CVE-2026-48978
ADVISORY - githubSummary
Summary
oras-go's auth.Client follows the realm URL from a registry's WWW-Authenticate: Bearer challenge without validating its scheme or host. The realm field is server-controlled by design in the OCI/distribution spec — registries legitimately point token requests at a separate auth endpoint (e.g. Docker Hub's registry-1.docker.io -> auth.docker.io), so cross-host realms on public DNS names are not in themselves a vulnerability. Two specific patterns, however, are never legitimate under any registry trust model and can be abused by a malicious or compromised registry (or a man-in-the-middle on a plaintext connection):
SSRF to internal networks. A realm of
http://169.254.169.254/...(AWS/Azure IMDS),http://10.0.0.x/...(RFC 1918), orhttp://127.0.0.1/...causes oras-go running on a cloud VM or corporate workstation to issue outbound HTTP requests from inside the user's trust boundary to an endpoint the user did not choose. The user's stored credentials are attached to those requests, but the principal harm is the network primitive — probing internal endpoints from the client. On IMDSv1 the response body is recoverable from log channels; on IMDSv2 the probe itself can still be used for service discovery.TLS downgrade. A registry contacted over
https://can return a realm with anhttp://scheme, causing oras-go to send the user's credentials over plaintext to the token endpoint. This defeats the transport security the user chose when typinghttps://.
What is NOT claimed
This advisory does not claim that credential forwarding to an arbitrary public attacker host through a server-controlled realm is, on its own, a vulnerability. The distribution spec defines realm as a server-controlled field; a strict same-host or same-eTLD+1 enforcement would deviate from the spec and break legitimate split-host deployments. Operators who want defense-in-depth against cross-host realm forwarding can use the opt-in Client.TrustedRealmHosts allowlist (added separately).
Affected versions
oras.land/oras-go/v2 <= v2.6.0
Severity
Medium. Network attack vector, low complexity, no privileges required, user interaction required (victim runs an oras command against the malicious or MITM'd registry), unchanged scope. Confidentiality impact is limited — IMDS probe responses can disclose information, and TLS downgrade exposes the realm request to passive observers — but the attacker does not obtain credentials beyond what the malicious endpoint already controls.
Affected code
registry/remote/auth/client.go—Client.Do()(bearer challenge handling)registry/remote/auth/client.go—Client.fetchBearerToken()/fetchDistributionToken/fetchOAuth2Token
The realm parameter from parseChallenge is threaded through to http.NewRequestWithContext without scheme or host validation.
CWE
- CWE-918: Server-Side Request Forgery (SSRF)
- CWE-319: Cleartext Transmission of Sensitive Information
Patch
registry/remote/auth/client.go now rejects realm URLs that:
- use a scheme other than
httporhttps - use
httpwhen the registry was contacted overhttps(TLS downgrade) - use an IP literal in a loopback, link-local, private, or unspecified range, unless the registry itself was reached at the same hostname (so loopback / in-cluster deployments are unaffected)
Cross-host realms on public DNS names continue to be accepted.
Credit
Reported by bugbunny.ai.
GitHub
CVSS SCORE
2.1lowChainguard
CGA-fv49-j265-mmrc
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