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Chandan Agrawal

Everyone suddenly works at Cloudflare now after the 18 Nov outage (me included 😄)

On 18 Nov 2025, Cloudflare faced a global outage - and the interesting part is:
 👉 it wasn’t a cyber attack
 👉 it wasn’t a DDoS
 👉 it wasn’t hardware failure

It happened because of a small configuration change that triggered a chain reaction.

𝗟𝗲𝘁 𝗺𝗲 𝘁𝗿𝘆 𝘁𝗼 𝗲𝘅𝗽𝗹𝗮𝗶𝗻 𝘁𝗵𝗲 𝗥𝗼𝗼𝘁 𝗖𝗮𝘂𝘀𝗲 𝗶𝗻 𝗺𝗼𝘀𝘁 𝘀𝗶𝗺𝗽𝗹𝗲𝗿 𝘄𝗮𝘆 -

Cloudflare uses a ClickHouse database with 𝘁𝘄𝗼 schemas:
𝗱𝗲𝗳𝗮𝘂𝗹𝘁 → main production data
𝗿𝟬 → internal metadata / backup tables

Before the change, the system could only read from the default schema.
So a query like 👇

𝙎𝙀𝙇𝙀𝘾𝙏 * 

𝙁𝙍𝙊𝙈 𝙨𝙮𝙨𝙩𝙚𝙢.𝙘𝙤𝙡𝙪𝙢𝙣𝙨 

𝙒𝙃𝙀𝙍𝙀 𝙩𝙖𝙗𝙡𝙚 = '𝙝𝙩𝙩𝙥_𝙧𝙚𝙦𝙪𝙚𝙨𝙩𝙨_𝙛𝙚𝙖𝙩𝙪𝙧𝙚𝙨';

returned only this:

𝗦𝗰𝗵𝗲𝗺𝗮               𝗖𝗼𝗹𝘂𝗺𝗻
-----------------------------
default                ip_address
default                user_agent
default                country

🔧 The Change
Engineers updated permissions:

𝐆𝐑𝐀𝐍𝐓 𝐒𝐄𝐋𝐄𝐂𝐓 𝐎𝐍 𝐫𝟎.* 𝐓𝐎 𝐛𝐨𝐭_𝐟𝐞𝐚𝐭𝐮𝐫𝐞_𝐠𝐞𝐧𝐞𝐫𝐚𝐭𝐨𝐫;

Meaning:
➡️ “Now the system can also access r0.”

This was likely intentional - maybe for migration or diagnostics.

⚠️ The Hidden Bug

The query didn’t filter by schema.

𝐓𝐡𝐞 𝐚𝐜𝐭𝐮𝐚𝐥 𝐪𝐮𝐞𝐫𝐲 𝐰𝐚𝐬:
-------------------------

SELECT name 
FROM system.columns
WHERE table = 'http_requests_features'

𝐈𝐭 𝐬𝐡𝐨𝐮𝐥𝐝 𝐡𝐚𝐯𝐞 𝐛𝐞𝐞𝐧:
--------------------------

SELECT name 
FROM system.columns
WHERE table = 'http_requests_features'
𝐀𝐍𝐃 𝐝𝐚𝐭𝐚𝐛𝐚𝐬𝐞 = '𝐝𝐞𝐟𝐚𝐮𝐥𝐭';

So after permission change, the system fetched both schemas - resulting in duplicate rows.

📈 After the Change
Row count went from:

3 rows → 6 rows
Example:

ip_address(from default)
user_agent(from default)
country(from default)
𝐢𝐩_𝐚𝐝𝐝𝐫𝐞𝐬𝐬  (𝐝𝐮𝐩𝐥𝐢𝐜𝐚𝐭𝐞, from r0)
𝐮𝐬𝐞𝐫_𝐚𝐠𝐞𝐧𝐭  (𝐝𝐮𝐩𝐥𝐢𝐜𝐚𝐭𝐞, from r0)
𝐜𝐨𝐮𝐧𝐭𝐫𝐲   (𝐝𝐮𝐩𝐥𝐢𝐜𝐚𝐭𝐞, from r0)

💥 Why Did It Break?
Cloudflare's proxy engine (FL2) was designed with a limit:
➡️ Max ~200 features allowed

After duplication, the feature file exceeded that limit.
💾 Memory overflow
😰 Panic
💥 Crash
🌍 Global outage

🧩 𝐊𝐞𝐲 𝐓𝐚𝐤𝐞𝐚𝐰𝐚𝐲𝐬
✔️ Small assumptions can break large systems
 ✔️ Configuration changes must be validated
 ✔️ Queries must define clear scope (schema, filters, limits)
 ✔️ Reliability is not just about smart design - it's about preventing unexpected interactions

Follow Chandan for more 💥💥

#design #tech #dsa #coding #cloudflare #maang #google #microsoft

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