act vulnerable to arbitrary file upload in artifact serverGHSA-pc99-qmg4-rcff
HIGHFix: nektos/act@63ae215GHSA-pc99-qmg4-rcff is a high-severity (CVSS 8.8) Unrestricted File Upload vulnerability in github.com/nektos/act. 2 public exploit or proof-of-concept references are indexed, so weaponization risk is real. A fix is available for github.com/nektos/act — see the affected versions and patch details below.
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA’s SSVC triage for GHSA-pc99-qmg4-rcff.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
github.com/nektos/actReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
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
- 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
- Clean the user-provided paths manually
Patches
Version 0.2.40 contains a patch.
Workarounds
Avoid use of artifact server with --artifact-server-path
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/nektos/act | all versions | 0.2.40go get github.com/nektos/act@v0.2.40 |
Affected Products
actact_projectResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/nektos/act, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/nektos/act to 0.2.40 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-pc99-qmg4-rcff is resolved across your whole dependency graph.
Workarounds
Treat uploaded files as untrusted until proven otherwise: validate the actual content type rather than the supplied extension, store uploads outside the web root on a volume mounted without execute permission, and rename them to server-generated identifiers so an attacker cannot choose the path a request will later resolve.
Frequently Asked Questions
Is GHSA-pc99-qmg4-rcff in your dependencies?
Find it across Go, including transitive dependencies.