Hi there, you’ve got a slick identity federation flow with Microsoft Entra External ID.
User hits login.yourbrand.com, gets redirected to Entra, auths like a champ, and…
BOOM — 403 Forbidden.
Why?
Because your Azure Web Application Firewall (WAF) is having a mental breakdown over a legit id_token
.
Yeah — your security layer just called your login system a malicious payload.
🤨 What’s the Real Problem?
Your federated OpenID Connect/OAuth2 flow passes stuff like this:
That id_token
?
Just a regular base64-encoded JWT.
To you? Harmless.
To WAF? A ticking SQLi timebomb disguised as a login.
Why does WAF flip out?
Because of high entropy, dots, slashes, equals, and God forbid — nested encodings.
-
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
→ “obfuscated attack!” -
code=0.AQIA...
→ “too long! Must be exfiltration!” -
redirect_uri=https://yourapp.com/callback?code=...
→ “wait… query inside query?”
In short, WAF sees JWT and OAuth as a stylish zero-day trying to sneak in wearing a tuxedo.
⚠️ The Usual Suspects — Which Rules Go Rogue?
Rule ID | Description |
---|---|
942430 |
SQL Injection (via libinjection) |
942440 |
SQLi patterns in encoded input (e.g., /* , -- , # ) |
941160 |
Suspicious comments or keyword obfuscation |
944100 |
Inbound anomaly score threshold (default block trigger) |
These rules trigger on base64 tokens, long strings, and symbols that JWTs love.
🛠 Step-by-Step: How to Un-break Your Login Without Breaking Security
1. Start in Detection Mode
Never go full block mode without seeing what the WAF hates.
Attach it to your Front Door.
2. Log Everything — Especially Blocks
Use Log Analytics to catch the WAF in action:
Look for:
-
id_token
-
code
-
redirect_uri
-
state
If you see rules like 942430
, 942440
, or 941160
— congrats, WAF is eating your auth flow alive.
3. Apply Exclusion Rules — Surgical Style
Don’t disable the whole ruleset.
Just tell WAF: “If the payload comes from these parameters, chill out.”
Example (JSON exclusion block):
Do this for:
-
id_token
-
code
-
state
-
redirect_uri
-
client_id
(if used in request body)
Pro tip: also exclude RequestArgNames
, not just POST body, depending on where your flow lands.
4. Move to Prevention Mode (Once Clean)
Test. Test. Test.
Then flip the switch:
Now WAF will block the bad stuff, and let legit tokens through like a polite bouncer with facial recognition.
🧠 Why All This Actually Matters
Let’s break it down like nerds:
Field | Looks Legit To You | WAF Thinks |
---|---|---|
id_token |
JWT from Entra | Obfuscated XSS |
code |
OAuth2 artifact | Command injection |
redirect_uri |
Encoded URL | LFI trap |
state |
CSRF token | Exfil payload |
Because WAF inspects everything after URL decode → HTML entity decode → Unicode normalize — even a comma in the wrong place starts a panic attack.
🛡 Bonus: Make It Even More Bulletproof
✅ Rate Limiting = Stop Brute Logins
✅ Bot Manager = Stop Script Kiddies
Enabled by default in Azure Front Door Premium. It filters:
-
Fake user agents
-
Malformed headers
-
Known bot networks
And if you’re not using a custom domain, you should.ciamlogin.com
is not enough.
Use login.yourbrand.com
for cookie control, TLS pinning, and redirect sanity.
📋 CI/CD Integration — What You Should Automate
Task | Why It Matters |
---|---|
WAF policy deployment via Bicep/Terraform | Repeatable, auditable |
Log scans via scheduled KQL jobs | Detect new false-positives |
GitHub Action to diff WAF logs weekly | Regression alerting |
Exclusion templates stored in version control | Traceability & rollback |
🎤 Final Words: Don’t Let WAF Be Your Worst Enemy
-
Microsoft Entra External ID is not the attacker. But WAF doesn’t know that.
-
Don’t neuter your firewall — teach it who the friends are.
-
False positives kill UX, kill revenue, kill trust.
-
Your job is to tune, not disable.
-
Your security posture shouldn’t come at the cost of “login doesn’t work.”
Want next level? I can throw together:
-
Terraform modules for exclusions
-
Dynamic KQL dashboards with rule heatmaps
-
JSON templates for all rule groups
-
Or a GitHub Copilot that tunes WAF in PRs
Just say the word.
Otherwise — go secure that flow like a legend.