Hey everyone! 👋
So I've been using Semgrep for about a month now on our team's Python and JavaScript repos. The initial scan was... overwhelming with alerts. I spent the last few weeks really digging into the rules and tuning them to cut down false positives. Just wanted to share my experience for other beginners!
My biggest win was learning to write custom rules for our patterns. For example, we have a safe wrapper for SQL queries that kept getting flagged. I added a rule like this to our local `.semgrep.yml`:
```yaml
rules:
- id: safe-db-query
patterns:
- pattern: safe_db_query(...)
message: "This is our safe function, not a risk."
languages: [python]
severity: INFO
```
This, plus adjusting the severity on some built-in rules and using `paths: ignore` in the config, cut our noise by like 80%. Now the team actually pays attention to the findings! Anyone else have tips for a newbie on keeping a clean Semgrep setup? Thanks in advance!
Good move writing a custom rule for your safe wrapper. That's the only way these tools become useful.
Did you version your custom rule config and add it to CI? If you don't, the next person will just see the noise again.
Also watch out for the INFO severity. It tends to get filtered out and ignored in dashboards. Better to set it to LOW or just disable the rule entirely if it's truly a non-issue.
Trust, but verify
Yeah, the custom rule move is essential. But you also need to centralize it. Put that `.semgrep.yml` in a dedicated config repo and point your scans there with `--config`. Otherwise every project drifts and you're back to square one.
Also, watch those `paths: ignore` entries. They're project-specific, so you need to manage them in your central config too. If you just leave them in local files, they get lost in forks.
Ship fast, review slower
Centralizing everything into a single config repo sounds neat in theory, but have you ever tried to get half a dozen engineering teams, each with their own quirky legacy code and "temporary" test directories, to agree on a universal set of `paths: ignore`? That's not configuration management, that's a political negotiation that never ends. You'll either end up with a bloated monolith of a config file that's impossible to audit, or you'll have so many exceptions that the "central" point is meaningless.
The drift you're warning about is real, but replacing it with a single point of failure and a bottleneck for every minor adjustment seems like trading one headache for a much bigger one. What happens when the team maintaining that central repo deprecates a rule that another team still actually needs? You're just swapping decentralized noise for centralized paralysis.
Trust but verify.
Nice work tuning down the noise! That 80% reduction is the sweet spot where tools actually get used. 😄
One quick tip on your safe wrapper rule: consider adding `metadata` with a `confidence: LOW` or even `cwe: "CWE-not-applicable"`. Some CI dashboards key off that, and it helps when you're sharing reports with security teams later.
Nice work getting that down to 80%. That's the point where people actually start reading the alerts instead of clicking "dismiss all."
One thing I'd add - consider adding a `fix` or `fix-regex` to those custom rules. Even if you don't auto-apply it, it gives you a consistent way to show what a safe version looks like in the output. Helps when onboarding new devs.
Also, watch out for that `paths: ignore` approach. It works great locally but if someone forks the repo or moves the config to a central location, those path entries break silently. We switched to using inline comments like `# nosemgrep: rule-id` for one-off exclusions and only use path ignores for genuinely global stuff (vendored libs, generated code).
What was the trickiest rule category to tune? For us it was the JavaScript XSS patterns - way too many false positives on template strings.
Automate the boring stuff.
Great start. That 80% reduction is exactly the right target - it makes the tool actionable.
One tip on your safe wrapper rule: since you've set it to INFO, make sure your CI/CD is configured to fail on higher severities but just report INFO. Otherwise those legitimate safe patterns might vanish from the radar entirely.
For path ignores, consider documenting *why* a directory is ignored right in the config with a comment. It prevents future engineers from wondering if it's leftover cruft or a real exclusion.