file store write outside workingDir via symlink traversal in oras-go

Hi Oras,

the file content store in oras-go attempts to confine writes to workingDir when AllowPathTraversalOnWrite=false, but the guard is lexical and does not account for symlink traversal. if workingDir contains a symlink path component and an attacker-controlled blob title (via ocispec.AnnotationTitle) targets a path under that symlink, pushFile() can create a file outside workingDir.

severity

MEDIUM

cvss (estimate): 6.5-8.0 (no vector provided)

relevant links

• repository: https://github.com/oras-project/oras-go
• commit: 0324380
• callsite: content/file/file.go:609 resolveWritePath() (used by pushFile())

vulnerability details

pins: 0324380

as-of: 2026-02-17

policy: GitHub Security Advisory (oras-project/oras-go)

callsite: content/file/file.go:609 resolveWritePath() → pushFile()

attacker control: attacker controls the pushed name (ocispec.AnnotationTitle) and can select a path with a symlink path component under workingDir → resolveWritePath() blocks .. via filepath.Rel but does not prevent symlink traversal → pushFile() opens/creates the final path and follows the symlink → a file is created outside workingDir

root cause

resolveWritePath() enforces the write boundary using a filepath.Rel-style check against workingDir. this prevents ../ escapes but is purely lexical and does not resolve symlinks. if a path component under workingDir is a symlink to an external location, the subsequent filesystem operation in pushFile() follows that symlink and performs the write outside workingDir while still passing the lexical boundary check.

attack path

1. attacker provides a blob title (via ocispec.AnnotationTitle) that contains a path like out/pwn.txt.
2. victim uses oras-go file store with AllowPathTraversalOnWrite=false and a workingDir that contains a symlink directory out -> /some/outside/dir.
3. the lexical boundary check accepts out/pwn.txt as being under workingDir.
4. the write follows the symlink and creates /some/outside/dir/pwn.txt.

impact

this is a filesystem boundary bypass that permits writes outside workingDir when a symlink path component exists under workingDir. the concrete security impact depends on the runtime environment (what filesystem locations are writable by the process and what downstream consumers do with the written file), but the intended confinement guarantee is violated.

proof of concept

the attached poc.zip contains a small, self-contained go harness that demonstrates:

• canonical (vulnerable): prints [CALLSITE_HIT] and [PROOF_MARKER] and shows the file is created outside workingDir
• control (no symlink component): prints [NC_MARKER] and confirms no outside write occurs

run:

unzip -q -o poc.zip -d /tmp
cd /tmp/poc-F-ORAS-SYMLINK-WRITE-001
make test

expected: when AllowPathTraversalOnWrite=false, file store writes should not be able to escape workingDir, including via symlink traversal.

actual: a symlink path component under workingDir allows writes to escape workingDir even when AllowPathTraversalOnWrite=false.

recommended fix

ensure confinement checks account for symlink traversal. options include rejecting symlinks in any path component (walk components with os.Lstat), validating the resolved parent directory via EvalSymlinks and enforcing it remains under the resolved workingDir, or using an openat()-style approach so the check and open happen relative to a trusted directory file descriptor.

fix accepted when: the canonical PoC no longer prints [PROOF_MARKER] for the same attacker-controlled inputs.

poc.zip
PR_DESCRIPTION.md

cheers,
Oleh