Portainer Has an Arbitrary File Read via Git Symlink Injection in Stack Auto-Update
Summary
Portainer supports deploying stacks from Git repositories. When a Git-backed stack is created or updated, Portainer clones the repository using go-git v5, which translates Git blob entries with mode 0o120000 (symlink) into real OS symlinks on the host filesystem via os.Symlink. The only entry blocked from becoming a symlink is .gitmodules; every other path — including docker-compose.yml, which Portainer treats as the stack entry point — is created as a symlink without validation.
Portainer's GET /api/stacks/{id}/file endpoint then reads the stack entry point with os.ReadFile, which follows OS symlinks transparently. A repository containing docker-compose.yml as a symlink to an arbitrary filesystem path (for example /etc/passwd or a mounted Kubernetes service account token) causes the symlink target's contents to be returned verbatim in the HTTP response. Any authenticated user with rights to create or update a Git-backed stack — the default configuration in Portainer CE — can read arbitrary files accessible to the Portainer process.
The issue is amplified by Git-stack auto-update: an attacker can create a stack from a legitimate repository, pass initial review, and later push a commit that replaces docker-compose.yml with a symlink; the file read is then triggered on the next scheduled update cycle with no further interaction required.
Severity
High
Attack complexity is Low: the attacker needs only the ability to host a Git repository and the default-granted permission to create a Git-backed stack. Privilege required is Low in typical CE deployments, where non-admin users can manage their own stacks; administrators retain the same attack surface regardless of the setting. Impact on confidentiality is High — the Portainer process commonly runs as root (required for Docker socket access), so arbitrary file read includes /etc/shadow, Kubernetes service account tokens, Docker secrets, environment variables, and the Portainer database itself. Integrity and availability are not directly affected, but the leaked contents (service account tokens, registry credentials, database session keys) frequently enable onward compromise of the host and managed environments.
Affected Versions
The vulnerability exists in every Portainer release since the introduction of Git-based stack deployment support — Git-backed stacks have always performed an unrestricted go-git checkout and subsequently read the entry-point file through os.ReadFile without resolving symlinks.
Fixes are included in the following releases:
| Branch | First vulnerable | Fixed in |
|--------------|------------------|------------|
| 2.33.x (LTS) | 2.33.0 | 2.33.8 |
| 2.39.x (LTS) | 2.39.0 | 2.39.2 |
| 2.40.x (STS) | all prior | 2.41.0 |
Portainer releases prior to 2.33.0 are end-of-life and will not receive a fix. Users on EOL versions should upgrade to a supported LTS branch.
Workarounds
Administrators who cannot immediately upgrade can reduce exposure by:
- Restricting who can create Git-backed stacks. Disable Allow non-admin users to manage their stacks in environment settings so that only administrators can submit a Git repository URL. This reduces the attack to an administrator-only surface but does not remove it.
- Avoiding untrusted repositories. Do not deploy Git-backed stacks from repositories you do not control or review, and do not grant stack-management rights to users who can supply an arbitrary repository URL.
- Disabling auto-update on existing stacks. Auto-update re-clones the repository on a schedule, which allows a repository that was safe at creation time to later become malicious. Disabling auto-update removes the deferred-exploitation path.
- Auditing existing stack working directories. Search project paths under
/data/compose/ (or your configured data directory) for symlink entries — find /data/compose -type l — and treat any unexpected results as potential evidence of past exploitation.
None of these replace the fix.
Affected Code
The vulnerability is the combination of two primitives. go-git translates Git symlink entries into OS symlinks unconditionally (except .gitmodules):
```go
// go-git v5 — Worktree.checkoutFileSymlink
func (w *Worktree) checkoutFileSymlink(f *object.File) (err error) {
if strings.EqualFold(f.Name, gitmodulesFile) {
return ErrGitModulesSymlink
}
// ... reads blob content as raw bytes ...
err = w.Filesystem.Symlink(string(bytes), f.Name)
return
}
```
Relative symlink targets (../../etc/passwd) are passed through to os.Symlink as-is and escape the worktree at OS resolution time. (Absolute targets are chrooted to the worktree by go-billy's ChrootHelper.Symlink and are not useful to the attacker.)
On the read side, GetFileContent in `api/filesystem/filesys
