| Package | Affected versions | Patched versions |
|---|---|---|
| js-yaml | ≥ 3.0.0, < 3.15.1≥ 4.0.0, < 4.3.1 | 3.15.14.3.1 |
!!omap resolution (js-yaml 3.x and 4.x)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.
| 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.
lib/type/omap.js (js-yaml 4.3.0):
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):
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carrier.seen.add(key)
// 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`);
}
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.
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.
Mirror the 5.x fix — replace the linear scan with a Set:
// 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.
lib/type/omap.js (3.x, 4.x) — the affected resolverlib/schema/default.js — registers !!omap in the default schemaFound 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.