PostCSS: Quadratic complexity in flat selector parsing allows CPU exhaustion

### Impact `.` and `#` are not word delimiters in the tokenizer, so a flat selector such as `.a.a.a...` reaches `splitWord()` as a single word token carrying n class or id indexes. Three passes scanned those index arrays linearly for every index, making the parse O(n^2) in the number of indexes rather than in input length: `uniqs()`, the `indices.forEach` loop, and the Sass-interpolation filter. Parsing a 400 KB flat selector took ~34 s on a modern laptop, fully occupying a single thread. A benign selector of identical byte size parses in tens of milliseconds, so the cost is driven by the index count, not the input size. The nesting depth of such a selector is 0, so the `maxNestingDepth` guard added in 7.1.3 offers no protection. Reachability is deployment dependent. Only consumers that parse untrusted, attacker-supplied selectors synchronously in a request path are exposed, for example CSS sanitizers, CSS-in-JS services and online playgrounds. Ordinary build-time use on trusted sources is not affected. ### Patches Fixed in 7.1.6. The three passes now use Set membership tests, making parsing linear in the number of indexes. There is no behaviour change: parsing is byte-identical on a differential corpus of 8413 selectors. ### Workarounds Cap the size of selectors accepted from untrusted sources before parsing.

Source published

5 Oct 2026 22:53:54 UTC

Source modified

26 Oct 2026 14:16:59 UTC

Observed here

26 Oct 2026 15:35:41 UTC

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GitHub Advisory Database · CC-BY-4.0

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

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