CVE-2026-121437
Mistune 3.3.0 before 3.3.4 Quadratic-Time DoS via _process_emphasis_delimiters
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.
Attack profile
How easy it is to exploit
- Attack vector
- Network
- Complexity
- Low
- Privileges needed
- None
- User interaction
- None
What an attacker can affect
- Confidentiality
- None
- Integrity
- None
- Availability
- High
Exploit likelihood
What each source reports
CVE List Score used
Mistune 3.3.0 before 3.3.4 Quadratic-Time DoS via _process_emphasis_delimiters
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.
- Record
CVE-2026-121437- CVSS 4.0
- 8.7 High
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N - CVSS 3.1
- 7.5 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H - Weakness
- CWE-407
- Updated
- 2026-11-03
EUVDEUVD-2026-115859
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.
- Record
EUVD-2026-115859- CVSS 4.0
- 8.7 High
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N - Updated
- 2026-11-03
GitHub AdvisoriesGHSA-xr4w-hw52-gjv9
Mistune from 3.3.0 before 3.3.4 contains a denial of service vulnerability that allows remote...
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.
- Record
GHSA-xr4w-hw52-gjv9- CVSS 4.0
- 8.7 High
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/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 - CVSS 3.1
- 7.5 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H - Weakness
- CWE-407
- Updated
- 2026-11-03
NVD
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.
- Record
CVE-2026-121437- CVSS 4.0
- 8.7 High
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/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 - CVSS 3.1
- 7.5 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H - Weakness
- CWE-407
- Updated
- 2026-11-03
OSV
Mistune 3.3.0 before 3.3.4 Quadratic-Time DoS via _process_emphasis_delimiters
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.
- Record
CVE-2026-121437- CVSS 4.0
- 8.7 High
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N - Weakness
- CWE-407
- Updated
- 2026-11-05
OSVDEBIAN-CVE-2026-121437
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.
- Record
DEBIAN-CVE-2026-121437- CVSS 4.0
- 8.7 High
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/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 - Updated
- 2026-11-04
OSVUBUNTU-CVE-2026-121437
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.
- Record
UBUNTU-CVE-2026-121437- CVSS 4.0
- 8.7 High
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N - CVSS 3.1
- 7.5 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H - Updated
- 2026-11-05
Affected products
Affected packages
| Package | Affected | Fixed |
|---|---|---|
mistuneDebian:14 |
All versions, plus listed versions 3.1.3-13.1.4-13.3.4-13.4.0-1 |
None known |
mistuneUbuntu:16.04:LTS |
All versions, plus listed versions 0.6-20.7.1-1 |
None known |
mistuneUbuntu:18.04:LTS |
All versions, plus listed versions 0.7.4-10.8-10.8.1-10.8.3-2 |
None known |
mistuneUbuntu:20.04:LTS |
All versions, plus listed versions 0.8.4-10.8.4-2 |
None known |
mistuneUbuntu:22.04:LTS |
All versions, plus listed versions 0.8.4-40.8.4-52.0.0-1+really0.8.4-1 |
None known |
mistuneUbuntu:24.04:LTS |
All versions, plus listed versions 2.0.4-23.0.2-1 |
None known |
mistuneUbuntu:26.10 |
All versions, plus listed versions 3.1.4-1 |
None known |
mistuneUbuntu:26.04:LTS |
All versions, plus listed versions 3.1.3-13.1.4-1 |
None known |
History
| Recorded | Changed | Sources |
|---|---|---|
| affected, aliases, references | OSV | |
| affected, aliases, references | OSV | |
| aliases, references | NVD, OSV | |
| published | EUVD, NVD | |
| references | CVE List | |
| facts, published | EUVD | |
| aliases, published | EUVD | |
| first indexed | CVE List, GitHub Advisories |
Dates are when Every CVE recorded each change. Upstream dates may differ.
In short
CVE-2026-121437 is a high vulnerability (CVSS 8.7) in Lepture Mistune, published November 3, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.62%, the 49th percentile. No source names a fixed version.
References
github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9
vulncheck.com/advisories/mistune-3.3.0-before-3.3.4-quadratic-time-dos-via-process-emphasis-delimiters
nvd.nist.gov/vuln/detail/CVE-2026-121437
security-tracker.debian.org/tracker/CVE-2026-121437
ubuntu.com/security/CVE-2026-121437
cve.org/CVERecord?id=CVE-2026-121437