DDEV incomplete tar symlink traversal fix allows host file overwrite - Incomplete fix of CVE-2026-32885

Summary

The CVE-2026-32885 ZipSlip fix added a symlink-target escape check to Untar, but the check rebases an ABSOLUTE symlink target into dest before validating (filepath.Join(dest, "/etc") stays lexically inside dest and passes), while the symlink is then created with the RAW absolute target via os.Symlink(file.Linkname, ...), so a malicious archive plants a host-absolute symlink and a following regular-file entry writes through it, restoring arbitrary host file create/overwrite (reaching $HOME rc-files, hence RCE). Confirmed at HEAD.

The defect (the symlink check is a no-op for absolute targets)

CVE-2026-32885 (fixed v1.25.2, commit 05cbe299) added a lexical containment check on the entry path and a NEW symlink-target validation. pkg/archive/archive.go, Untar, tar.TypeSymlink case:

if filepath.IsAbs(file.Linkname) {
    resolvedTarget = filepath.Join(dest, file.Linkname)   // line 298: absolute target REBASED into dest
} else {
    resolvedTarget = filepath.Join(fullPathDir, file.Linkname)
}
if !strings.HasPrefix(filepath.Clean(resolvedTarget)+sep, filepath.Clean(dest)+sep) {  // line 302
    return ...escapes destination...                       // line 303
}
_ = os.Remove(fullPath)                                     // line 307
err = os.Symlink(file.Linkname, fullPath)                  // line 310: writes the RAW Linkname

For an absolute Linkname like /Users/victim, line 298 computes filepath.Join(dest, "/Users/victim") = dest/Users/victim (lexically inside dest), so the escape check at line 302 PASSES. But line 310 creates the symlink with the raw file.Linkname, so the on-disk symlink dest/link points at the real host path /Users/victim. The value validated (rebased) is not the value written (raw): a check-one-thing / do-another divergence.

Write-through: the archive carries a second regular-file entry whose Name traverses the planted link (e.g. link/.zshrc). Its fullPath is lexically inside dest so the line-238 check passes (that check is purely lexical and does not resolve the symlink), then os.Create follows the planted symlink and overwrites /Users/victim/.zshrc on the host. Overwriting a shell rc-file, git hook, or cron entry yields code execution. Reachable via ddev add-on get <repo|url> (the exact remote-add-on vector the advisory names), and via ddev import-db / import-files. GitHub codeload tarballs preserve symlinks.

Verification

EXECUTED (go test driving the real Untar): a file was written OUTSIDE dest and Untar returned nil (VULNERABLE); a pre-existing host file was truncated and overwritten with the attacker payload; the negative controls (a relative ../ symlink escape, and a plain ../ file entry) are both correctly rejected, isolating the bug to the absolute-symlink branch. NOT verified: the end-to-end add-on get over the network (the caller chain is source-confirmed); Windows (reliable on macOS/Linux, DDEV's primary hosts). Unzip is NOT affected (tar-only).

Suggested fix

Reject absolute symlink targets outright (a dev-environment archive has no need to plant host-absolute symlinks), or validate the RAW target that will actually be written rather than a rebased copy. Additionally, resolve symlinks when validating each regular-file entry's parent so a write cannot traverse a previously-planted link (or extract to a staging dir and reject any entry whose realpath escapes). The lexical line-238 check does not defend against a planted symlink.

Tooling

AI assistance was used while investigating. The write-through is executed-verified with negative controls (an absolute-symlink entry plus a follow-up file entry wrote and overwrote files OUTSIDE dest while relative-escape and plain-ZipSlip controls were correctly rejected), and I confirmed at HEAD that Untar rebases an absolute symlink target into dest for the escape check (line 298) but creates the symlink with the raw absolute target (line 310).