xmldom is a pure JavaScript W3C standard-based (XML DOM Level 2 Core) DOMParser and XMLSerializer module. Prior to @xmldom/xmldom versions 0.8.15 and 0.9.12, and in xmldom version 0.6.0 and earlier, Document.createProcessingInstruction(target, data) in lib/dom.js accepts an unvalidated target, while the requireWellFormed: true serializer checks only for a colon and the reserved case-insensitive xml name on 0.9.x and performs no target check on 0.8.x. Because serialization emits <?target data?>, a target containing >, ?, whitespace, or another invalid XML-name character can break the processing-instruction boundary and inject XML structure. This issue is fixed in @xmldom/xmldom versions 0.8.15 and 0.9.12; no fixed version is available for xmldom.

Affected: npm: @xmldom/xmldom, npm: xmldom

## Summary

`Document.createProcessingInstruction()` in `@xmldom/xmldom` performs no validation on the `target` parameter. The `requireWellFormed: true` serializer option validates only for `:` in the target and a case-insensitive `xml` prefix, but does not check for `>` characters. A `>` in the target breaks the processing instruction boundary (`<?...?>`), allowing injection of arbitrary content into the serialized XML output.

## Details

`Document.createProcessingInstruction(target, data)` at `lib/dom.js` around line 2413 accepts any string as the `target` parameter and stores it on the PI node without validation.

During serialization, the `requireWellFormed` code path (around line 3286) performs two checks on PI targets:

1. Rejects targets containing `:` (namespace prefix check) 2. Rejects targets matching `xml` case-insensitively (reserved prefix)

However, it does NOT validate that the target conforms to the XML Name production, and critically does NOT check for `>` characters. Since processing instructions are serialized as `<?target data?>`, a `>` in the target prematurely closes the PI, causing the remaining content to be interpreted as document content by any downstream XML parser.

### Root Cause

1. `createProcessingInstruction()` performs no validation on `target` 2. The serializer's `requireWellFormed` check is incomplete -- it only checks for `:` and `xml`, missing characters that break PI syntax (`>`, `?`, whitespace) 3. The serializer emits the target verb…

undici: undici: Denial of Service via unrequested WebSocket subprotocol encoding/asn1: golang: Go encoding/asn1: Denial of Service via excessive recursion in Unmarshal ip-address: ip-address: Server-Side Request Forgery via IPv4-mapped/NAT64 IPv6 address misclassification urllib: urllib: Credential leakage via cross-origin redirects net/http: golang: Go net/http: Unencrypted HTTP/2 connections vulnerable to Denial of Service html/template: golang: Go html/template: Cross-Site Scripting via pathological input encoding/xml: golang: Go: Denial of Service via XML decoding recursion depth issue net/url: golang: golang net/url: Denial of Service from quadratic complexity in path resolution crypto/tls: golang: Golang crypto/tls: Denial of Service via indefinite KeyUpdate messages nanoid: nanoid: Denial of Service via negative size input in non-secure module functions axios: axios: Denial of Service via uncontrolled recursion in form data processing pymdown-extensions: Pymdown-extensions: Denial of Service via Regular Expression Vulnerability brace-expansion: DoS via unbounded intermediate arrays, bypassing the CVE-2026-14257 mitigation ip-address: ip-address: Inconsistent IP address parsing leads to Server-Side Request Forgery (SSRF) and trust-boundary bypass nanoid: nanoid: Predictable ID generation due to integer overflow fast-uri: fast-uri: Server-Side Request Forgery via repeated hostname percent-decoding fast-uri: fast-uri: Host confusion via skipped IDN canonicalization fast-…

Affected: Ubuntu:20.04:LTS: node-xmldom, Ubuntu:22.04:LTS: node-xmldom, Ubuntu:24.04:LTS: node-xmldom, Ubuntu:26.04:LTS: node-xmldom

xmldom is a pure JavaScript W3C standard-based (XML DOM Level 2 Core) DOMParser and XMLSerializer module. Prior to @xmldom/xmldom versions 0.8.15 and 0.9.12, and in xmldom version 0.6.0 and earlier, Document.createProcessingInstruction(target, data) in lib/dom.js accepts an unvalidated target, while the requireWellFormed: true serializer checks only for a colon and the reserved case-insensitive xml name on 0.9.x and performs no target check on 0.8.x. Because serialization emits <?target data?>, a target containing >, ?, whitespace, or another invalid XML-name character can break the processing-instruction boundary and inject XML structure. This issue is fixed in @xmldom/xmldom versions 0.8.15 and 0.9.12; no fixed version is available for xmldom.