OWASP WAF Naxsi suffers from a bypass vulnerability.
086ae504afd9243fc50ad06efe7ad3f4780533c5b6293a8ed1470c10d736b667
OWASP WAF Naxsi bypass Vulnerability
Certain unspecified input is not properly handed in
naxsi_src/naxsi_utils.c naxsi_unescape_uri(u_char **dst, u_char **src,
size_t size, ngx_uint_t type) before being used to filtered. This can
be exploited to bypass some WAF rules.
Naxsi site
https://code.google.com/p/naxsi/
Affected
All the version
My site
http://safe3.com.cn/
My nick name is Safe3
It happens like that,the naxsi_unescape_uri function process the % url
decode,if the next char after the % is a hex char and not after the
hex char,then it will drop the % and the next char.So if we input a
sql keyword "s%elect",it will come to "slect" instead,this is not the
standard url decode way.Such as the IIS asp,it will process the
"s%elect" as a result of "select",so we can bypass some
WAF rules just like that.
I afford a standard url decode function patch as the follow to fix this issue:
void
ngx_unescape_uri(u_char **dst, u_char **src, size_t size, ngx_uint_t type)
{
u_char *d, *s, ch, c, decoded;
enum {
sw_usual = 0,
sw_quoted,
sw_quoted_second
} state;
d = *dst;
s = *src;
state = 0;
decoded = 0;
while (size--) {
ch = *s++;
switch (state) {
case sw_usual:
if (ch == '?'
&& (type & (NGX_UNESCAPE_URI|NGX_UNESCAPE_REDIRECT)))
{
*d++ = ch;
goto done;
}
if (ch == '%'&&size>1) {
ch=*s;
c = (u_char) (ch | 0x20);
if ((ch >= '0' && ch <= '9')||(c >= 'a' && c <= 'f')) {
ch=*(s+1);
c = (u_char) (ch | 0x20);
if ((ch >= '0' && ch <= '9')||(c >= 'a' && c <= 'f')) {
state = sw_quoted;
break;
}
}
*d++ = '%';
break;
}
if (ch == '+') {
*d++ = ' ';
break;
}
*d++ = ch;
break;
case sw_quoted:
if (ch >= '0' && ch <= '9') {
decoded = (u_char) (ch - '0');
state = sw_quoted_second;
break;
}
c = (u_char) (ch | 0x20);
if (c >= 'a' && c <= 'f') {
decoded = (u_char) (c - 'a' + 10);
state = sw_quoted_second;
break;
}
/* the invalid quoted character */
state = sw_usual;
*d++ = ch;
break;
case sw_quoted_second:
state = sw_usual;
if (ch >= '0' && ch <= '9') {
ch = (u_char) ((decoded << 4) + ch - '0');
if (type & NGX_UNESCAPE_REDIRECT) {
if (ch > '%' && ch < 0x7f) {
*d++ = ch;
break;
}
*d++ = '%'; *d++ = *(s - 2); *d++ = *(s - 1);
break;
}
*d++ = ch;
break;
}
c = (u_char) (ch | 0x20);
if (c >= 'a' && c <= 'f') {
ch = (u_char) ((decoded << 4) + c - 'a' + 10);
if (type & NGX_UNESCAPE_URI) {
if (ch == '?') {
*d++ = ch;
goto done;
}
*d++ = ch;
break;
}
if (type & NGX_UNESCAPE_REDIRECT) {
if (ch == '?') {
*d++ = ch;
goto done;
}
if (ch > '%' && ch < 0x7f) {
*d++ = ch;
break;
}
*d++ = '%'; *d++ = *(s - 2); *d++ = *(s - 1);
break;
}
*d++ = ch;
break;
}
/* the invalid quoted character */
break;
}
}
done:
*dst = d;
*src = s;
}