GHSA-mf7q-r4rv-jv94: Crossplane's TOCTOU between cosign verification and image fetch in xpkg.CachedClient allows tag-based package install to bypass signature check

Crossplane allows package signature verification to be configured via the ImageConfig mechanism. When enabled, the package manager uses cosign to verify that packages are correctly signed before pulling and installing them.

When a package is installed using a tag reference (e.g., a semantic version), a malicious OCI registry could serve a correctly signed image for verification, then subsequently serve an unsigned image for installation. This is possible because Crossplane resolves the tag reference separately for each step.

This vulnerability is relevant only for users who do all three of the following:

1. Configure signature verification for packages,
2. Install packages using tag references rather than digests, and
3. Install packages from registries they do not control.