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 === 'tag:yaml.org,2002:merge') { if (Array.isArray(valueNode)) { for (index = 0, quantity = valueNode.length; index < quantity; index += 1) { mergeMappings(state, _result, valueNode[index], overridableKeys); } } else { mergeMappings(state, _result, valueNode, overridableKeys); } } When the merge value is a sequence (YAML 1.1 <<: [ *a, *a, ... ]), each element is handed to mergeMappings() without deduplication. mergeMappings() then does sourceKeys = Object.keys(source); for (index = 0; index < sourceKeys.length; index += 1) { key = sourceKeys[index]; if (!_hasOwnProperty.call(destination, key)) { setProperty(destination, key, source[key]); overridableKeys[key] = true; } } Every alias reference in the sequence resolves (by design) to the SAME object via state.anchorMap. After the first merge, every subsequent merge of that same reference is a pure no-op semantically, but still performs: * one Object.keys(source) call (O(K)) * K _hasOwnProperty.call checks on the destination Total: M * K hasOwnProperty checks + M Object.keys allocations, while the final object and all observable side effects are identical to a single merge. YAML semantics for "<<:" are idempotent and commutative over duplicate sources, so collapsing duplicates preserves behavior exactly; this isn't a spec trade-off. PoC Environment: js-yaml version: 4.1.1 Node.js: v24.5.0 Platform: arm64 macOS (reproduced consistently) Reproduction script: Create many keys in one anchored map (&a). Merge that same alias repeatedly via <<: [*a, *a, ...]. Measure parse time and compare with control payload using single merge (<<: *a). Observed repeated runs (same machine): K=M=1000, input 9,909 bytes: ~33–36 ms K=M=2000, input 20,909 bytes: ~121–123 ms K=M=4000, input 42,909 bytes: ~524–537 ms K=M=6000, input 64,909 bytes: ~1,608–1,829 ms K=M=8000, input 86,909 bytes: ~3,395–3,565 ms Control (single merge, similar key counts): K=2000: ~1–2 ms K=4000: ~3 ms K=8000: ~5 ms Also verified: repeated-merge output equals single-merge output (same key count and same JSON), confirming excess time is redundant computation. Impact This is a denial-of-service vulnerability (CPU exhaustion / algorithmic complexity). Any service parsing untrusted YAML with js-yaml can be impacted, including API backends, CI tools, config processors, and automation services. An attacker can submit crafted YAML to significantly increase CPU time and reduce availability. Suggested fix: Dedupe the merge source list by reference before invoking mergeMappings. Any of the following are minimal and preserve YAML 1.1 merge semantics: dedupe in storeMappingPair: if (keyTag === 'tag:yaml.org,2002:merge') { if (Array.isArray(valueNode)) { var seen = new Set(); for (index = 0, quantity = valueNode.length; index < quantity; index += 1) { var src = valueNode[index]; if (seen.has(src)) continue; // idempotent; skip redundant alias seen.add(src); mergeMappings(state, _result, src, overridableKeys); } } else { mergeMappings(state, _result, valueNode, overridableKeys); } }