Content spoofing via decoded language whitespace
Published Dec 1, 2025 · Updated Dec 2, 2025
Improper input validation in syntax-tree mdast-util-to-hast 13.0.0 through 13.2.0 allows remote attackers to spoof rendered content. The code handler copies a character-reference-decoded node.lang value into one className entry, so embedded whitespace becomes extra unprefixed classes. Applications must render attacker-supplied Markdown and apply matching CSS or JavaScript; the result can disguise code as ordinary page content.
Summary
What happened
Improper input validation in syntax-tree mdast-util-to-hast 13.0.0 through 13.2.0 allows remote attackers to spoof rendered content. The code handler copies a character-reference-decoded node.lang value into one className entry, so embedded whitespace becomes extra unprefixed classes. Applications must render attacker-supplied Markdown and apply matching CSS or JavaScript; the result can disguise code as ordinary page content.
The record
- CVE
- CVE-2025-66400
- Published
- Dec 1, 2025
- Updated
- Dec 2, 2025
- Vendor
- syntax-tree
- Product
- mdast-util-to-hast
- Classifications
- CWE-915, CWE-20, T1036
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Dec 1, 2025CVE publishedPublication date reported by the CVE source.
- Dec 2, 2025Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=>= 13.0.0, < 13.2.1
What conditions does exploitation require?
What is affected?
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Upstream crafted Markdown reproducerproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
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Your stack
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