CVSS 4.0
8.7 highCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:NCVE-2026-121437
High (8.7). EPSS 0.6% estimated probability of exploitation activity in the next 30 days.
On this page
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.
Severity
CVSS 3.1
7.5 highCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H- EPSS
- 0.6% estimated probability of exploitation activity in the next 30 days; 50th percentile. Scored 2026-11-04. A forecast for this CVE across the internet, not for your own systems.
- CISA's assessment (SSVC)
- Exploitation: PoC · Automatable: no · Technical impact: partial, assessed 2026-11-03. From CISA's Vulnrichment in the CVE List: whether it is exploited, whether exploiting it can be automated, and how much control it gives.
- CWE
- CWE-407 Inefficient Algorithmic Complexity
- Published
- Modified
- Record from
- the NVD, synced
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Explain CVE-2026-121437 for an engineer deciding whether to act. Use https://cve.bz/api/public/cves/CVE-2026-121437?via=ask-ai-copy and https://cve.bz/cve/CVE-2026-121437?via=ask-ai-copy as the source of truth: what is affected and which versions fix it, whether it is in CISA KEV, public exploits, and its EPSS (an internet-wide 30-day estimate of exploitation activity, not a forecast for my systems). If they give no fixed versions, use the CVE record https://cveawg.mitre.org/api/cve/CVE-2026-121437 or the vendor advisory, not third-party mirrors. Tell me how to check whether I am affected and how to verify a fix. Cite the sources and say where they are uncertain.Opens a third-party assistant; its answer is its own, not cve.bz's. The prompt holds only this CVE's id and public links.
Versions
From the CNA's record in the CVE List.
| Product | Affected |
|---|---|
| lepture mistune | from 3.3.0 before 3.3.4 |
Linux distributions
| Distribution | Release | Package | Fixed in | Advisory |
|---|---|---|---|---|
| Debian | 14 | mistune | no fix listed | DEBIAN-CVE-2026-121437 |
From the distributions' data via OSV.dev: the Debian Security Tracker. Modified: reformatted and linked to CVE records. Credits. On the distributions' own pages: Debian.
Red Hat
Red Hat rates this Important.
| Product | Package | State | Advisory |
|---|---|---|---|
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-feature-server-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-pipeline-runtime-datascience-cpu-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-pipeline-runtime-minimal-cpu-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-pipeline-runtime-pytorch-cuda-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-pipeline-runtime-pytorch-llmcompressor-cuda-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-pipeline-runtime-pytorch-rocm-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-pipeline-runtime-tensorflow-cuda-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-pipeline-runtime-tensorflow-rocm-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-th06-cpu-torch210-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-th06-cpu-torch291-py312-rhel9 | Not affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-th06-cuda130-torch210-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-th06-cuda130-torch291-py312-rhel9 | Not affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-th06-rocm64-torch291-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-th-torch-cpu-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-th-torch-cuda-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-th-torch-rocm-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-workbench-codeserver-datascience-cpu-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-workbench-jupyter-datascience-cpu-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-workbench-jupyter-minimal-cpu-py312-rhel9 | Affected | |
| Red Hat OpenShift AI (RHOAI) | rhoai/odh-workbench-jupyter-minimal-cuda-py312-rhel9 | Affected | |
| 8 more in the JSON and on Red Hat's page. | |||
Contains Red Hat Security Data © Red Hat, Inc., licensed under CC BY 4.0. Source: https://access.redhat.com/security/data. Modified: reformatted. Red Hat's page for CVE-2026-121437.