INCIDENT RUNBOOK

Fix: Middleware Infinite Redirect Loop in FastAPI on Docker

Comprehensive architectural post-mortem and verified configuration patch for fastapi deployed in docker production environments.

Automated Remediation ProtocolUpdated for 2026 Production

Facing middleware-infinite-redirect-loop in CI/CD or runtime containers?

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● Observed Terminal Error / Stack Trace

ERROR:fastapi.middleware: Middleware 'auth_middleware' caused an infinite redirect loop
ERROR:fastapi.app: Exception in ASGI application
Traceback (most recent call last):
  File '/app/main.py', line 35, in 
    app.add_middleware(auth_middleware)
  File '/app/fastapi/middleware.py', line 123, in add_middleware
    await middleware(app, scope, receive, send)
  File '/app/auth_middleware.py', line 50, in 
    return await redirect('/login')
  File '/app/fastapi/responses.py', line 200, in redirect
    raise HTTPException(status_code=status, headers=headers)

Root Cause Analysis

The infinite redirect loop is caused by a mismatch in the middleware configuration, where a redirect is triggered recursively without a proper termination condition. This can occur when the middleware is not properly configured to handle redirects, or when there are conflicting redirect rules. In a Docker environment, this issue can be exacerbated by container limits and lifecycle mismatches, where the container's runtime environment is not properly synchronized with the application's expectations.

● Configuration & Code Resolution

diff --git a/main.py b/main.py
--- a/main.py
+++ b/main.py
@@ -32,7 +32,7 @@
 from fastapi import FastAPI, Request
 from fastapi.responses import RedirectResponse
-
+from fastapi.middleware import Middleware
+
 app = FastAPI()
-
-app.add_middleware(auth_middleware)
+app.add_middleware(
+    Middleware(
+        auth_middleware,
+        exclude={/login}
+    )
+)

Verification & Build Command

docker exec -it <container_id> bash -c 'curl -I http://localhost:8000/' | grep -q 'HTTP/1.1 200 OK'