CVE-2026-33540: Distribution affected by pull-through cache credential exfiltration via www-authenticate bearer realm

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-04-06 · updated 2026-06-17
Weakness: CWE-918

Description

Distribution is a toolkit to pack, ship, store, and deliver container content. Prior to 3.1.0, 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 vulnerability is fixed in 3.1.0.

Affected products

Distribution Project Distribution: before 3.1.0 — fixed in 3.1.0

How to fix

Upgrade to a fixed version: Distribution Project Distribution 3.1.0. Check the vendor advisory for mitigations.

References

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