Disallow Dockerfile symlinks escaping build context by mahendrarathore1742 · Pull Request #6885 · podman-container-tools/buildah

mahendrarathore1742 · 2026-06-01T10:59:09Z

Summary

This change aligns Buildah with Docker/BuildKit behavior by rejecting Dockerfile paths
inside the build context that resolve (via symlink) to a target outside the context.

What Changed

Validate Dockerfile paths in imagebuildah.BuildDockerfiles to ensure any symlink
resolves within ContextDirectory.
Add a regression test that reproduces the symlink-escape scenario.

Why

Builds should not be able to read files outside the declared build context.
Docker/BuildKit fails in this case; Buildah previously allowed it. This fixes the
inconsistency and closes the security gap.

Testing

go test ./imagebuildah \
  -run TestDockerfileSymlinkOutsideContext \
  -tags "containers_image_openpgp exclude_graphdriver_btrfs"

Note: Full go test ./... fails in this container due to conformance tests
requiring privileged operations (overlay mounts and chown).

Closes

Closes #6861

nalind

This looks broadly correct to me. I tend to run conformance tests under podman unshare if I need to run them locally, but they're passing in CI. There are a couple of tweaks I'd like to see, though.

nalind · 2026-06-02T20:50:13Z

+	assert.NoError(t, os.Mkdir(contextDir, 0o755))
+	dockerfile := filepath.Join(tmpDir, "Dockerfile")
+	assert.NoError(t, os.WriteFile(dockerfile, []byte("FROM scratch"), 0o644))
+	assert.NoError(t, os.Symlink("../Dockerfile", filepath.Join(contextDir, "Dockerfile")))


Might want to add a second case where the target is an absolute path (dockerfile instead of "../Dockerfile") to be sure.

nalind · 2026-06-02T21:04:36Z

+						return "", nil, fmt.Errorf("resolving containerfile %q: %w", dfile, err)
+					}
+					resolvedRel, err := filepath.Rel(contextAbs, resolved)
+					if err != nil || resolvedRel == ".." || strings.HasPrefix(resolvedRel, ".."+string(filepath.Separator)) {


Can we split this block into two blocks, so that for cases where err != nil the error we return can wrap the error that filepath.Rel() returned?

Thanks for the review! I split the filepath.Rel() error handling to preserve the wrapped error and added an absolute-path symlink case to the test.

nalind

LGTM, thanks!

eriksjolund · 2026-06-03T15:03:12Z

+					return "", nil, fmt.Errorf("resolving containerfile %q: %w", dfile, err)
+				}
+				if rel, err := filepath.Rel(contextAbs, dfileAbs); err == nil && rel != ".." && !strings.HasPrefix(rel, ".."+string(filepath.Separator)) {
+					resolved, err := filepath.EvalSymlinks(dfileAbs)


Could this dereference symlinks outside of the context directory?
(EvalSymlinks() on line 158)

Would rel not be "..", and also not have "../" as a prefix if dfileAbs was outside of contextAbs?

Symlinks might first take us out of the context directory and then back in again.

It could also be single symlink that contains something like ../context/Dockerfile as in the
the example #6861 (comment) where docker fails but podman succeeds.

Thanks for raising this. The current check only verifies dfileAbs is under the context, so a symlink could still resolve outside (or escape then re-enter). I’ll add a second guard to ensure the resolved path is also within contextAbs (via filepath.Rel on the resolved path) and push an update.

I added another docker behavior example #6861 (comment)

Thanks for adding the additional Docker behavior example! I've tested both scenarios and the fix now correctly handles:

context/Dockerfile -> ../context/file - Now correctly fails (matches Docker)

context/Dockerfile -> ../file - Now correctly succeeds (matches Docker)

The implementation has been updated and tested to ensure both the security fix and Docker behavior alignment are working correctly. The changes are pushed to the fix/dockerfile-symlink-context branch and ready for review.

TomSweeneyRedHat · 2026-06-10T21:33:47Z

LGTM

TomSweeneyRedHat · 2026-06-11T15:31:52Z

Some unhappy tests which I restarted.

nalind · 2026-06-23T21:51:04Z

You're going to want to rebase to pick up missing packages that are causing those "pivot_root: command not found" test failures, added to the VM images that we updated to in #6911.

Signed-off-by: Mahendra Singh Rathore <49229348+mahendrarathore1742@users.noreply.github.com>

- Prevent Dockerfile symlinks from escaping build context (security fix) - Match Docker behavior for symlink resolution: * context/Dockerfile -> ../context/file: fail (both Docker and Buildah) * context/Dockerfile -> ../file: succeed (Docker behavior, now Buildah matches) - Add comprehensive tests for main issue and Docker behavior examples - Resolve symlink validation to check both original and resolved paths Closes podman-container-tools#6861 Signed-off-by: Mahendra Singh Rathore <49229348+mahendrarathore1742@users.noreply.github.com>

mahendrarathore1742 force-pushed the fix/dockerfile-symlink-context branch from 2ba89e0 to f935293 Compare June 1, 2026 11:02

nalind requested changes Jun 2, 2026

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nalind approved these changes Jun 3, 2026

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eriksjolund reviewed Jun 3, 2026

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nalind mentioned this pull request Jun 5, 2026

Reject symlinked Containerfile and ignore files escaping build context #6886

Open

mahendrarathore1742 added 3 commits July 9, 2026 15:38

Disallow Dockerfile symlinks escaping build context

a823f5d

Signed-off-by: Mahendra Singh Rathore <49229348+mahendrarathore1742@users.noreply.github.com>

Cover absolute symlink targets in context checks

5997ac9

Signed-off-by: Mahendra Singh Rathore <49229348+mahendrarathore1742@users.noreply.github.com>

mahendrarathore1742 force-pushed the fix/dockerfile-symlink-context branch from ed06de3 to 09e6b2c Compare July 9, 2026 15:41

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mahendrarathore1742 commented Jun 1, 2026

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TomSweeneyRedHat commented Jun 10, 2026

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TomSweeneyRedHat commented Jun 11, 2026

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nalind commented Jun 23, 2026

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