ORAS CLI: Cyclic Referrer Graph Can Cause Unbounded Recursion and Resource Consumption

Summary

A malicious OCI registry can return a cyclic referrer graph (e.g. A -> A or A -> B -> A). The ORAS CLI's recursive referrer traversal does not track visited descriptors, so a cycle causes unbounded recursion and memory growth — a client-side denial of service.

This affects oras discover (recursive referrer traversal) and the recursive referrer counting used by oras backup and oras restore. Because oras discover --depth defaults to 0 (unlimited), discover is affected out of the box.

Note: This advisory covers only the ORAS CLI (oras.land/oras). The related Referrers API pagination loop in the oras-go library is tracked in a separate advisory on the oras-go repository.

Details

In oras discover, recursive referrer traversal follows discovered referrers without maintaining a visited-descriptor set.

Simplified flow:

runDiscover
  -> fetchAllReferrers(root)
     -> registry.Referrers(root)
     -> for each referrer:
          fetchAllReferrers(referrer)

A malicious registry can create cyclic graphs such as:

A -> A

or

A -> B -> A

Because traversal does not track previously visited descriptors, recursion never reaches a terminating condition.

Similarly, the recursive counting logic (graph.RecursiveFindReferrers, used by the backup and restore workflows) accumulates descriptors without cycle detection, causing unbounded memory growth.

PoC

Create valid descriptors where:

referrers(A) = [A]

or:

referrers(A) = [B]
referrers(B) = [A]