Distribution is a toolkit for shipping container content that retrieves authentication token endpoints from upstream registries by parsing WWW‑Authenticate challenges. The bearer challenge contains a realm URL, which Distribution uses to obtain access tokens without verifying that the realm matches the intended upstream host. This weakness, identified as CWE‑918, means an attacker who can control or intercept the upstream registry’s challenge can supply a malicious realm URL. Distribution will then send the configured upstream credentials by basic authentication to the attacker‑controlled endpoint, revealing credentials that can be used to pull private images or impersonate the upstream registry.

All releases of Distribution before version 3.1.0 that enable pull‑through caching are affected. The issue occurs when the upstream registry presents a bearer challenge and the realm component is taken at face value.

The CVSS score is 7.5, indicating high severity, while the EPSS score is below 1 %, suggesting that exploitation is not currently common and the vulnerability is not listed in CISA’s KEV catalog. The likely attack vector is network‑based, requiring the attacker to control the upstream registry or position themselves as a man‑in‑the‑middle to inject a rogue challenge. Successful exploitation results in credential exfiltration, allowing unauthorized access to downstream registry resources.