Append-only is a storage mode where data can only be added — modification and deletion are not permitted. This principle is commonly used in backup systems, log files, and blockchain technologies.
In an ISMS, append-only is relevant to ISO 27001 Annex A controls A.8.15 (Logging) and A.8.13 (Information Backup). Append-only backups (also called immutable backups) protect against ransomware that attempts to encrypt or delete backup copies. For audit trails, the mode ensures that logged events cannot be tampered with after the fact — an essential requirement for evidence preservation. Cloud providers such as AWS (S3 Object Lock) and Azure (Immutable Blob Storage) offer native append-only functionality.