Update npm package js-yaml to v4.2.0 [SECURITY]

[hash-worker]

This PR contains the following updates:

Package	Change	Age	Confidence
js-yaml	4.1.1 → 4.2.0

---

Warning

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GitHub Vulnerability Alerts

CVE-2026-53550

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);
  }
}

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Release Notes

nodeca/js-yaml (js-yaml)

v4.2.0

Compare Source

Added

• Added docs/safety.md with notes about processing untrusted YAML.
• Added maxDepth (100) loader option. Not a problem, but gives a better
  exception instead of RangeError on stack overflow.
• Added a loader option limiting merge sequence length. Not a problem after
  merge fix, but an additional restriction for safety.
• Added sourcemaps to dist/ builds.

Changed

• Stop resolving numbers with underscores as numeric scalars, #​627.
• Switched dev toolchains to Vite / neostandard.
• Updated demo.
• Reorganized tests.
• dist/ files are no longer kept in the repository.

Fixed

• Fix parsing of properties on the first implicit block mapping key, #​62.
• Fix trailing whitespace handling when folding flow scalar lines, #​307.
• Reject top-level block scalars without content indentation, #​280.
• Ensure numbers survive round-trip, #​737.
• Fix test coverage for issue #​221.
• Fix flow scalar trailing whitespace folding, #​307.
• Fix digits in YAML named tag handles.

Security

• Fix potential DoS via quadratic complexity in merge - deduplicate repeated
  elements (makes sense for malformed files > 10K).

---

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Project	Deployment	Actions	Updated (UTC)
hash	Error		Jun 29, 2026 5:59pm

2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
hashdotdesign-tokens	Ignored	Preview	Jun 29, 2026 5:59pm
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⚠️ Artifact update problem

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The artifact failure details are included below:

File name: yarn.lock

error This project's package.json defines "packageManager": "yarn@4.16.0". However the current global version of Yarn is 1.22.22.

Presence of the "packageManager" field indicates that the project is meant to be used with Corepack, a tool included by default with all official Node.js distributions starting from 16.9 and 14.19.
Corepack must currently be enabled by running corepack enable in your terminal. For more information, check out https://yarnpkg.com/corepack.

[cursor]

PR Summary

Low Risk
Dependency-only bump with no code changes; main residual risk is subtle YAML parsing behavior changes in packages that load YAML (e.g. repo skill scripts, API email dump).

Overview
Bumps js-yaml from 4.1.1 to 4.2.0 in @apps/hash-api, @local/repo-chores, and @tests/hash-playwright. No application source changes—only dependency version pins.

This addresses CVE-2026-53550 (DoS via quadratic merge-key/<< alias processing). 4.2.0 also adds loader safety options (maxDepth, merge sequence limits) and minor parsing fixes that could affect edge-case YAML if those code paths parse untrusted input.

^{Reviewed by Cursor Bugbot for commit 880c8fc. Bugbot is set up for automated code reviews on this repo. Configure here.}