fix: prevent zip-slip/tar-slip path traversal in gcs-fetcher#1086
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mohammadmseet-hue wants to merge 1 commit intoGoogleCloudPlatform:masterfrom
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fix: prevent zip-slip/tar-slip path traversal in gcs-fetcher#1086mohammadmseet-hue wants to merge 1 commit intoGoogleCloudPlatform:masterfrom
mohammadmseet-hue wants to merge 1 commit intoGoogleCloudPlatform:masterfrom
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The archive extraction functions in fetcher.go use filepath.Join(dest, untrustedName) without verifying that the result stays within the destination directory. An archive entry named "../../../etc/cron.d/evil" resolves to /etc/cron.d/evil, escaping the extraction directory. Three independent vectors are affected: - unzip() line 777: ZIP entry file.Name - fetchFromTarGz() line 887: tar header h.Name - fetchObject() line 289: manifest j.filename This adds a safeJoin() helper that validates the resolved path remains under the base directory, matching the pattern used in Google's own go-containerregistry (mutate.go:298-320). Ref: CVE-2018-1002200 (Zip Slip class)
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Summary
The archive extraction functions in
gcs-fetcher/pkg/fetcher/fetcher.gousefilepath.Join(dest, untrustedName)without verifying the result stays within the destination directory. Archive entries with../in the filename escape the extraction directory and write files to arbitrary paths.Affected Code (3 vectors)
filepath.Join(dest, file.Name)filepath.Join(gf.DestDir, h.Name)filepath.Join(dest, j.filename)None of these validate that the resolved path remains under the base directory.
Why filepath.Join does NOT prevent this
filepath.Joincleans paths but does NOT constrain them. This is the well-known Zip Slip class (CVE-2018-1002200).The Fix
Adds a
safeJoin()function that validates the resolved path stays under the base directory usingstrings.HasPrefixafterfilepath.Clean. Applied to all 3 vectors.This matches the pattern used in Google's own
go-containerregistry(mutate.go:298-320).Impact
gcs-fetcher runs as the first step in Google Cloud Build pipelines. A malicious source archive (zip/tar.gz) with traversal entries can write files outside
/workspace/, overwriting build scripts or injecting code into downstream build steps that execute with the build service account's credentials.