Docker Image Signature

note

Docker images are currently signed with both Docker Content Trust (DCT) and Cosign. Support for DCT ends on December 8, 2026. After that date, images will be signed only with Cosign. For more details, see Transition to Cosign.

Overview

Cosign is a Sigstore tool that verifies container image signatures. Verifying these signatures ensures that the Docker images running Cortex XSOAR/XSIAM and platform content such as content packs (connectors), integrations (capabilities), and scripts are authentic and untampered with. Cosign will replace the deprecated Docker Content Trust (based on Notary v1) with a modern, OCI-native image signing workflow.

This article explains how to use Cosign to verify Docker images running Cortex XSOAR/XSIAM and platform content items.

Transition to Cosign

Currently, all images are currently dual-signed with both DCT and Cosign to enable a seamless transition to Cosign. After December 8, we will stop signing images with DCT and move exclusively to Cosign.

If your system currently relies on docker trust inspect / DOCKER_CONTENT_TRUST=1, switch to cosign verify before December 8.

Task	DCT (legacy)	Cosign (new)
Verify before pull	DOCKER_CONTENT_TRUST=1 docker pull <image>:<tag> (verification runs as part of the pull)	COSIGN_REPOSITORY=<org>/sig-<image> cosign verify --key cosign.pub --insecure-ignore-tlog=true <image>@sha256:<digest>, then docker pull <image>@sha256:<digest>
Inspect signatures	docker trust inspect <image>	COSIGN_REPOSITORY=<org>/sig-<image> cosign tree <image>:<tag>

Prerequisite

Ensure cosign binary is installed in your environment.

On macOS (or any environment with Homebrew):

brew install cosign

cosign version

On Linux, download the release binary directly:

COSIGN_VERSION=v3.0.2

curl -sSfL "https://github.com/sigstore/cosign/releases/download/${COSIGN_VERSION}/cosign-linux-amd64" \

-o /usr/local/bin/cosign

chmod +x /usr/local/bin/cosign

cosign version

Note:

Both cosign v2 and v3 work for verification. For other platforms and installation methods, see the cosign installation docs.

Get the Public Key

The Cosign public key (cosign.pub) is all you need to verify, it is not secret.

Note:

--key cosign.pub is a file path. Cosign reads a file named cosign.pub in the directory where the command is run from. If that file is missing, the error open cosign.pub: no such file or directory is returned, so save the key first.

Create cosign.pub with the published key (run all later commands from the same directory, or pass the full path to the file):

cat > cosign.pub <<'EOF'

-----BEGIN PUBLIC KEY-----

MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEH1arbOyIQuZLdy8bF5ss2LHCDB9M

DlUs3beCk/90l2LQyOLWSLEAHsCTv43LxKhhOn+Cqvot8rpjBFrb7UEj7g==

-----END PUBLIC KEY-----

EOF

Confirm the file was saved correctly:

cat cosign.pub # prints the PEM block saved above

Verify the Image Signature

Images are signed by digest and can be verified using the image tag (:<tag>) or the image digest (@sha256:<digest>). When a tag is passed, Cosign resolves it to the digest it currently points to and verifies the signature for that digest.

We encourage using the Example Verification Script, which only needs the image tag or digest and handles the signature repository automatically. To verify manually with Cosign instead:

1. Make sure cosign.pub is saved in the current directory (see Get the Public Key).
2. Get the image reference to verify, either the image tag or the image digest (see Get the Image Digest).
3. Set COSIGN_REPOSITORY to the sibling <org>/sig-<image> repository where the signature is stored (for example, the signature for demisto/python3 is stored in demisto/sig-python3).
4. Run cosign verify with --insecure-ignore-tlog=true.

For example, to verify demisto/python3 by tag:

COSIGN_REPOSITORY=demisto/sig-python3 \

cosign verify --key cosign.pub --insecure-ignore-tlog=true \

demisto/python3:<tag>

To verify demisto/python3 by digest:

COSIGN_REPOSITORY=demisto/sig-python3 \

cosign verify --key cosign.pub --insecure-ignore-tlog=true \

demisto/python3@sha256:<digest>

Get the Image Digest

A tag can be moved to point to a different image, while a digest always identifies the same image. To get the digest of an image tag, pull the image and read its repository digest:

docker pull demisto/python3:<tag>

docker inspect --format='{{index .RepoDigests 0}}' demisto/python3:<tag>

# prints demisto/python3@sha256:<digest>

Pass the printed demisto/python3@sha256:<digest> reference to cosign verify. Pulling and running the image by the same digest guarantees that the image in use is the one that was verified.

Example Verification Script

For convenience, the ready-to-run wrapper utils/verify_signature.sh is available in the demisto/dockerfiles repository. It derives the sibling <org>/sig-<image> signature repository automatically, so only the image reference is passed.

To use the script:

1. Make sure cosign is installed and on the PATH (see Prerequisite).

2. Download the script and make it executable:

   curl -sSfL https://raw.githubusercontent.com/demisto/dockerfiles/master/utils/verify_signature.sh \

   -o verify_signature.sh

   chmod +x verify_signature.sh

3. Save cosign.pub in the same directory as the script (see Get the Public Key), or set PUBLIC_KEY=/path/to/cosign.pub to point to it elsewhere.

4. Run the script with the image tag or digest to verify:

   ./verify_signature.sh demisto/<image>:<tag>

   ./verify_signature.sh demisto/<image>@sha256:<digest>

   A bare image name (for example, python3:<tag>) defaults to the demisto org. References without a tag or digest are rejected.

Troubleshooting

Issue	Cause	Fix
open cosign.pub: no such file or directory	--key cosign.pub points at a file that is not in the current directory.	Save the key as shown in Get the Public Key, or pass the full path (e.g. --key /path/to/cosign.pub).
no matching signatures on verify	Verifying against the image repository instead of the signature repository.	Set COSIGN_REPOSITORY=<org>/sig-<image> to match where the signature is stored.
Verify fails looking for a transparency-log entry	Images are signed without a public transparency log entry.	Add --insecure-ignore-tlog=true to cosign verify.