A denial of service vulnerability exists in the Footnote extension of league/commonmark from version 1.5.0 up to but not including 2.9.0. The vulnerability arises because the extension appends the entire list of backreferences for every footnote definition without deduplication, causing quadratic growth in output size, parse time, and memory usage when duplicate footnote definitions are present. An unauthenticated attacker can exploit this by submitting a crafted document that expands a small input (~10 KB) into a very large output (~62 MB) and high memory consumption (~440 MB), potentially causing a PHP worker to be killed due to out-of-memory conditions. This impacts availability only, with no confidentiality or integrity effects. The Footnote extension must be enabled for the vulnerability to be exploitable.