Which of sprintf/snprintf is more secure?

c security unix printf secure-coding
Arpit picture Arpit · Sep 6, 2011 · Viewed 66.7k times · Source

I wish to know which of these two options is the more secure one to use:

#define MAXLEN 255
char buff[MAXLEN + 1]

  1. sprintf(buff, "%.*s", MAXLEN, name)

  2. snprintf(buff, MAXLEN, "%s", name)

My understanding is that both are same. Please suggest.

Answer

azernik picture azernik · Sep 6, 2011

The two expressions you gave are not equivalent: sprintf takes no argument specifying the maximum number of bytes to write; it simply takes a destination buffer, a format string, and a bunch of arguments. Therefore, it may write more bytes than your buffer has space for, and in so doing write arbitrary code. The %.*s is not a satisfactory solution because:

  1. When the format specifier refers to length, it's referring to the equivalent of strlen; this is a measure of the number of characters in the string, not its length in memory (i.e. it doesn't count the null terminator).
  2. Any change in the format string (adding a newline, for example) will change the behavior of the sprintf version with respect to buffer overflows. With snprintf, a fixed, clear maximum is set regardless of changes in the format string or input types.

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