js-yaml: Exponential parsing time in flow collections leads to denial of service

### 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.

Source published

24 Jul 2026 16:47:36 UTC

Source modified

10 Sep 2026 03:51:11 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Publisher rating · HIGH