CVE-2026-121438
Description
A flaw was found in mistune. A remote attacker could cause a Denial of Service (DoS) by submitting specially crafted Markdown input containing deeply nested brackets. Because the link parser performs repetitive scans when evaluating these nested brackets, processing the crafted input consumes excessive processor resources and can cause the parsing application to hang.
Statement
This vulnerability is rated as having a Moderate impact because an attacker can induce high CPU utilization and application unresponsiveness, but the flaw does not allow unauthorized code execution, privilege elevation, or data leakage. In Red Hat OpenShift and Red Hat OpenShift AI environments, exposure is limited to workloads and applications that parse untrusted, user-supplied Markdown. The blast radius is isolated to the container or parsing process handling the malformed input without affecting host node stability.
Mitigation
Upgrade mistune to 3.3.3 or later (fix commit fe02f40), which bounds nested-bracket link scanning in InlineParser.parse_link. Workaround: limit accepted Markdown input size/nesting depth, or parse untrusted Markdown in an isolated worker with resource limits.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 5.3 | N/A | 5.3 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | Low | N/A | Low |
| Privileges Required | None | N/A | None |
| User Interaction | None | N/A | None |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | None | N/A | None |
| Integrity Impact | None | N/A | None |
| Availability Impact | Low | N/A | Low |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
cve.org: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Resource Consumption (CPU)
Frequently Asked Questions
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