Mistune 3.3.0 before 3.3.4 Quadratic-Time DoS via _process_emphasis_delimiters
Mistune 3.3.0 before 3.3.4 is affected through quadratic-time emphasis delimiter processing. The fix is in Mistune 3.3.4.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- Proof of concept public, per CISA
- Fix
- Fixed in 3.3.4
- Who is exposed
- Default setups
Do first: Check every install: default setups are affected.
- Exploitation15/40CISA rates a proof of concept as public
- Exposure22/25Default setups, reachable over the network without a login
- Reach?/20Reach: unknown (data gap). No tracked technology or package matched
- Severity8.7/10CVSS 8.7
- No patch0/5No signal
What would change the score
- Would score about 71 if added to CISA KEV
- Would score about 66 if CISA's SSVC rates exploitation active
CVE-2026-121437 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.7 (high) and affects mistune; fixed in 3.3.4.
- Last modified
- Where the record stands: published, rejected by its publisher, or superseded by another id.What do these mean? →
- Published
- Whether CISA lists it in its Known Exploited Vulnerabilities catalog: proof that attackers are using it.What do these mean? →
- Not listed in CISA KEV
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 46/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Default setups
- Fix
- Fixed in 3.3.4
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
- Exploitation: poc
- Automatable: no
- Technical impact: partial
Summary
Mistune from 3.3.0 before 3.3.4 contains a denial of service vulnerability that allows remote attackers to exhaust CPU through quadratic-time emphasis delimiter processing. Attackers can submit small markdown documents with repeated patterns like "*a_b" to trigger an O(n²) index-fixup loop, stalling workers for seconds.
What to do
Check every install: default setups are affected.
Upgrade to a fixed version: 3.3.4.
Timeline
- CVE published
- First seen by cve.now
- CISA SSVC assessment
Exploitation: poc · Automatable: no · Technical impact: partial
- Record last updated
Affected versions
- lepture mistune
From 3.3.0 · fixed in 3.3.4
Evidence 3
- Advisory
- Patch
- Link
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-121437
Is CVE-2026-121437 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. CISA's SSVC assessment notes a proof of concept exists. No public exploit code is tracked.Source: cve.now feed data
What does this mean?
Emphasis delimiter processing in Mistune takes quadratic time, so crafted Markdown input can drive CPU use high and stall processing for seconds, which is a denial of service.Source: github.com, vulncheck.com
Which versions are affected by CVE-2026-121437?
mistune: from 3.3.0 before 3.3.4.Source: cve.now feed data
Am I affected, and how do I check?
Mistune versions 3.3.0 up to but not including 3.3.4 are affected. The reported behavior was measured with the default renderer and no plugins. To check, look at the installed Mistune version, for example with pip show mistune.Source: github.com, vulncheck.com
How do I fix CVE-2026-121437?
Upgrade to Mistune 3.3.4 or later, which the vendor states fixes the issue. No workaround is listed in the facts.Source: github.com, vulncheck.com