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Computer program or file valid in multiple programming languages or file formats

In computing, a polyglot is a computer program or script (or other file) written in a valid form of multiple programming languages or file formats. The name was coined by analogy to multilingualism. A polyglot file is composed by combining syntax from two or more different formats. When the file formats are to be compiled or interpreted as source code, the file can be said to be a polyglot program, though file formats and source code syntax are both fundamentally streams of bytes, and exploiting this commonality is key to the development of polyglots. Polyglot files have practical applications in compatibility, but can also present a security risk when used to bypass validation or to exploit a vulnerability.

Polyglot programs have been crafted as challenges and curios in hacker culture since at least the early 1990s. A notable early example, named simply polyglot was published on the Usenet group rec. puzzles in 1991, supporting 8 languages, though this was inspired by even earlier programs. In 2000, a polyglot program was named a winner in the International Obfuscated C Code Contest. In the 21st century, polyglot programs and files gained attention as a covert channel mechanism for propagation of malware. Such documents can be parsed as either HTML (which is SGML compatible) or XML, and will produce the same DOM structure either way. For example, in order for an HTML5 document to meet these criteria, the two requirements are that it must have an HTML5 doctype, and be written in well formed XHTML. The same document can then be served as either HTML or XHTML, depending on browser support and MIME type. As expressed by the html polyglot recommendation, For example, to add an empty textarea to a page, one cannot use <textarea/>, but has to use <textarea></textarea> instead.

The DICOM medical imaging format was designed to allow polyglotting with TIFF files, allowing efficient storage of the same image data in a file that can be interpreted by either DICOM or TIFF viewers.

The Python 2 and Python 3 programming languages were not designed to be compatible with each other, but there is sufficient commonality of syntax that a polyglot Python program can be written than runs in both versions.

A Graphics Interchange Format Java Archives (GIFAR) is a polyglot file that is simultaneously in the GIF and JAR file format. This technique can be used to exploit security vulnerabilities, for example through uploading a GIFAR to a website that allows image uploading (as it is a valid GIF file), and then causing the Java portion of the GIFAR to be executed as though it were part of the website's intended code, being delivered to the browser from the same origin. Java was patched in JRE 6 Update 11, with a CVE published in December 2008. GIFARs are possible because GIF images store their header in the beginning of the file, and JAR files (as with any ZIP archive based format) store their data at the end.

Polyglot programming, referring to the practise of building systems using multiple programming languages, but not necessarily in the same file. Polyglot persistence is similar, but about databases.