Why is this trivial C program detected as a Virus?

Question

I'm a bit rusty in C, but I've come across this strange problem:

I wrote this program:

#include <stdio.h>

main()
{
  int n;
  n=1;
  while (n>0)
  {
    puts("Write a number: ");
    scanf(" %d",&n);
    printf("This is the number you wrote: %d\n", n);
  }

}

Apparently there are absolutely no syntax errors, and for what I could understand, neither was any compiling error. It compiled and built perfectly. Now, if I switch this line:

puts("Write a number: ");

with this one:

printf("Write a number: ");

it compiles with no errors but when the compiled object launches, it immediately stops and an anti-virus warning pops up saying it identified a trojan horse. Before taking any conclusions, I built it several times and after getting the same message I scanned it in virustotal.com and this was the result.

Well I know puts is actually more correct than printf given the objective, but still it should work just fine...

What's wrong here?

I'm using AVG anti-virus, and Pelles C to compile.

Thanks in advance.

Answer 1

It's a false positive, obviously. The generated machine code just happens to resemble code that is in the malware database. This has nothing to do with the use of puts().

Answer 2

Anti virus software work on signatures which are basically known patterns in executable code used by virus software.

Some virus in the wild has a similar pattern to the printf version of code you wrote (I searched all of the people who did flag you as a virus, unfortunately none of them publish what their signature files are checking for). Due to the fact you should never call printf with one argument it is likely many anti-virus software providers may use that as part of their signature process.

The two options you have are don't call printf with a single argument (which you shouldn't anyway) or submit your program as a false positive to the antivirus vendors that said your program was a virus and they may update their signatures to rule out your program as a false positive.

Answer 3

printf() has a Uncontrolled format string security risk

you should use puts()

also found this:

see the comments in What is the difference between printf() and puts() in C?

Just a note on using printf instead of puts: never, ever do a printf(variable) to print a string. Use puts(variable) or printf("%s', variable). There's a security risk in using a variable format string: if the variable can be written by an attacker they can attack the program by using format strings. – Zan Lynx Dec 1 '12 at 9:05