org.xwiki.commons:xwiki-commons-xml's HTML sanitizer allows form elements in restricted

### Impact The HTML sanitizer that is included in XWiki since version 14.6RC1 allowed form and input HTML tags. In the context of XWiki, this allows an attacker without script right to either create forms that can be used for phishing attacks or also in the context of a sheet, the attacker could add an input like `{{html}}<input type="hidden" name="content" value="{{groovy}}println(&quot;Hello from Groovy!&quot;)" />{{/html}}` that would allow remote code execution when it is submitted by an admin (the sheet is rendered as part of the edit form). The attacker would need to ensure that the edit form looks plausible, though, which can be non-trivial as without script right the attacker cannot display the regular content of the document. ### Patches This has been patched in XWiki 14.10.6 and 15.2RC1 by removing the central form-related tags from the list of allowed tags. ### Workarounds An admin can manually disallow the tags by adding `form, input, select, textarea, button` to the configuration option `xml.htmlElementSanitizer.forbidTags` in the `xwiki.properties` configuration file. ### References * https://jira.xwiki.org/browse/XCOMMONS-2634 * https://github.com/xwiki/xwiki-commons/commit/99484d48e899a68a1b6e33d457825b776c6fe8c3

Quoted source text, attributed separately from HOL analysis.