js-yaml is a JavaScript YAML parser and dumper. From 3.0.0 until 3.15.2 and 4.3.2, 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. This issue is fixed in versions 3.15.2 and 4.3.2.

A denial-of-service vulnerability exists in the js-yaml JavaScript YAML parser affecting versions 3.0.0 through 3.15.1 and 4.0.0 through 4.3.1. The flaw lies in the maxTotalMergeKeys limit implementation in loader.js, which fails to count empty mapping sources when processing the YAML merge key '<<'. An attacker can craft a small malicious YAML document that aliases large sequences of empty mappings into many merge targets, triggering O(N * K) CPU processing while the configured resource limit counter remains unchanged. This allows prolonged CPU consumption in applications parsing untrusted YAML input, effectively causing a denial-of-service condition. Merge processing is enabled by default on affected release lines, broadening the attack surface. The vulnerability has been patched in versions 3.15.2 and 4.3.2. Users are strongly advised to upgrade to the fixed versions immediately.