CVE-2026-53550: JS-YAML Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-h67p-54hq-rp68",
  "modified" : "2026-06-29T15:05:57Z",
  "published" : "2026-06-15T17:15:07Z",
  "aliases" : [ "CVE-2026-53550" ],
  "summary" : "JS-YAML: Quadratic-complexity DoS in merge key handling via repeated aliases",
  "details" : "### Summary\nA 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.  \nThis 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.\n\n### Details\nThe issue is in merge handling inside `lib/loader.js`:\n\n- `storeMappingPair(...)` iterates every element of a merge sequence when key tag is `tag:yaml.org,2002:merge`.\n- For each element, it calls `mergeMappings(...)`.\n- `mergeMappings(...)` computes `Object.keys(source)` and performs `_hasOwnProperty.call(destination, key)` checks for each key.\n\nWhen input is of the form:\n\na: &a {k0:0, k1:0, ..., kK:0}\nb: {<<: [*a, *a, *a, ... repeated M times ...]}\nall *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.\nResulting work is O(K * M), while input size is O(K + M), giving quadratic scaling as payload grows.\nRelevant code path:\nlib/loader.js in storeMappingPair(...) merge branch (keyTag === 'tag:yaml.org,2002:merge')\nlib/loader.js mergeMappings(...)\n\n\n### Root cause\nFile:       lib/loader.js\nFunction:   storeMappingPair(state, _result, overridableKeys, keyTag, keyNode,\n                             valueNode, startLine, startLineStart, startPos)\nLines:      ~359-366\n\n    if (keyTag === 'tag:yaml.org,2002:merge') {\n      if (Array.isArray(valueNode)) {\n        for (index = 0, quantity = valueNode.length; index < quantity; index += 1) {\n          mergeMappings(state, _result, valueNode[index], overridableKeys);\n        }\n      } else {\n        mergeMappings(state, _result, valueNode, overridableKeys);\n      }\n    }\n\nWhen the merge value is a sequence (YAML 1.1 <<: [ *a, *a, ... ]), each element\nis handed to mergeMappings() without deduplication. mergeMappings() then does\n\n    sourceKeys = Object.keys(source);\n    for (index = 0; index < sourceKeys.length; index += 1) {\n      key = sourceKeys[index];\n      if (!_hasOwnProperty.call(destination, key)) {\n        setProperty(destination, key, source[key]);\n        overridableKeys[key] = true;\n      }\n    }\n\nEvery alias reference in the sequence resolves (by design) to the SAME object\nvia state.anchorMap. After the first merge, every subsequent merge of that same\nreference is a pure no-op semantically, but still performs:\n\n  * one Object.keys(source) call (O(K))\n  * K _hasOwnProperty.call checks on the destination\n\nTotal: M * K hasOwnProperty checks + M Object.keys allocations, while the final\nobject and all observable side effects are identical to a single merge.\n\nYAML semantics for `<<:` are idempotent and commutative over duplicate sources,\nso collapsing duplicates preserves behavior exactly; this isn't a spec trade-off.\n\n\n### PoC\nEnvironment:\njs-yaml version: 4.1.1\nNode.js: v24.5.0\nPlatform: arm64 macOS (reproduced consistently)\nReproduction script:\nCreate many keys in one anchored map (&a).\nMerge that same alias repeatedly via <<: [*a, *a, ...].\nMeasure parse time and compare with control payload using single merge (<<: *a).\nObserved repeated runs (same machine):\nK=M=1000, input 9,909 bytes: ~33–36 ms\nK=M=2000, input 20,909 bytes: ~121–123 ms\nK=M=4000, input 42,909 bytes: ~524–537 ms\nK=M=6000, input 64,909 bytes: ~1,608–1,829 ms\nK=M=8000, input 86,909 bytes: ~3,395–3,565 ms\nControl (single merge, similar key counts):\nK=2000: ~1–2 ms\nK=4000: ~3 ms\nK=8000: ~5 ms\nAlso verified: repeated-merge output equals single-merge output (same key count and same JSON), confirming excess time is redundant computation.\n\n\n### Impact\nThis is a denial-of-service vulnerability (CPU exhaustion / algorithmic complexity).\nAny 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.\n\n### Suggested fix:\nDedupe the merge source list by reference before invoking mergeMappings. Any of\nthe following are minimal and preserve YAML 1.1 merge semantics:\n\ndedupe in storeMappingPair:\n\n    if (keyTag === 'tag:yaml.org,2002:merge') {\n      if (Array.isArray(valueNode)) {\n        var seen = new Set();\n        for (index = 0, quantity = valueNode.length; index < quantity; index += 1) {\n          var src = valueNode[index];\n          if (seen.has(src)) continue;   // idempotent; skip redundant alias\n          seen.add(src);\n          mergeMappings(state, _result, src, overridableKeys);\n        }\n      } else {\n        mergeMappings(state, _result, valueNode, overridableKeys);\n      }\n    }",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "js-yaml"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.0.0"
      }, {
        "fixed" : "4.2.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 4.1.1"
    }
  }, {
    "package" : {
      "ecosystem" : "npm",
      "name" : "js-yaml"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.15.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/nodeca/js-yaml/security/advisories/GHSA-h67p-54hq-rp68"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-53550"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nodeca/js-yaml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-407" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-15T17:15:07Z",
    "nvd_published_at" : "2026-06-22T16:16:38Z"
  }
}