diff --git a/book/source/04-certificates.md b/book/source/04-certificates.md
index bc1335a..9f88ca0 100644
--- a/book/source/04-certificates.md
+++ b/book/source/04-certificates.md
@@ -290,6 +290,12 @@ Wiktor suggests to check: https://blogs.gentoo.org/mgorny/2018/08/13/openpgp-key
 (unbound_user_ids)=
 ### Adding unbound User IDs to a certificate
 
+```{admonition} TODO
+:class: warning
+
+references/links missing
+```
+
 Some OpenPGP subsystems may add User IDs to a certificate, which are not bound to the primary key by the certificate's owner. This can be useful to store local identity information (e.g., Sequoia's public store attaches "pet-names" to certificates, in this way).
 
 ## Zooming in: Packet structure