remote: validate foreign layer URLs to prevent SSRF (fixes #2259)#2293
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Foreign layer descriptors in OCI/Docker manifests may carry arbitrary URLs in the descriptor's "urls" field. When the registry blob endpoint returns 404, the client fetches these URLs directly with no validation, allowing a malicious registry to point them at internal services (e.g. the cloud instance-metadata endpoint 169.254.169.254). Add validateForeignURL, which applies the same scheme and private-IP checks as transport.validateRealmURL to every foreign layer URL before making a network request. HTTP is only permitted when the registry itself was reached over HTTP (insecure=true). DNS-based SSRF remains out of scope, consistent with the design decision in validateRealmURL. Fixes google#2259.
The previous fix validates each foreign layer URL with validateForeignURL before adding it to the fetch list. However, http.Client follows redirects by default: an attacker can host a foreign layer URL on a public domain that passes the initial check, then redirect the client to http://169.254.169.254/... (AWS/GCP instance-metadata), leaking cloud credentials. Add fetchForeignBlobURL (on *fetcher) that reuses the existing transport but sets a CheckRedirect hook calling validateForeignURL on every redirect hop. Compressed() now routes foreign layer fetches through this method instead of the shared fetchBlobURL so that the validation happens at both the initial URL and each redirect destination. New tests: - TestPullingForeignLayerSSRFViaRedirect: attacker httptest server issues a 302 to a loopback victim; confirms Compressed() returns an error before any credentials are returned.
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May 13, 2026
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Looks good, but needs some code cleanup before merge.
Move validateForeignURL and fetchForeignBlobURL (plus their tests) next to the fetcher receiver, trim verbose function comments, and rework Compressed() to call fetchBlobURL on the single registry URL with foreign URLs collected and iterated separately.
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## main #2293 +/- ##
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+ Coverage 56.73% 56.75% +0.02%
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Files 165 165
Lines 11259 11299 +40
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+ Hits 6388 6413 +25
- Misses 4112 4121 +9
- Partials 759 765 +6 ☔ View full report in Codecov by Sentry. 🚀 New features to boost your workflow:
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… (google#2293) * remote: validate foreign layer URLs to prevent SSRF Foreign layer descriptors in OCI/Docker manifests may carry arbitrary URLs in the descriptor's "urls" field. When the registry blob endpoint returns 404, the client fetches these URLs directly with no validation, allowing a malicious registry to point them at internal services (e.g. the cloud instance-metadata endpoint 169.254.169.254). Add validateForeignURL, which applies the same scheme and private-IP checks as transport.validateRealmURL to every foreign layer URL before making a network request. HTTP is only permitted when the registry itself was reached over HTTP (insecure=true). DNS-based SSRF remains out of scope, consistent with the design decision in validateRealmURL. Fixes google#2259. * remote: block SSRF redirect bypass for foreign layer URLs The previous fix validates each foreign layer URL with validateForeignURL before adding it to the fetch list. However, http.Client follows redirects by default: an attacker can host a foreign layer URL on a public domain that passes the initial check, then redirect the client to http://169.254.169.254/... (AWS/GCP instance-metadata), leaking cloud credentials. Add fetchForeignBlobURL (on *fetcher) that reuses the existing transport but sets a CheckRedirect hook calling validateForeignURL on every redirect hop. Compressed() now routes foreign layer fetches through this method instead of the shared fetchBlobURL so that the validation happens at both the initial URL and each redirect destination. New tests: - TestPullingForeignLayerSSRFViaRedirect: attacker httptest server issues a 302 to a loopback victim; confirms Compressed() returns an error before any credentials are returned. * remote: move foreign layer SSRF helpers to fetcher.go per review Move validateForeignURL and fetchForeignBlobURL (plus their tests) next to the fetcher receiver, trim verbose function comments, and rework Compressed() to call fetchBlobURL on the single registry URL with foreign URLs collected and iterated separately. * remote: rename unused params to satisfy revive linter
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Summary
OCI and Docker manifests may include a `urls` field in layer descriptors
specifying alternative sources for foreign layers. Without validation, a
malicious registry can set these URLs to private or link-local addresses
(e.g. `http://169.254.169.254/latest/meta-data/iam/security-credentials/\`),
causing the client to exfiltrate cloud credentials when pulling an image.
Attack scenario 1 — direct private IP in `urls`
```json
{
"mediaType": "application/vnd.docker.image.rootfs.foreign.diff.tar.gzip",
"digest": "sha256:...",
"size": 1024,
"urls": ["http://169.254.169.254/latest/meta-data/iam/security-credentials/my-role"]
}
```
The client calls `Compressed()`, which iterates `d.URLs` and fetches each
URL via `fetchBlobURL`. On AWS/GCP/Azure the metadata service returns IAM
tokens.
Attack scenario 2 — redirect-based bypass (added in this update)
Initial URL validation checks only the URL literal. A public CDN under
attacker control can pass validation and then issue a redirect:
Fix
IP literals. HTTP is only allowed when the registry itself uses HTTP
(insecure mode), matching the precedent in `transport.validateRealmURL`.
sets a `CheckRedirect` hook that passes every redirect destination through
`validateForeignURL`, closing the redirect-bypass path.
(not the shared `fetchBlobURL`) so both the initial URL and each redirect
hop are validated.
Test plan
(169.254.169.254), RFC-1918, unspecified, disallowed schemes, insecure HTTP.
URL is rejected before any request reaches 169.254.169.254.
attacker server redirects to loopback victim; confirms `Compressed()` returns
a "private or link-local" error before any data is returned.
Fixes #2259.