The previous app name validation regex permitted shell metacharacters such as `;`, `$`, backticks, `|`, and `&`. These names were embedded unquoted into the generated git pre-receive hook script, allowing an authenticated user to execute arbitrary commands as the dokku user simply by pushing to a remote with a crafted app name. App names are now restricted to lowercase alphanumerics, dots, and hyphens, and the hook script also quotes the app variable as a defense-in-depth measure.
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