Various uses of the Go toolchain in untrusted VCS repositories can result in
unexpected code execution. When using the Go toolchain in directories fetched
using various VCS tools (such as directly cloning Git or Mercurial repositories)
can cause the toolchain to execute unexpected commands, if said directory
contains multiple VCS configuration metadata (such as a '.hg' directory in a Git
repository). This is due to how the Go toolchain attempts to resolve which VCS
is being used in order to embed build information in binaries and determine
module versions.

The toolchain will now abort attempting to resolve which VCS is being used if it
detects multiple VCS configuration metadata in a module directory or nested VCS
configuration metadata (such as a '.git' directoy in a parent directory and a
'.hg' directory in a child directory). This will not prevent the toolchain from
building modules, but will result in binaries omitting VCS related build
information.

If this behavior is expected by the user, the old behavior can be re-enabled by
setting GODEBUG=allowmultiplevcs=1. This should only be done in trusted
repositories.

Thanks to RyotaK (https://ryotak.net) of GMO Flatt Security Inc for reporting
this issue.

This is CVE-2025-4674.

/cc @golang/security and @golang/release