[lbipng15] Fixed the recently reported 1's complement security issue by

  replacing the value that is illegal in the PNG spec, in both signed and
  unsigned values, with 0. Illegal unsigned values (anything greater than or
  equal to  0x80000000) can still pass through, but since these are not illegal
  in ANSI-C (unlike 0x80000000 in the signed case) the checking that
  occurs later can catch them (John Bowler).

Safely convert num_bytes to a png_byte in png_set_sig_bytes() (Robert
  Seacord).

1 files changed, 7 insertions, 1 deletions

diff --git a/pngrutil.c b/pngrutil.c
index dccca52c4..ae1727a52 100644
--- a/pngrutil.c
+++ b/pngrutil.c
@@ -89,7 +89,13 @@ png_get_int_32)(png_const_bytep buf)
       return uval;
 
    uval = (uval ^ 0xffffffff) + 1;  /* 2's complement: -x = ~x+1 */
-   return -(png_int_32)uval;
+   if ((uval & 0x80000000) == 0) /* no overflow */
+       return -(png_int_32)uval;
+   /* The following has to be safe; this function only gets called on PNG data
+    * and if we get here that data is invalid.  0 is the most safe value and
+    * if not then an attacker would surely just generate a PNG with 0 instead.
+    */
+   return 0;
 }
 
 /* Grab an unsigned 16-bit integer from a buffer in big-endian format. */