Fix an s_server arbitrary file read issue on Windows

Running s_server in WWW mode on Windows can allow a client to read files
outside the s_server directory by including backslashes in the name, e.g.

GET /..\myfile.txt HTTP/1.0

There exists a check for this for Unix paths but it is not sufficient
for Windows.

Since s_server is a test tool no CVE is assigned.

Thanks to Jobert Abma for reporting this.
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10215)

(cherry picked from commit 0a4d6c67480a4d2fce514e08d3efe571f2ee99c9)

apps/s_server.c

@@ -3205,6 +3205,12 @@ static int www_body(int s, int stype, int prot, unsigned char *context)
                 if (e[0] == ' ')
                     break;
 
+                if (e[0] == ':') {
+                    /* Windows drive. We treat this the same way as ".." */
+                    dot = -1;
+                    break;
+                }
+
                 switch (dot) {
                 case 1:
                     dot = (e[0] == '.') ? 2 : 0;
@@ -3213,11 +3219,11 @@ static int www_body(int s, int stype, int prot, unsigned char *context)
                     dot = (e[0] == '.') ? 3 : 0;
                     break;
                 case 3:
-                    dot = (e[0] == '/') ? -1 : 0;
+                    dot = (e[0] == '/' || e[0] == '\\') ? -1 : 0;
                     break;
                 }
                 if (dot == 0)
-                    dot = (e[0] == '/') ? 1 : 0;
+                    dot = (e[0] == '/' || e[0] == '\\') ? 1 : 0;
             }
             dot = (dot == 3) || (dot == -1); /* filename contains ".."
                                               * component */
@@ -3231,11 +3237,11 @@ static int www_body(int s, int stype, int prot, unsigned char *context)
 
             if (dot) {
                 BIO_puts(io, text);
-                BIO_printf(io, "'%s' contains '..' reference\r\n", p);
+                BIO_printf(io, "'%s' contains '..' or ':'\r\n", p);
                 break;
             }
 
-            if (*p == '/') {
+            if (*p == '/' || *p == '\\') {
                 BIO_puts(io, text);
                 BIO_printf(io, "'%s' is an invalid path\r\n", p);
                 break;