CURLcode Curl_init_userdefined(struct Curl_easy *data)
{
  struct UserDefined *set = &data->set;
  CURLcode result = CURLE_OK;

  set->out = stdout; /* default output to stdout */
  set->in_set = stdin;  /* default input from stdin */
  set->err  = stderr;  /* default stderr to stderr */

  /* use fwrite as default function to store output */
  set->fwrite_func = (curl_write_callback)fwrite;

  /* use fread as default function to read input */
  set->fread_func_set = (curl_read_callback)fread;
  set->is_fread_set = 0;
  set->is_fwrite_set = 0;

  set->seek_func = ZERO_NULL;
  set->seek_client = ZERO_NULL;

  /* conversion callbacks for non-ASCII hosts */
  set->convfromnetwork = ZERO_NULL;
  set->convtonetwork   = ZERO_NULL;
  set->convfromutf8    = ZERO_NULL;

  set->filesize = -1;        /* we don't know the size */
  set->postfieldsize = -1;   /* unknown size */
  set->maxredirs = -1;       /* allow any amount by default */

  set->httpreq = HTTPREQ_GET; /* Default HTTP request */
  set->rtspreq = RTSPREQ_OPTIONS; /* Default RTSP request */
  set->ftp_use_epsv = TRUE;   /* FTP defaults to EPSV operations */
  set->ftp_use_eprt = TRUE;   /* FTP defaults to EPRT operations */
  set->ftp_use_pret = FALSE;  /* mainly useful for drftpd servers */
  set->ftp_filemethod = FTPFILE_MULTICWD;

  set->dns_cache_timeout = 60; /* Timeout every 60 seconds by default */

  /* Set the default size of the SSL session ID cache */
  set->general_ssl.max_ssl_sessions = 5;

  set->proxyport = 0;
  set->proxytype = CURLPROXY_HTTP; /* defaults to HTTP proxy */
  set->httpauth = CURLAUTH_BASIC;  /* defaults to basic */
  set->proxyauth = CURLAUTH_BASIC; /* defaults to basic */

  /* SOCKS5 proxy auth defaults to username/password + GSS-API */
  set->socks5auth = CURLAUTH_BASIC | CURLAUTH_GSSAPI;

  /* make libcurl quiet by default: */
  set->hide_progress = TRUE;  /* CURLOPT_NOPROGRESS changes these */

  Curl_mime_initpart(&set->mimepost, data);

  /*
   * libcurl 7.10 introduced SSL verification *by default*! This needs to be
   * switched off unless wanted.
   */
  set->ssl.primary.verifypeer = TRUE;
  set->ssl.primary.verifyhost = TRUE;
#ifdef USE_TLS_SRP
  set->ssl.authtype = CURL_TLSAUTH_NONE;
#endif
  set->ssh_auth_types = CURLSSH_AUTH_DEFAULT; /* defaults to any auth
                                                      type */
  set->ssl.primary.sessionid = TRUE; /* session ID caching enabled by
                                        default */
  set->proxy_ssl = set->ssl;

  set->new_file_perms = 0644;    /* Default permissions */
  set->new_directory_perms = 0755; /* Default permissions */

  /* for the *protocols fields we don't use the CURLPROTO_ALL convenience
     define since we internally only use the lower 16 bits for the passed
     in bitmask to not conflict with the private bits */
  set->allowed_protocols = CURLPROTO_ALL;
  set->redir_protocols = CURLPROTO_ALL &  /* All except FILE, SCP and SMB */
                          ~(CURLPROTO_FILE | CURLPROTO_SCP | CURLPROTO_SMB |
                            CURLPROTO_SMBS);

#if defined(HAVE_GSSAPI) || defined(USE_WINDOWS_SSPI)
  /*
   * disallow unprotected protection negotiation NEC reference implementation
   * seem not to follow rfc1961 section 4.3/4.4
   */
  set->socks5_gssapi_nec = FALSE;
#endif

  /* Set the default CA cert bundle/path detected/specified at build time.
   *
   * If Schannel (WinSSL) is the selected SSL backend then these locations
   * are ignored. We allow setting CA location for schannel only when
   * explicitly specified by the user via CURLOPT_CAINFO / --cacert.
   */
  if(Curl_ssl_backend() != CURLSSLBACKEND_SCHANNEL) {
#if defined(CURL_CA_BUNDLE)
    result = Curl_setstropt(&set->str[STRING_SSL_CAFILE_ORIG], CURL_CA_BUNDLE);
    if(result)
      return result;

    result = Curl_setstropt(&set->str[STRING_SSL_CAFILE_PROXY],
                            CURL_CA_BUNDLE);
    if(result)
      return result;
#endif
#if defined(CURL_CA_PATH)
    result = Curl_setstropt(&set->str[STRING_SSL_CAPATH_ORIG], CURL_CA_PATH);
    if(result)
      return result;

    result = Curl_setstropt(&set->str[STRING_SSL_CAPATH_PROXY], CURL_CA_PATH);
    if(result)
      return result;
#endif
  }

  set->wildcard_enabled = FALSE;
  set->chunk_bgn      = ZERO_NULL;
  set->chunk_end      = ZERO_NULL;
  set->tcp_keepalive = FALSE;
  set->tcp_keepintvl = 60;
  set->tcp_keepidle = 60;
  set->tcp_fastopen = FALSE;
  set->tcp_nodelay = TRUE;
  set->ssl_enable_npn = TRUE;
  set->ssl_enable_alpn = TRUE;
  set->expect_100_timeout = 1000L; /* Wait for a second by default. */
  set->sep_headers = TRUE; /* separated header lists by default */
  set->buffer_size = READBUFFER_SIZE;
  set->upload_buffer_size = UPLOADBUFFER_DEFAULT;
  set->happy_eyeballs_timeout = CURL_HET_DEFAULT;
  set->fnmatch = ZERO_NULL;
  set->upkeep_interval_ms = CURL_UPKEEP_INTERVAL_DEFAULT;
  set->maxconnects = DEFAULT_CONNCACHE_SIZE; /* for easy handles */
  set->httpversion =
#ifdef USE_NGHTTP2
    CURL_HTTP_VERSION_2TLS
#else
    CURL_HTTP_VERSION_1_1
#endif
    ;
  Curl_http2_init_userset(set);
  return result;
}

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.