CVE-2026-121438
Mistune before 3.3.3 Denial of Service via Balanced Bracket Link Parsing
Mistune before 3.3.3 contains a denial of service vulnerability in InlineParser.parse_link that allows attackers to cause quadratic CPU growth by supplying balanced nested brackets. Attackers can submit crafted Markdown with thousands of nested bracket pairs not forming a valid link, triggering repeated _contains_nested_link scans that exhaust CPU in parsing applications.
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
- Low
Exploit likelihood
What each source reports
CVE List Score used
Mistune before 3.3.3 Denial of Service via Balanced Bracket Link Parsing
Mistune before 3.3.3 contains a denial of service vulnerability in InlineParser.parse_link that allows attackers to cause quadratic CPU growth by supplying balanced nested brackets. Attackers can submit crafted Markdown with thousands of nested bracket pairs not forming a valid link, triggering repeated _contains_nested_link scans that exhaust CPU in parsing applications.
- Record
CVE-2026-121438- CVSS 4.0
- 6.9 Medium
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N - CVSS 3.1
- 5.3 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L - Weakness
- CWE-400
- Updated
- 2026-11-07
EUVDEUVD-2026-115860
Mistune before 3.3.3 contains a denial of service vulnerability in InlineParser.parse_link that allows attackers to cause quadratic CPU growth by supplying balanced nested brackets. Attackers can submit crafted Markdown with thousands of nested bracket pairs not forming a valid link, triggering repeated _contains_nested_link scans that exhaust CPU in parsing applications.
- Record
EUVD-2026-115860- CVSS 4.0
- 6.9 Medium
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N - Updated
- 2026-11-07
GitHub AdvisoriesGHSA-296h-qmjr-9mv2
Mistune before 3.3.3 contains a denial of service vulnerability in InlineParser.parse_link that...
Mistune before 3.3.3 contains a denial of service vulnerability in InlineParser.parse_link that allows attackers to cause quadratic CPU growth by supplying balanced nested brackets. Attackers can submit crafted Markdown with thousands of nested bracket pairs not forming a valid link, triggering repeated _contains_nested_link scans that exhaust CPU in parsing applications.
- Record
GHSA-296h-qmjr-9mv2- CVSS 4.0
- 6.9 Medium
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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
- 5.3 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L - Weakness
- CWE-400
- Updated
- 2026-11-03
NVD
Mistune before 3.3.3 contains a denial of service vulnerability in InlineParser.parse_link that allows attackers to cause quadratic CPU growth by supplying balanced nested brackets. Attackers can submit crafted Markdown with thousands of nested bracket pairs not forming a valid link, triggering repeated _contains_nested_link scans that exhaust CPU in parsing applications.
- Record
CVE-2026-121438- CVSS 4.0
- 6.9 Medium
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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
- 5.3 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L - Weakness
- CWE-400
- Updated
- 2026-11-07
OSV
Mistune before 3.3.3 Denial of Service via Balanced Bracket Link Parsing
Mistune before 3.3.3 contains a denial of service vulnerability in InlineParser.parse_link that allows attackers to cause quadratic CPU growth by supplying balanced nested brackets. Attackers can submit crafted Markdown with thousands of nested bracket pairs not forming a valid link, triggering repeated _contains_nested_link scans that exhaust CPU in parsing applications.
- Record
CVE-2026-121438- CVSS 4.0
- 6.9 Medium
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N - Weakness
- CWE-400
- Updated
- 2026-11-07
OSVDEBIAN-CVE-2026-121438
Mistune before 3.3.3 contains a denial of service vulnerability in InlineParser.parse_link that allows attackers to cause quadratic CPU growth by supplying balanced nested brackets. Attackers can submit crafted Markdown with thousands of nested bracket pairs not forming a valid link, triggering repeated _contains_nested_link scans that exhaust CPU in parsing applications.
- Record
DEBIAN-CVE-2026-121438- CVSS 4.0
- 6.9 Medium
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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
- 2026-11-07
OSVUBUNTU-CVE-2026-121438
Mistune before 3.3.3 contains a denial of service vulnerability in InlineParser.parse_link that allows attackers to cause quadratic CPU growth by supplying balanced nested brackets. Attackers can submit crafted Markdown with thousands of nested bracket pairs not forming a valid link, triggering repeated _contains_nested_link scans that exhaust CPU in parsing applications.
- Record
UBUNTU-CVE-2026-121438- CVSS 4.0
- 6.9 Medium
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N - CVSS 3.1
- 5.3 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L - Updated
- 2026-11-05
OSVopenSUSE-SU-2026:12476-1 No score
python313-mistune-3.4.0-3.1 on GA media
These are all security issues fixed in the python313-mistune-3.4.0-3.1 package on the GA media of openSUSE Tumbleweed.
- Record
openSUSE-SU-2026:12476-1- Updated
- 2026-11-07
Affected products
Affected packages
| Package | Affected | Fixed |
|---|---|---|
mistuneDebian:12 |
All versions, plus listed versions 2.0.4-12.0.4-23.0.2-13.0.2-23.1.3-13.1.4-13.3.4-13.4.0-1 |
None known |
mistuneDebian:13 |
All versions, plus listed versions 3.1.3-13.1.4-13.3.4-13.4.0-1 |
None known |
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 |
python-mistuneopenSUSE:Tumbleweed |
before 3.4.0-3.1 | 3.4.0-3.1 |
History
| Recorded | Changed | Sources |
|---|---|---|
| affected, aliases, facts, references | EUVD, NVD, OSV | |
| references | CVE List | |
| affected, aliases, references | OSV | |
| affected, aliases, references | OSV | |
| aliases, references | OSV | |
| published | EUVD, NVD | |
| 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-2026-121438 is a medium vulnerability (CVSS 6.9) 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 0.29%, the 21st percentile. Fixed in python-mistune 3.4.0-3.1.
References
github.com/lepture/mistune/security/advisories/GHSA-47p6-w2rc-5qf7
vulncheck.com/advisories/mistune-before-3.3.3-denial-of-service-via-balanced-bracket-link-parsing
nvd.nist.gov/vuln/detail/CVE-2026-121438