Improper Certificate Validation in Cosign

Impact

Cosign can be manipulated to claim that an entry for a signature exists in the Rekor transparency log even if it doesn't. This requires the attacker to have pull and push permissions for the signature in OCI. This can happen with both standard signing with a keypair and "keyless signing" with Fulcio.

Signing with a keypair

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To reproduce this vulnerability, create a keypair and sign an image. Don't store the signature in Rekor:

$ cosign generate-key-pair       
$ cosign sign --key cosign.key IMAGE

If an attacker has access to the signature in OCI, they can manipulate cosign into believing the entry was stored in Rekor even though it wasn't. To accomplish this, the attacker needs to:

1. Pull the signature image
2. Annotate the signature image with any valid Rekor bundle under the dev.sigstore.cosign/bundle annotation
3. Push the signature image back to OCI

Verification then prints out the following:

$ cosign verify [--key]  [IMAGE]

Verification for [IMAGE] --
The following checks were performed on each of these signatures:
  - The cosign claims were validated
  - Existence of the claims in the transparency log was verified offline
  - The signatures were verified against the specified public key
  - Any certificates were verified against the Fulcio roots.

[…]

The claim that Existence of the claims in the transparency log was verified offline is inaccurate since an entry for this image doesn't exist in the log. The claim that Any certificates were verified against the Fulcio roots. is technically correct but since there were no certificates that should be explicitly called out.

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"Keyless signing" with an OIDC flow

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To reproduce this vulnerability, sign the image with a Fulcio identity. Don't store the signature in Rekor:

$ COSIGN_EXPERIMENTAL=1 cosign sign IMAGE
[...]
IMAGE appears to be a private repository, please confirm uploading to the transparency log at "https://rekor.sigstore.dev" [Y/N]: n

Then, create a keypair and sign the image again. Store an entry for the signature in Rekor:

$ cosign generate-key-pair       
$ COSIGN_EXPERIMENTAL=1 cosign sign --key cosign.key IMAGE

Enter password for private key: IMAGE appears to be a private repository, please confirm uploading to the transparency log at "https://rekor.sigstore.dev" [Y/N]: y

If an attacker has access to the signature in OCI, they can manipulate cosign into believing the entry was stored in Rekor even though it wasn't. To accomplish this, the attacker needs to:

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