Georgi Guninski security advisory #70, 2004

DoS in apache httpd 2.0.49, yet still apache much better than windows

Systems affected:
apache httpd 2.0.49, earlier 2.x versions were not tested. 1.3.x is safe.

Risk:  low for 32 bit systems, unknown for 64 bit systems with more than
4GB virtual memory

Date: 28 June 2004

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Description:

There is denial of service in apache httpd 2.0.49. It is possible to
consume arbitrary amount of memory.
On 64 bit systems with more than 4GB virtual memory this may lead to heap
based buffer overflow whose exploitation is unclear at the moment.

Details:
The problem is in server/protocol.c ap_get_mime_headers_core:
------
      if (last_field != NULL) {
            if ((len > 0) && ((*field == '\t') || *field == ' ')) {
...
fold_buf = (char *)apr_palloc(r->pool, alloc_len);
-----
If header lines starts with TAB or SPACE, apache allocates memory for it.

This allows making arbitrary long header lines.

The following applies to 64 bit systems with a lot of virtual memory if
sizeof(long)==8 and sizeof(int)==4.

This code can be hit on line 743:
ap_escape_html(r->pool, last_field),
last_field can be arbitrary long.
Looking into ap_escape_html shows:
----
int i, j;

for (i = 0, j = 0; s[i] != '\0'; i++)
        if (s[i] == '<' || s[i] == '>')
            j += 3;
        else if (s[i] == '&')
            j += 4;

    if (j == 0)
        return apr_pstrmemdup(p, s, i);

    x = apr_palloc(p, i + j + 1);

----
(i+j+1) can be made almost arbitraty because of int signedness.
On linux x86_64 it was confirmed that sending about 820MB of data overflows
(i+j+1) which leads to a crash in memcpy, but with good heap layout more
can be done.

Probably only (i) can wrap, but because of the way in which apache leaks
memory this is not tested yet.


Fix:

Unofficial fix from an Apache httpd developer, use at your risk.
----------------------------------
Index: server/protocol.c
===============================================
RCS file: /home/cvspublic/httpd-2.0/server/protocol.c,v
retrieving revision 1.148
diff -u -r1.148 protocol.c
--- server/protocol.c    22 Apr 2004 22:38:03 -0000    1.148
+++ server/protocol.c    13 Jun 2004 19:47:36 -0000
@@ -716,6 +716,23 @@
                  * continuations that span many many lines.
                  */
                 apr_size_t fold_len = last_len + len + 1; /* trailing null */
+
+                if ((fold_len - 1) > r->server->limit_req_fieldsize) {
+                    r->status = HTTP_BAD_REQUEST;
+                    /* report what we have accumulated so far before the
+                     * overflow (last_field) as the field with the problem
+                     */
+                    apr_table_setn(r->notes, "error-notes",
+                                   apr_pstrcat(r->pool,
+                                               "Size of a request header field "
+                                               "after folding "
+                                               "exceeds server limit.<br />\n"
+                                               "<pre>\n",
+                                               ap_escape_html(r->pool, last_field),
+                                               "</pre>\n", NULL));
+                    return;
+                }
+
                 if (fold_len > alloc_len) {
                     char *fold_buf;
                     alloc_len += alloc_len;
----------------------------------
Workaround:
not known.

Vendor status:
security@apache.org are aware of the bug

Georgi Guninski
http://www.guninski.com