CVE-2026-59870

js-yaml is a JavaScript YAML parser and dumper. From 3.0.0 until 3.15.2, 4.3.2, and 5.4.1, maxTotalMergeKeys in lib/js-yaml/loader.js and lib/loader.js does not count empty mapping sources while processing the merge key <<. An attacker can alias a large sequence of empty mappings into many merge targets, causing O(N * K) processing while totalMergeKeys remains unchanged and the configured resource limit is never reached. A relatively small YAML document can therefore cause prolonged CPU consumption in applications that parse untrusted YAML, and merge processing is enabled by default on these release lines. In v3 & v4, merge is enabled by default so the severity score is higher. This issue is fixed in versions 3.15.2, 4.3.2, and 5.4.1.

js-yaml is a JavaScript YAML parser and dumper. From 5.0.0 until 5.2.2, parsing a small YAML document can take exponential time when an application calls load() or loadAll() on untrusted input. In src/parser/parser.ts, readFlowCollection uses restoreState and calls parseNode a second time when a flow-sequence entry is recognized as a key: value pair. If the key is a nested flow sequence of the same shape, every level is parsed twice, causing O(2^n) work and allowing an input under 200 bytes to keep one CPU busy for minutes, block the Node.js event loop, and stall the process. No anchors, aliases, merges, tags, or nondefault options are required. Fixed in version 5.2.2.

A vulnerability in the js-yaml library (JavaScript YAML parser and dumper) causes quadratic CPU time growth when processing YAML documents containing a chain of mappings with merge keys. The issue affects versions from 3.0.0 to 3.14.0 and from 4.0.0 to 4.2.0.

A vulnerability in the js-yaml library (versions 5.0.0 through 5.2.0) causes quadratic CPU time when parsing YAML documents with merge keys enabled. A chain of mappings where each entry merges the previous one leads to excessive CPU load.

A vulnerability in js-yaml (YAML parser for JavaScript) before versions 4.2.0 and 3.15.0 allows a DoS attack via a crafted YAML document that causes quadratic time complexity during merge-key (<<) processing. Repeatedly using the same alias in a merge sequence leads to CPU exhaustion and blocks the Node.js event loop for seconds with a relatively small payload (tens of KB).

js-yaml is a JavaScript YAML parser and dumper. In js-yaml before 4.1.1 and 3.14.2, it's possible for an attacker to modify the prototype of the result of a parsed yaml document via prototype pollution (`__proto__`). All users who parse untrusted yaml documents may be impacted. The problem is patched in js-yaml 4.1.1 and 3.14.2. Users can protect against this kind of attack on the server by using `node --disable-proto=delete` or `deno` (in Deno, pollution protection is on by default).