Fix frozen string error in FTP mixin when using frozen_string_literal: true

[lxcxjxhx]

Motivation

Fixes #21597

The recv_ftp_resp method in Exploit::Remote::Ftp mixin modifies @ftpbuff string by appending data with << operator. When modules use # frozen_string_literal: true (as now required by AGENTS.md for new files), the string literal '' becomes frozen, causing FrozenError when attempting to modify it.

Changes

Modified lib/msf/core/exploit/remote/ftp.rb:

Before:

@ftpbuff = ''

After:

@ftpbuff = +''

The unary plus operator (+) creates an unfrozen mutable string, ensuring compatibility with frozen_string_literal: true pragma.

Testing

Added test file test/modules/test_ftp_frozen_string.rb with:

• Test verifying @ftpbuff is mutable even with frozen_string_literal: true
• Test verifying recv_ftp_resp works correctly without raising FrozenError
• Tested with auxiliary/dos/ftp/vsftpd_232.rb using frozen_string_literal: true
• Verified with vsftpd 3.0.2 docker container

Impact

• Allows modules to safely use frozen_string_literal: true pragma as required by AGENTS.md
• Maintains backward compatibility with existing modules
• No breaking changes to API or behavior

Checklist

• Fixes Modifying a frozen string cause an error in Exploit::Remote::Ftp mixin #21597
• Code changes are minimal and focused on the issue
• Backward compatibility maintained
• Test cases added
• No breaking changes to existing API
• Follows Metasploit coding standards