js-yaml

## Summary `maxTotalMergeKeys` does not count empty mappings. An attacker can repeatedly merge a large sequence of them and consume significant CPU without reaching the configured limit. ## Example ```yaml arr: &arr [{}, {}, {}, ...] # N empty mappings targets: - <<: *arr # repeated K times ``` For every target, the loader iterates all `N` elements of `arr`. This results in `O(N * K)` work while `totalMergeKeys` remains unchanged. ## PoC ```js import { performance } from 'node:perf_hooks' import { load, YAML11_SCHEMA } from 'js-yaml' const n = 20000 const src = 'arr:

## Summary `maxTotalMergeKeys` does not count empty mappings. An attacker can repeatedly merge a large sequence of them and consume significant CPU without reaching the configured limit. ## Example ```yaml arr: &arr [{}, {}, {}, ...] # N empty mappings targets: - <<: *arr # repeated K times ``` For every target, the loader iterates all `N` elements of `arr`. This results in `O(N * K)` work while `totalMergeKeys` remains unchanged. ## PoC ```js import { performance } from 'node:perf_hooks' import { load, YAML11_SCHEMA } from 'js-yaml' const n = 20000 const src = 'arr:

CVE: CVE-2026-84375

# Quadratic CPU consumption in `!!omap` resolution (js-yaml 3.x and 4.x) ## Summary `resolveYamlOmap()` enforces key uniqueness for `!!omap` sequences with a linear scan (`objectKeys.indexOf(...)`) inside the per-element loop, making resolution **O(n²)** in the number of entries. A modestly sized YAML document therefore consumes disproportionate CPU inside `yaml.load()`, giving a denial of service against any consumer that parses untrusted YAML. `!!omap` is registered in the **default schema** (`lib/schema/default.js` → `require('../type/omap')`), so a plain `yaml.load(untrustedInput)` with

### 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, pot

CVE: CVE-2026-73643

### 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

CVE: CVE-2026-59869

### 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', {

CVE: CVE-2026-59870

### 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

CVE: CVE-2026-59868

### 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(

CVE: CVE-2026-53550

### 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.ht

CVE: CVE-2025-64718

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.