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+# DRAFT Appendices
+
+## Security Considerations
+
+MessageFormat 2.0 _patterns_ are meant to allow a _message_ to include any string value
+which users might normally wish to use in their environment.
+Programming languages and other environments vary in what characters are permitted
+to appear in a valid string.
+In many cases, certain types of characters, such as invisible control characters,
+require escaping by these host formats.
+In other cases, strings are not permitted to contain certain characters at all.
+Since _messages_ are subject to the restrictions and limitations of their 
+host environments, their serializations and resource formats,
+that might be sufficient to prevent most problems.
+However, MessageFormat itself does not supply such a restriction.
+
+MessageFormat _messages_ permit nearly all Unicode code points,
+with the exception of surrogates, 
+to appear in _literals_, including the text portions of a _pattern_.
+This means that it can be possible for a _message_ to contain invisible characters
+(such as bidirectional controls, 
+ASCII control characters in the range U+0000 to U+001F,
+or characters that might be interpreted as escapes or syntax in the host format)
+that abnormally affect the display of the _message_
+when viewed as source code, or in resource formats or translation tools,
+but do not generate errors from MessageFormat parsers or processing APIs.
+
+Bidirectional text containing right-to-left characters (such as used for Arabic or Hebrew) 
+also poses a potential source of confusion for users. 
+Since MessageFormat 2.0's syntax makes use of 
+keywords and symbols that are left-to-right or consist of neutral characters 
+(including characters subject to mirroring under the Unicode Bidirectional Algorithm), 
+it is possible to create messages that,
+when displayed in source code, or in resource formats or translation tools, 
+have a misleading appearance or are difficult to parse visually.
+
+For more information, see \[[UTS#55](https://unicode.org/reports/tr55/)\] 
+<cite>Unicode Source Code Handling</cite>.
+
+MessageFormat 2.0 implementations might allow end-users to install
+_selectors_, _functions_, or _markup_ from third-party sources.
+Such functionality can be a vector for various exploits,
+including buffer overflow, code injection, user tracking,
+fingerprinting, and other types of bad behavior.
+Any installed code needs to be appropriately sandboxed.
+In addition, end-users need to be aware of the risks involved.