Kernel boot fails due to missing environment variable in Contao deployment

Required Environment Variable

For the Symfony kernel to initialize in Contao 4.13, the APP_SECRET environment variable must be defined and non-empty. This variable is critical for the security component's bootstrap process; if it is absent, the kernel throws a runtime exception, resulting in a generic 500 error.

Isolating the Variable via Logs

To determine if APP_SECRET is the cause without altering deployment files, examine the application logs. Symfony logs bootstrap failures before the application fully loads. You can isolate the responsible variable by searching for specific keywords in the var/log directory:

grep -i "secret" var/log/prod.log
# Or for development environments
grep -i "secret" var/log/dev.log

If the output contains phrases such as "Missing secret key" or "The secret key is missing or empty", the APP_SECRET variable is confirmed as the culprit. If these phrases are absent, the boot failure may be caused by other variables (such as DATABASE_URL) or file permission issues masking the error.

Verification and Resolution

To confirm that correcting this variable resolves the boot failure, follow these steps:

1. Define a non-empty string for APP_SECRET in your .env or .env.local file.
2. Clear the Symfony cache to ensure the new environment configuration is loaded:

   php bin/console cache:clear --env=prod

3. Reload the web server if using a process manager (e.g., PHP-FPM) to refresh environment variables.
4. Verify the var/log output again; the "Missing secret key" entry should no longer appear, and the kernel should boot successfully.

Diagnostic Note

If the logs are completely empty despite the 500 error, please verify if the web server has write permissions to the var/log directory, as this would mask the underlying environment variable error.