CORS for Google Tag Manager
medium4 checks
CORS decides which origins may read your API responses. A wildcard turns an authenticated API into a public one.
What Google Tag Manager must have
- Access-Control-Allow-Origin limited to known origins
- No wildcard combined with credentials
- No reflection of arbitrary Origin headers
- Preflight responses only allow the methods actually used
How to fix it
- 1Replace wildcards with an explicit allowlist
- 2Validate Origin before echoing it and reject unknown origins early
Google Tag Manager context
Tag injection container for marketing and analytics scripts.
Anyone with GTM publish rights can inject scripts on every page — a supply-chain trust decision.