Ceph CephX: malleable, unauthenticated tickets let a low-privilege key be forged into Manager, MDS or OSD access

Impact

CephX encrypts tickets with AES-128-CBC using a hard-coded IV and no MAC. An attacker holding one low-privilege key who can also observe CephX traffic can use the monitor as an encryption oracle - it will encrypt attacker-chosen entity names - and splice ciphertext blocks into valid tickets for privileged entities such as Manager, MDS and OSD. Separately, flipping a single bit in a service ticket sets its allow_all field, escalating any CephX principal. On a GPU cluster this collapses the tenancy boundary of the shared storage tier: a per-tenant CephX key becomes cluster-wide storage authority over other tenants' datasets and checkpoints. The fix is protocol-level, so it lands only when both ends are upgraded.

Who can reach it

An attacker on the Ceph cluster or public network (CVSS adjacent) who holds one low-privilege CephX key and can observe CephX traffic. The allow_all bit-flip variant needs only an existing CephX credential.

What to do

Upgrade to Ceph 20.2.4 or 19.2.6 across monitors, OSDs, MDS, Manager and clients, then restart the daemons - a rolling restart of the storage fleet, with client-side updates on GPU nodes that mount CephFS or RBD. Since this is a protocol weakness, mixed-version clusters stay exposed until clients are also on a fixed release; plan the client rollout rather than stopping at the servers. Ensure the cluster network is not reachable from tenant workloads in the meantime.

References

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