Bug 864612 (CVE-2012-4505)

An insufficient input validation flaw, leading to a heap-based buffer overflow was found in libproxy 0.3.x in the px_pac_reload() function.  libproxy allocates a memory to store downloaded proxy.pac proxy auto-configuration file content using the Content-Length size from the remote server's HTTP response header.  Allocation size is content length + 1.  A malicious host hosting proxy.pac, or a man in the middle attacker, could use this flaw to make libproxy allocate insufficient amount of memory and subsequently overflow a heap-based buffer.

http://code.google.com/p/libproxy/source/browse/tags/libproxy-0.3.1/src/lib/pac.c#165

165      self->cache = px_malloc0(content_length+1);
166      for (int recvd=0 ; recvd != content_length ; )
167              recvd += recv(sock, self->cache + recvd, content_length - recvd, 0);

This issue was confirmed with libproxy 0.3.x.  Earlier 0.2.x versions seems to be affected too.  It does not affect 0.4.x versions, where fixed size buffer is used.