Affected: Go: github.com/distribution/distribution/v3, Go: github.com/distribution/distribution

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 conne…