Medium)


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

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-08-24 18:15 UTC

CWE

CWE-80 - Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)

Assigner

VulnCheck mitre root

References

2 references

URL	Tags
https://github.com/EmilStenstrom/justhtml/securit…	vendor-advisory
https://www.vulncheck.com/advisories/justhtml-bef…	third-party-advisory

Impacted products

1 product

Vendor	Product	Version	CPE status
EmilStenstrom	justhtml	Affected: 0 , < 1.13.0 (semver) Unaffected: 1.13.0 (semver)	guessed

Date Public

2026-03-21 00:00

Credits

AlfinJ0se

Show details on NVD website

JSON

To clipboard

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}

FKIE_CVE-2026-5389

Vulnerability from fkie_nvd - Published: 2026-08-23 14:16 - Updated: 2026-08-24 19:16

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
	disclosure@vulncheck.com	https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-5vp3-3cg6-2rq3
	disclosure@vulncheck.com	https://www.vulncheck.com/advisories/justhtml-before-xss-via-code-fence-breakout
	134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-5vp3-3cg6-2rq3

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "defaultStatus": "unaffected",
          "packageURL": "pkg:pypi/justhtml",
          "product": "justhtml",
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  "lastModified": "2026-08-24T19:16:40.547",
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        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
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          "modifiedSubConfidentialityImpact": "NOT_DEFINED",
          "modifiedSubIntegrityImpact": "NOT_DEFINED",
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            },
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    {
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      "url": "https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-5vp3-3cg6-2rq3"
    },
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      "source": "disclosure@vulncheck.com",
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}

GHSA-Q3JF-9WV7-P29W

Vulnerability from github – Published: 2026-08-23 15:33 – Updated: 2026-08-23 15:33

Details

Severity

6.1 (Medium)


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

5.1 (Medium)


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

Show details on source website

JSON

To clipboard

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  "affected": [],
  "aliases": [
    "CVE-2026-5389"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-80"
    ],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-08-23T14:16:53Z",
    "severity": "MODERATE"
  },
  "details": "justhtml versions before 1.13.0 contain a cross-site scripting vulnerability in the to_markdown() function when serializing attacker-controlled pre content. Attackers can place backticks inside sanitized pre elements to break out of fixed-length code fences, allowing raw HTML to execute when the generated Markdown is rendered by CommonMark or GFM-style renderers.",
  "id": "GHSA-q3jf-9wv7-p29w",
  "modified": "2026-08-23T15:33:04Z",
  "published": "2026-08-23T15:33:04Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-5vp3-3cg6-2rq3"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-5389"
    },
    {
      "type": "WEB",
      "url": "https://www.vulncheck.com/advisories/justhtml-before-xss-via-code-fence-breakout"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    },
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/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",
      "type": "CVSS_V4"
    }
  ]
}

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.

Detection rules are retrieved from Rulezet.

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.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.

Action not permitted

CVE-2026-5389 (GCVE-0-2026-5389)

FKIE_CVE-2026-5389

GHSA-Q3JF-9WV7-P29W

Tags

Sightings

Nomenclature