Mistune: Prior to 3.2.1, HTMLRenderer.heading() builds the opening <hN> tag by string-concatenating the id attribute
Affects mistuneSource GCVE Database
Not known to be exploited · CVSS 6.1 Medium · fixed in 3.2.1 : update mistune when convenient.
Data as of 9 Nov 2026, 14:18 UTC
- Patch
- Available – fixed in 3.2.1
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:NExploitation evidence
Disclosed: evidence found.
CVE published 26 May 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors
A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 9 Nov 2026.
What to do
Affected software
| Product | Affected | Fixed in |
|---|---|---|
Ubuntu 26.10mistunepkg:deb/ubuntu/mistune?arch=source&distro=stonkingOther threats | Affected
| Fixed in– |
Ubuntu 26.10mistune0pkg:deb/ubuntu/mistune0?arch=source&distro=stonkingOther threats | Affected
| Fixed in– |
Ubuntu 26.04 LTSmistunepkg:deb/ubuntu/mistune?arch=source&distro=resoluteOther threats | Affected
| Fixed in– |
Ubuntu 25.10mistunepkg:deb/ubuntu/mistune?arch=source&distro=questingOther threats | Affected
| Fixed in– |
Ubuntu 25.10mistune0pkg:deb/ubuntu/mistune0?arch=source&distro=questingOther threats | Affected
| Fixed in– |
Ubuntu 24.04 LTSmistunepkg:deb/ubuntu/mistune?arch=source&distro=nobleOther threats | Affected
| Fixed in– |
Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.
AI-suggested
Upgrade Mistune to version 3.2.1 or later, where this vulnerability is fixed. No other mitigation is required as the fix properly sanitizes the id attribute in the HTMLRenderer.heading() method.
Description
Mistune versions prior to 3.2.1 have a vulnerability in the HTMLRenderer.heading() function where the id attribute is concatenated directly into the HTML without sanitization. This allows an attacker to inject arbitrary additional attributes into heading elements by including a double-quote character in the id value. The issue is fixed in version 3.2.1.