🛡️ Sentinel: Fix shell injection and password handling in entrypoint

[google-labs-jules]

🛡️ Sentinel: [CRITICAL/HIGH] Fix command injection and password truncation

🚨 Severity: HIGH
💡 Vulnerability:

1. Unquoted adduser arguments allowed passwords with spaces to be truncated (only first word used).
2. Unquoted variable expansion allowed globbing (*), causing file names to be injected as passwords or command arguments.
3. read without -r consumed backslashes in passwords.

🎯 Impact:

• Users setting strong passwords/phrases (e.g., "my secret pass") end up with truncated passwords ("my").
• Passwords containing * could be replaced by file names from the container's file system, leading to unauthorized access or denial of service.
• Backslashes in passwords were silently removed.

🔧 Fix:

• Added proper quoting to shell variables.
• Added read -r flag.
• Wrapped dynamic command execution in set -f / set +f to disable globbing safely.

✅ Verification:

• Added tests/test_entrypoint_injection.sh which mocks vpncmd and verifies that spaces, backslashes, and glob characters are preserved correctly.

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PR created automatically by Jules for task 16140805324010560972 started by @bluPhy

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