The league/commonmark library's XmlRenderer component can cause a denial of service via excessive memory and output size when rendering deeply nested XML. This occurs because indentation whitespace grows quadratically with the depth of the XML tree, which can be triggered by deeply nested Markdown blockquotes or attacker-influenced ASTs. The vulnerability affects all 2.x versions from 2.0.0 up to but not including 2.9.0. There is no impact on confidentiality or integrity, only availability due to resource exhaustion. Mitigations include lowering the parser's max_nesting_level, capping input size, constraining XML consumers, or preferring HTML rendering for untrusted content. A patch is available.