CVE-2026-59868
Description
A flaw was found in js-yaml, a JavaScript YAML parser and dumper. When merge keys are enabled, a remote attacker could exploit this vulnerability by crafting a YAML document where a chain of mappings uses merge keys, each merging the previous one. This can lead to a significant increase in CPU time required to parse the document, resulting in a Denial of Service (DoS) for the affected system.
Statement
js-yaml versions 5.0.0 through 5.1.x are vulnerable to a denial of service. When merge keys ('<<') are enabled during YAML parsing, js-yaml re-merges the accumulated mapping into each subsequent mapping in a chain. A document consisting of a linear chain of mappings that each merge the previous one causes quadratic (O(n^2)) CPU consumption relative to input size, even though the document size itself grows only linearly. This can be triggered via yaml.load() on untrusted YAML input that uses merge keys. Fixed in js-yaml 5.2.0.
Mitigation
Upgrade to js-yaml 5.2.0 or later. Where an immediate upgrade is not possible, avoid parsing untrusted YAML documents that use merge keys ('<<'), or apply size and time limits when parsing untrusted YAML input.
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).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 7.5 | 7.5 | 5.3 |
| Attack Vector | Network | Network | Network |
| Attack Complexity | Low | Low | Low |
| Privileges Required | None | None | None |
| User Interaction | None | None | None |
| Scope | Unchanged | Unchanged | Unchanged |
| Confidentiality | None | None | None |
| Integrity Impact | None | None | None |
| Availability Impact | High | High | Low |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
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)
Attackers can create crafted inputs that intentionally cause the regular expression to use excessive backtracking in a way that causes the CPU consumption to spike.
Frequently Asked Questions
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