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There are several national variants of ASCII. As a result, the original ASCII is often referred as *US-ASCII*. The international standard _ISO 646_ \[[ISO/IEC 646-1991|AA. C References#ISO/IEC 646-1991]\] defines a character set similar to US-ASCII, but with code positions corresponding to US-ASCII characters {{@\[\]\{\|\}}} as _national use positions_. It also gives some liberties with the characters {{\#$^`\~}}. In _ISO 646_, several national variants of ASCII have been defined, assigning different letters and symbols to the national use positions. Consequently, the characters that appear in those positions, including those in *US-ASCII*, are less portable in international data transfer. Consequently, due to the national variants, some characters are less portable than others --- they might be transferred or interpreted incorrectly. |
In addition to the letters of the English alphabet ("A" through "Z" and "a" through "z"), the digits ("0" through "9"), and the space, only the following characters are portable:
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This non-compliant code example is derived from FIO30-C. Exclude user input from format strings, except that a newline is removed on the assumption that fgets() will include it.
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No checks are performed on the file name to prevent troublesome characters. If an attacker knew this code was in a program used to create or rename files that would later be used in a script or automated process of some sort, they could choose particular characters in the output file name to confuse the later process for malicious purposes.
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char myFilename[1000];
char const elimNewln[] = "\n";
char const badChars[] = "-\n\r ,;'\\<\"";
do {
fgets(myFilename, sizeof(myFilename)-1, stdin);
myFilename[sizeof(myFilename)-1] ='\0';
myFilename[strcspn(myFilename, elimNewln)]='\0';
} while ( (strcspn(myFilename, badChars))
< (strlen(myFilename)));
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Similarly, you must provide validate all file names originating from untrusted sources to ensure they contain only safe characters.
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Failing to use only the subset of ASCII that is guaranteed to work can result in misinterpreted data.
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