CVE-2026-33540 is a high-severity (CVSS 7.5) Server-Side Request Forgery (SSRF) vulnerability in github.com/distribution/distribution/v3. A fix is available for github.com/distribution/distribution/v3 — see the affected versions and patch details below.
Distribution affected by pull-through cache credential exfiltration via www-authenticate bearer realm
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-33540.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
github.com/distribution/distribution/v3🐹github.com/distribution/distributionReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
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 bearerrealmfrom upstreamWWW-Authenticatewithout validating destinationinternal/client/auth/session.go:485-510(fetchToken): uses the realm URL directly for token fetchinternal/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.
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 poc.zip PR_DESCRIPTION.md RUNNABLE_POC.md
best, oleh
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/distribution/distribution/v3 | all versions | 3.1.0go get github.com/distribution/distribution/v3@v3.1.0 |
| 🐹Go | github.com/distribution/distribution | all versions | No fix |
Affected Products
distributiondistribution_projectDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/distribution/distribution/v3, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/distribution/distribution/v3 to 3.1.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-33540 is resolved across your whole dependency graph.
Workarounds
Restrict outbound requests from the affected component to an allowlist of hosts, block access to link-local and internal address ranges at the network layer, and require authentication on internal services so a forged request cannot reach them unauthenticated.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
To mitigate this issue, ensure that all upstream registries configured for Distribution in pull-through cache mode are trusted and secured. Implement network segmentation and firewall rules to restrict outbound connections from the Distribution instance to only known and trusted upstream registry endpoints. This reduces the risk of an attacker-controlled upstream or a Man-in-the-Middle attack redirecting authentication.Source: Red Hat security advisory for CVE-2026-33540 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Openshift Data Foundation 4.22 | odf4/cephcsi-rhel9:1782932114 | RHSA-2026:37387 |
Frequently Asked Questions
Is CVE-2026-33540 in your dependencies?
Find it across Go, including transitive dependencies.