CVE-2026-33540

hi guys, commit: 40594bd98e6d6ed993b5c6021c93fdf96d2e5851 (as-of 2026-01-31) contact: GitHub Security Advisory (https://github.com/distribution/distribution/security/advisories/new) ## summary in pull-through cache mode, distribution discovers token auth endpoints by parsing `WWW-Authenticate` challenges returned by the configured upstream registry. the `realm` URL from a bearer challenge is used without validating that it matches the upstream registry host. as a result, an attacker-controlled upstream (or an attacker with MitM position to the upstream) can cause distribution to send the configured upstream credentials via basic auth to an attacker-controlled `realm` URL. this is the same vulnerability class as CVE-2020-15157 (containerd), but in distribution’s pull-through cache proxy auth flow. ## severity HIGH note: the baseline impact is credential disclosure of the configured upstream credentials. if a deployment uses broader credentials for upstream auth (for example cloud iam credentials), the downstream impact can be higher; i am not claiming this as default for all deployments. ## impact credential exfiltration of the upstream authentication material configured for the pull-through cache. attacker starting positions that make this realistic: - supply chain / configuration: an operator configures a proxy cache to use an upstream that becomes attacker-controlled (compromised registry, stale domain, or a malicious mirror) - network: MitM on the upstream connection in environments where the upstream is reachable over insecure transport or a compromised network path ## affected components - `registry/proxy/proxyauth.go:66-81` (`getAuthURLs`): extracts bearer `realm` from upstream `WWW-Authenticate` without validating destination - `internal/client/auth/session.go:485-510` (`fetchToken`): uses the realm URL directly for token fetch - `internal/client/auth/session.go:429-434` (`fetchTokenWithBasicAuth`): sends credentials via basic auth to the realm URL ## reproduction attachment: `poc.zip` (local harness) with canonical and control runs. the harness is local and does not contact a real registry: it uses two local HTTP servers (upstream + attacker token service) to demonstrate whether basic auth is sent to an attacker-chosen realm. ```bash unzip -q -o poc.zip -d poc cd poc make canonical make control ``` expected output (excerpt): ``` [CALLSITE_HIT]: getAuthURLs::configureAuth [PROOF_MARKER]: basic_auth_sent=true realm_host=127.0.0.1 account_param=user authorization_prefix=Basic ``` control output (excerpt): ``` [CALLSITE_HIT]: getAuthURLs::configureAuth [NC_MARKER]: realm_validation=PASS basic_auth_sent=false ``` ## suggested remediation validate that the token `realm` destination is within the intended trust boundary before associating credentials with it or sending any authentication to it. one conservative option is strict same-host binding: only accept a realm whose host matches the configured upstream host. ## fix accepted when - distribution does not send configured upstream credentials to an attacker-chosen realm URL - a regression test covers the canonical and blocked cases [addendum.md](https://github.com/user-attachments/files/24984637/addendum.md) [poc.zip](https://github.com/user-attachments/files/24984638/poc.zip) [PR_DESCRIPTION.md](https://github.com/user-attachments/files/24984639/PR_DESCRIPTION.md) [RUNNABLE_POC.md](https://github.com/user-attachments/files/24984640/RUNNABLE_POC.md) best, oleh

Distribution affected by pull-through cache credential exfiltration via www-authenticate bearer realm in github.com/distribution/distribution