Junglewise Threat Intelligence

CVE-2026-83606: xmldom PI grammar regex ReDoS in unterminated processing instructions

CVE-2026-83606 · Severity: high · CVSS 8.7 · Published 2026-09-08

Executive brief

@xmldom/xmldom is a popular XML parsing library used to read and process XML documents in Node.js applications. The library contains a regular expression vulnerability that causes the parser to consume excessive CPU time when processing maliciously crafted XML with unterminated processing instructions. An attacker can send a single small XML document to a server to stall the Node.js event loop for multiple seconds, causing denial of service and preventing the server from handling legitimate requests.

Technical details

The vulnerability is a ReDoS (Regular Expression Denial of Service) affecting the PI (processing instruction) production in lib/grammar.js. The regex pattern uses both a greedy separator (S+) and a lazy quantifier (Char*?) that can both match XML whitespace. When parsing an unterminated processing instruction (missing the closing `?>`), the regex engine must try every partition of the whitespace between these two quantifiers before ultimately failing, resulting in O(n²) time complexity. The regex is invoked in parsePI() and parseProcessingInstruction() functions in lib/sax.js, executing against the entire remaining source string. Only versions 0.9.0-beta.9 through 0.9.10 are affected; versions 0.8.x and earlier use a different bounded PI parsing approach. The fix in 0.9.11 inserts a fixed-width negative lookahead (?!\\s) after the greedy S+ to prevent backtracking while maintaining correct behavior on valid XML.

Affected products

  • xmldom @xmldom/xmldom 0.9.0-beta.9 through 0.9.10

Timeline

  • 2026-08-12: disclosed
  • 2026-09-08: patched: Fixed in version 0.9.11 via PR #1039

References

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