CVE-2021-43827: Inline footnotes wrapped in <a> tags can cause errors in discourse-footnotes

Severity: medium (CVSS 4.0 v2.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.8% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-12-14 · updated 2026-06-17
Weakness: CWE-755

Description

discourse-footnote is a library providing footnotes for posts in Discourse. ### Impact When posting an inline footnote wrapped in `<a>` tags (e.g. `<a>^[footnote]</a>`, the resulting rendered HTML would include a nested `<a>`, which is stripped by Nokogiri because it is not valid. This then caused a javascript error on topic pages because we were looking for an `<a>` element inside the footnote reference span and getting its ID, and because it did not exist we got a null reference error in javascript. Users are advised to update to version 0.2. As a workaround editing offending posts from the rails console or the database console for self-hosters, or disabling the plugin in the admin panel can mitigate this issue.

Affected products

discourse-footnote: before 0.2 — fixed in 0.2

How to fix

Upgrade to a fixed version: discourse-footnote 0.2. Check the vendor advisory for mitigations.

References

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