static char *detect_proxy(struct connectdata *conn)
{
  char *proxy = NULL;

  /* If proxy was not specified, we check for default proxy environment
   * variables, to enable i.e Lynx compliance:
   *
   * http_proxy=http://some.server.dom:port/
   * https_proxy=http://some.server.dom:port/
   * ftp_proxy=http://some.server.dom:port/
   * no_proxy=domain1.dom,host.domain2.dom
   *   (a comma-separated list of hosts which should
   *   not be proxied, or an asterisk to override
   *   all proxy variables)
   * all_proxy=http://some.server.dom:port/
   *   (seems to exist for the CERN www lib. Probably
   *   the first to check for.)
   *
   * For compatibility, the all-uppercase versions of these variables are
   * checked if the lowercase versions don't exist.
   */
  char proxy_env[128];
  const char *protop = conn->handler->scheme;
  char *envp = proxy_env;
  char *prox;

  /* Now, build <protocol>_proxy and check for such a one to use */
  while(*protop)
    *envp++ = (char)tolower((int)*protop++);

  /* append _proxy */
  strcpy(envp, "_proxy");

  /* read the protocol proxy: */
  prox = curl_getenv(proxy_env);

  /*
   * We don't try the uppercase version of HTTP_PROXY because of
   * security reasons:
   *
   * When curl is used in a webserver application
   * environment (cgi or php), this environment variable can
   * be controlled by the web server user by setting the
   * http header 'Proxy:' to some value.
   *
   * This can cause 'internal' http/ftp requests to be
   * arbitrarily redirected by any external attacker.
   */
  if(!prox && !strcasecompare("http_proxy", proxy_env)) {
    /* There was no lowercase variable, try the uppercase version: */
    Curl_strntoupper(proxy_env, proxy_env, sizeof(proxy_env));
    prox = curl_getenv(proxy_env);
  }

  envp = proxy_env;
  if(prox) {
    proxy = prox; /* use this */
  }
  else {
    envp = (char *)"all_proxy";
    proxy = curl_getenv(envp); /* default proxy to use */
    if(!proxy) {
      envp = (char *)"ALL_PROXY";
      proxy = curl_getenv(envp);
    }
  }
  if(proxy)
    infof(conn->data, "Uses proxy env variable %s == '%s'\n", envp, proxy);

  return proxy;
}

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.