PVS-Studio warning V618 for WinSCP

bool CAsyncSslSocketLayer::CreateSslCertificate(....)
{
  ....
  char buffer[1001];
  int len;
  while ((len = pBIO_read(bio, buffer, 1000)) > 0)
  {
    buffer[len] = 0;
    fprintf(file, buffer);
  }
  ....
}

This suspicious code was found in WinSCP project by PVS-Studio static code analyzer.
Warning message is:
V618 It's dangerous to call the 'fprintf' function in such a manner, as the line being passed could contain format specification. The example of the safe code: printf("%s", str); asyncsslsocketlayer.cpp 2247

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