A blob is a binary object stored in a registry or similar system. In Docker and other container platforms, a blob usually holds an image layer, configuration data, or another chunk of content that a manifest points to. Blobs are the raw payloads behind an image, while the manifest describes how those payloads fit together.
Blobs matter in cyber security because they can preserve more than code. Build layers often contain environment files, embedded credentials, certificates, or other secrets that were added during development and later forgotten. If a registry is public or poorly protected, attackers can enumerate repositories, fetch manifests, and inspect blobs to recover sensitive data. Defenders reduce this risk by restricting registry access, scanning images before publication, removing secrets from builds, and rotating any credentials that may have been exposed in historical layers.