xmldom: End-tag Whitespace-Trim Regex ReDoS — quadratic backtracking in the 0.8.x end-tag parser

Summary

On the @xmldom/xmldom 0.8.x line, parsing an XML end tag whose name is followed by a long run of whitespace and then a non-whitespace character triggers quadratic-time regular-expression backtracking (ReDoS), so a single small crafted end tag stalls the Node.js event loop. It is reachable from DOMParser.parseFromString under default options, unauthenticated, before any validity check — an availability-only denial of service. The 0.9.x line is not affected.

Details

lib/sax.js (release-0.8.x, commit e5c1480) trims trailing whitespace from a captured end-tag name with an unanchored global regex:

• lib/sax.js line 120: https://github.com/xmldom/xmldom/blob/e5c14802592685bb872c042c54c3f73758875c85/lib/sax.js#L120

/[ \t\n\r]+$/g

Applied to a string shaped whitespace-run + one non-whitespace char (e.g. the content of an end tag </ … x>), the engine must, for every starting position, extend [ws]+ to the end and then fail the $ anchor when the trailing non-whitespace char is present — classic O(n²) backtracking in the length of the whitespace run. The trimmed substring is delimited only by indexOf('>'), so the attacker controls its length directly.

Proof of Concept

const { DOMParser } = require('@xmldom/xmldom'); // 0.8.x
const n = 64 * 1024;
const payload = '<r></' + ' '.repeat(n) + 'x>';
console.time('parse');
new DOMParser().parseFromString(payload, 'text/xml');
console.timeEnd('parse');

Measured (Node 18) — time quadruples per doubling of the whitespace run (canonical O(n²)):

| Whitespace run | Isolated regex | End-to-end parseFromString (0.8.13) | |---|---|---| | 4 KB | 5.6 ms | 5.7 ms | | 8 KB | 22.7 ms | 22.5 ms | | 16 KB | 88.6 ms | 92 ms | | 32 KB | 354 ms | 361 ms | | 64 KB | 1434 ms | 1452 ms | | 128 KB | 5761 ms | — |

Impact

Availability only: a single parse of a small crafted document blocks the Node.js event loop for the duration of the quadratic scan (≈1.4 s at 64 KB; multi-second with larger inputs). No memory blow-up, no data exposure, no integrity impact. Because XML is routinely accepted from untrusted sources and parsed with default options, one request can stall a server.

Affected Versions

Affected on the 0.7.x and 0.8.x lines (the trailing-whitespace trim was added in 0.7.0, present through 0.8.14); the fix targets the 0.8.x LTS patch. The 0.9.x line rewrote end-tag parsing to an anchored linear matcher and never had this regex, so it is not affected. No published unscoped xmldom is affected — the vulnerable code exists only in a 0.7.0 git tag that was never released to npm (npm view xmldom → latest = 0.6.0).

Fix Applied

Anchors the end-tag trailing-whitespace trim so it runs in linear time instead of backtracking quadratically on a long whitespace run. Byte-identical output. Non-breaking; 0.8.x-only.

Severity note

The complexity is quadratic, not exponential, so a multi-second stall requires tens-to-hundreds of KB of input. VA:H reflects that xmldom applies no input-size limit and the path runs on default-options parsing, so a single unbounded parse can fully stall the event loop.