OpenBao's System Backend allows Unauthorized Management of the containing Namespace

Summary

A user that is granted namespace management (/sys/namespaces) capabilities within a non-root namespace ("the victim namespace") can abuse special handling of the literal path "root" in namespace path canonicalization to manage the victim namespace itself.

Details

Several endpoints under /sys/namespaces/* accept a namespace path segment that is canonicalized and then appended to the path of the sys mount's containing namespace (set via path prefix or X-Vault-Namespace header) to determine the absolute path of the namespace to operate on.

Given the special namespace path "root" canonicalizes to en empty path (""), when passed as /sys/namespaces/root, the resulting absolute namespace path remains equal to the sys mount's containing namespace. Given ACLs are evaluated before namespace path canonicalization, this allows users with capabilities on /sys/namespaces/root within any given namespace to operate on the namespace itself instead.

Impact

Users that were granted the required capabilities can abuse this vulnerability to:

• Look up
• Delete
• Lock
• Patch custom metadata

against the namespace containing the system backend they can manage sys/namespaces/root in. The exact range of operations that can be performed depends on the specific capabilities granted on said path and any sub-paths such as /api-lock.

Notably, the root namespace is immutable and cannot be modified, deleted or locked, and is thus unaffected. Also note that users can only abuse this vulnerability to operate on the direct parent or "containing" namespace relative to their capabilities, not arbitrary namespaces.

Patch

This will be fixed in OpenBao v2.5.5.

PoC

Start a development server:

bao server -dev

Create a namespace:

bao namespace create victim

This will be the namespace we gain unauthorized management of.

Create a policy that allows management of namespaces, inside of the victim namespace.

bao policy write -namespace=victim namespace-management - <<EOF
path "sys/namespaces/*" {
	capabilities = ["read", "update", "patch", "delete"]
}
EOF

Then create a token with above policy attached:

export BAO_TOKEN=$(bao token create -namespace=victim -policy=namespace-management -field=token)