Impact
A Cosign bundle can be crafted to successfully verify an artifact even if the embedded Rekor entry does not reference the artifact's digest, signature or public key. When verifying a Rekor entry, Cosign verifies the Rekor entry signature, and also compares the artifact's digest, the user's public key from either a Fulcio certificate or provided by the user, and the artifact signature to the Rekor entry contents. Without these comparisons, Cosign would accept any response from Rekor as valid. A malicious actor that has compromised a user's identity or signing key could construct a valid Cosign bundle by including any arbitrary Rekor entry, thus preventing the user from being able to audit the signing event.
This vulnerability only affects users that provide a trusted root via --trusted-root or when fetched automatically from a TUF repository, when no trusted key material is provided via SIGSTORE_REKOR_PUBLIC_KEY. When using the default flag values in Cosign v3 to sign and verify (--use-signing-config=true and --new-bundle-format=true for signing, --new-bundle-format=true for verification), users are unaffected. Cosign v2 users are affected using the default flag values.
This issue had previously been fixed in GHSA-8gw7-4j42-w388 but recent refactoring caused a regression. We have added testing to prevent a future regression.
Steps to Reproduce
echo blob > /tmp/blob
cosign sign-blob -y --new-bundle-format=false --bundle /tmp/bundle.1 --use-signing-config=false /tmp/blob
cosign sign-blob -y --new-bundle-format=false --bundle /tmp/bundle.2 --use-signing-config=false /tmp/blob
jq ".rekorBundle |= $(jq .rekorBundle /tmp/bundle.2)" /tmp/bundle.1 > /tmp/bundle.3
cosign verify-blob --bundle /tmp/bundle.3 --certificate-identity-regexp='.*' --certificate-oidc-issuer-regexp='.*' /tmp/blob
Patches
Upgrade to Cosign v2.6.2 or Cosign v3.0.4. This does not affect Cosign v1.
Workarounds
You can provide trusted key material via a set of flags under certain conditions. The simplest fix is to upgrade to the latest Cosign v2 or v3 release.
Note that the example below works for cosign verify, cosign verify-blob, cosign verify-blob-attestation, and cosign verify-attestation`.
SIGSTORE_REKOR_PUBLIC_KEY=<path to Rekor pub key> cosign verify-blob --use-signing-config=false --new-bundle-format=false --bundle=<path to bundle> <artifact>
References
1seal Finder
Impact
A Cosign bundle can be crafted to successfully verify an artifact even if the embedded Rekor entry does not reference the artifact's digest, signature or public key. When verifying a Rekor entry, Cosign verifies the Rekor entry signature, and also compares the artifact's digest, the user's public key from either a Fulcio certificate or provided by the user, and the artifact signature to the Rekor entry contents. Without these comparisons, Cosign would accept any response from Rekor as valid. A malicious actor that has compromised a user's identity or signing key could construct a valid Cosign bundle by including any arbitrary Rekor entry, thus preventing the user from being able to audit the signing event.
This vulnerability only affects users that provide a trusted root via
--trusted-rootor when fetched automatically from a TUF repository, when no trusted key material is provided viaSIGSTORE_REKOR_PUBLIC_KEY. When using the default flag values in Cosign v3 to sign and verify (--use-signing-config=trueand--new-bundle-format=truefor signing,--new-bundle-format=truefor verification), users are unaffected. Cosign v2 users are affected using the default flag values.This issue had previously been fixed in GHSA-8gw7-4j42-w388 but recent refactoring caused a regression. We have added testing to prevent a future regression.
Steps to Reproduce
Patches
Upgrade to Cosign v2.6.2 or Cosign v3.0.4. This does not affect Cosign v1.
Workarounds
You can provide trusted key material via a set of flags under certain conditions. The simplest fix is to upgrade to the latest Cosign v2 or v3 release.
Note that the example below works for
cosign verify,cosign verify-blob,cosign verify-blob-attestation, andcosign verify-attestation`.References