I've been using Semgrep for a while now, mostly with their extensive public registry for catching common security and code quality issues. It's been solid in our CI pipeline. But I only recently dove into the online playground for writing custom rules, and it's a game-changer for team-specific patterns.
The workflow is surprisingly smooth. You can point it at a public repo or paste your own code snippet, then craft a rule in the editor with real-time feedback. It highlights matches instantly, which makes iterating on pattern logic much faster than running local test cycles. Once you're happy, you can export the rule as a YAML file ready for your semgrep-ci config.
Here's a simple, practical example from our codebase. We wanted to enforce a specific pattern for our service health checks in Go:
```yaml
rules:
- id: custom-http-health-check-endpoint
patterns:
- pattern: |
http.HandleFunc("/health", $HANDLER)
- metavariable-pattern:
metavariable: $HANDLER
pattern: |
func($W http.ResponseWriter, $R *http.Request) {
$BODY
}
message: "Health check endpoint should use `healthHandler` function"
languages: [go]
severity: WARNING
```
The playground let me test this against several of our services to ensure it only flagged the legacy inline handlers, not the refactored ones. This is perfect for codifying internal best practices or deprecation patterns that off-the-shelf rules can't cover.
For platform teams, this is a powerful way to scale governance. Think about patterns like:
- Ensuring specific resource labels are set in Terraform modules.
- Detecting non-standard logging formats in microservices.
- Enforcing namespace or network policy structures in Kubernetes manifests.
The ability to quickly prototype, share a playground link for team review, and then deploy has streamlined our internal rule creation process significantly. It turns Semgrep from just a security scanner into a unified code policy engine.
Custom rules are great, but you need to validate they work in your actual pipeline environment, not just the playground. The exported YAML still needs proper version control and a review process before it hits CI.
Have you considered the audit trail? Every custom rule change needs to be logged with a rationale, especially if you're in a regulated environment. If you can't trace who wrote a rule and why, you've got a governance gap.
Also, that health check pattern is too broad. It would flag any handler function, not just ones missing your specific `healthHandler`. You need tighter constraints or you'll create noise that teams will learn to ignore.
Where is your SOC 2?
Good point on the audit trail. I'd push that YAML into a repo with PRs required, so every change gets a diff and a comment thread. That covers the "who and why" for most teams.
On the broad pattern, you're totally right about noise. The playground's real-time highlighting is great for spotting that early. If a pattern lights up too much of your test code, you can tweak it right there before exporting.
What's your go-to method for testing exported rules in a staging CI run?
Webhooks or bust.
PRs for rule changes are fine, but they add friction. The real trick is getting teams to actually review those diffs. In my experience, they rubber stamp it unless you tie it to a concrete, painful bug that escaped into production.
>What's your go-to method for testing exported rules in a staging CI run?
We run them against a snapshot of the codebase from last quarter. If a new rule fires on more than, say, 2% of the total possible findings from that snapshot, it's probably too noisy and needs another pass. Blindly testing in staging means you're just waiting for the first annoyed Slack message from a dev team.
-- cost first