apko CVE-2026-42574

Impact

A crafted .apk could install a TypeSymlink tar entry whose target pointed outside the build root, and a subsequent directory-creation or file-write entry in the same or later archive could traverse that symlink to reach host paths the build user could write to. The root cause was the sanitizePath helper in pkg/apk/fs/rwosfs.go, which rejected only lexical .. traversal and did not resolve or refuse symlinks. Every disk-backed DirFS method that handed its caller-supplied path to a symlink-following stdlib call - ReadFile, WriteFile, Chmod, Chown, Chtimes, MkdirAll, Mkdir, and Mknod - was affected. The reachable primitive from a malicious APK during tar extraction is the MkdirAll / Mkdir / WriteFile chain via apko build-cpio and disk-backed consumers such as melange; the remaining sinks are reachable by direct callers of the pkg/apk/fs package. The in-memory tarfs install path used by apko build, apko publish, and apko build-minirootfs is not affected.

Patches

Fixed in apko v1.2.5 by #2187 / commit f5a96e1, which scopes all DirFS operations through a Go 1.24 *os.Root. The sanitizePath helper has been removed; *os.Root refuses traversal via .., absolute-target symlinks, relative-target symlinks, and hardlinks by construction. Regression tests in pkg/apk/apk/path_traversal_test.go cover each composite primitive.

Workarounds

No complete workaround. Operators running pre-1.2.5 apko (or downstream tools such as melange that embed pre-1.2.5 pkg/apk/fs) should upgrade. Consuming only APKs from trusted, signed sources reduces but does not eliminate exposure.

Resources

• https://github.com/chainguard-dev/apko/pull/2187
• https://github.com/chainguard-dev/apko/commit/f5a96e1299ac81c7ea9441705ec467688086f442
• https://github.com/chainguard-dev/apko/releases/tag/v1.2.5
• Related: GHSA-5g94-c2wx-8pxw (CVE-2026-25121) - prior lexical .. traversal fix

Credits

apko thanks Oleh Konko (@1seal from 1seal.org) for the initial report of the symlink-escape class, and to @Xh081iX for a follow-up set of reports covering additional reachable primitives (ReadFile, Chmod/Chown, Mknod, MkdirAll/Mkdir) that shaped the comprehensive fix.