GHSA-h67p-54hq-rp68 — JS-YAML: Quadratic-complexity DoS in merge key handling via repeated aliases

Affected: npm: js-yaml

### Summary A crafted YAML document can trigger algorithmic CPU exhaustion in `js-yaml` merge-key processing (`<<`) by repeating the same alias many times in a merge sequence. This causes quadratic parse-time behavior relative to input size and can block a Node.js worker/event loop for seconds with a relatively small payload (tens of KB), resulting in denial of service.

### Details The issue is in merge handling inside `lib/loader.js`:

- `storeMappingPair(...)` iterates every element of a merge sequence when key tag is `tag:yaml.org,2002:merge`. - For each element, it calls `mergeMappings(...)`. - `mergeMappings(...)` computes `Object.keys(source)` and performs `_hasOwnProperty.call(destination, key)` checks for each key.

When input is of the form:

a: &a {k0:0, k1:0, ..., kK:0} b: {<<: [*a, *a, *a, ... repeated M times ...]} all *a entries refer to the same anchored object. After the first merge, subsequent merges are semantically no-ops, but the parser still reprocesses all keys each time. Resulting work is O(K * M), while input size is O(K + M), giving quadratic scaling as payload grows. Relevant code path: lib/loader.js in storeMappingPair(...) merge branch (keyTag === 'tag:yaml.org,2002:merge') lib/loader.js mergeMappings(...)

### Root cause File: lib/loader.js Function: storeMappingPair(state, _result, overridableKeys, keyTag, keyNode, valueNode, startLine, startLineStart, startPos) Lines: ~359-366

if (keyTag === 't…

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GHSA-h67p-54hq-rp68 — JS-YAML: Quadratic-complexity DoS in merge key handling via repeated aliases