A flaw was found in the way samba implemented DCE/RPC. If a client to a Samba server sent a very large DCE/RPC request, and chose to fragment it, an attacker could replace later fragments with their own data, bypassing the signature requirements.
https://www.samba.org/samba/security/CVE-2021-23192.html
https://ubuntu.com/security/CVE-2021-23192