diff --git a/glib/ghostutils.c b/glib/ghostutils.c
index 66be5c9a6..6e671d64c 100644
--- a/glib/ghostutils.c
+++ b/glib/ghostutils.c
@@ -21,6 +21,10 @@
 
 #include <string.h>
 
+#ifdef G_OS_UNIX
+#include <unistd.h>
+#endif
+
 #include "ghostutils.h"
 
 #include "garray.h"
@@ -29,6 +33,10 @@
 #include "gstrfuncs.h"
 #include "glibintl.h"
 
+#ifdef G_PLATFORM_WIN32
+#include <windows.h>
+#endif
+
 
 /**
  * SECTION:ghostutils
@@ -405,6 +413,45 @@ idna_end_of_label (const gchar *str)
   return str;
 }
 
+static gsize
+get_hostname_max_length_bytes (void)
+{
+#if defined(G_OS_WIN32)
+  wchar_t tmp[MAX_COMPUTERNAME_LENGTH];
+  return sizeof (tmp) / sizeof (tmp[0]);
+#elif defined(_SC_HOST_NAME_MAX)
+  glong max = sysconf (_SC_HOST_NAME_MAX);
+  if (max > 0)
+    return (gsize) max;
+
+#ifdef HOST_NAME_MAX
+  return HOST_NAME_MAX;
+#else
+  return _POSIX_HOST_NAME_MAX;
+#endif /* HOST_NAME_MAX */
+#else
+  /* Fallback to some reasonable value
+   * See https://stackoverflow.com/questions/8724954/what-is-the-maximum-number-of-characters-for-a-host-name-in-unix/28918017#28918017 */
+  return 255;
+#endif
+}
+
+/* Returns %TRUE if `strlen (str) > comparison_length`, but without actually
+ * running `strlen(str)`, as that would take a very long time for long
+ * (untrusted) input strings. */
+static gboolean
+strlen_greater_than (const gchar *str,
+                     gsize        comparison_length)
+{
+  gsize i;
+
+  for (i = 0; str[i] != '\0'; i++)
+    if (i > comparison_length)
+      return TRUE;
+
+  return FALSE;
+}
+
 /**
  * g_hostname_to_ascii:
  * @hostname: a valid UTF-8 or ASCII hostname
@@ -413,8 +460,8 @@ idna_end_of_label (const gchar *str)
  * string containing no uppercase letters and not ending with a
  * trailing dot.
  *
- * Returns: an ASCII hostname, which must be freed, or %NULL if
- * @hostname is in some way invalid.
+ * Returns: (nullable) (transfer full): an ASCII hostname, which must be freed,
+ *    or %NULL if @hostname is in some way invalid.
  *
  * Since: 2.22
  **/
@@ -425,6 +472,32 @@ g_hostname_to_ascii (const gchar *hostname)
   GString *out;
   gssize llen, oldlen;
   gboolean unicode;
+  gsize hostname_max_length_bytes = get_hostname_max_length_bytes ();
+
+  /* Do an initial check on the hostname length, as overlong hostnames take a
+   * long time in the IDN cleanup algorithm in nameprep(). The ultimate
+   * restriction is that the IDN-decoded (i.e. pure ASCII) hostname cannot be
+   * longer than 255 bytes. That’s the least restrictive limit on hostname
+   * length of all the ways hostnames can be interpreted. Typically, the
+   * hostname will be an FQDN, which is limited to 253 bytes long. POSIX
+   * hostnames are limited to `get_hostname_max_length_bytes()` (typically 255
+   * bytes).
+   *
+   * See https://stackoverflow.com/a/28918017/2931197
+   *
+   * It’s possible for a hostname to be %-encoded, in which case its decoded
+   * length will be as much as 3× shorter.
+   *
+   * It’s also possible for a hostname to use overlong UTF-8 encodings, in which
+   * case its decoded length will be as much as 4× shorter.
+   *
+   * Note: This check is not intended as an absolute guarantee that a hostname
+   * is the right length and will be accepted by other systems. It’s intended to
+   * stop wildly-invalid hostnames from taking forever in nameprep().
+   */
+  if (hostname_max_length_bytes <= G_MAXSIZE / 4 &&
+      strlen_greater_than (hostname, 4 * MAX (255, hostname_max_length_bytes)))
+    return NULL;
 
   label = name = nameprep (hostname, -1, &unicode);
   if (!name || !unicode)
@@ -594,8 +667,8 @@ punycode_decode (const gchar *input,
  * Of course if @hostname is not an internationalized hostname, then
  * the canonical presentation form will be entirely ASCII.
  *
- * Returns: a UTF-8 hostname, which must be freed, or %NULL if
- * @hostname is in some way invalid.
+ * Returns: (nullable) (transfer full): a UTF-8 hostname, which must be freed,
+ *    or %NULL if @hostname is in some way invalid.
  *
  * Since: 2.22
  **/
@@ -604,6 +677,12 @@ g_hostname_to_unicode (const gchar *hostname)
 {
   GString *out;
   gssize llen;
+  gsize hostname_max_length_bytes = get_hostname_max_length_bytes ();
+
+  /* See the comment at the top of g_hostname_to_ascii(). */
+  if (hostname_max_length_bytes <= G_MAXSIZE / 4 &&
+      strlen_greater_than (hostname, 4 * MAX (255, hostname_max_length_bytes)))
+    return NULL;
 
   out = g_string_new (NULL);
 
diff --git a/glib/guri.c b/glib/guri.c
index 19fe4ac71..571cf10aa 100644
--- a/glib/guri.c
+++ b/glib/guri.c
@@ -604,9 +604,21 @@ parse_host (const gchar  *start,
     }
 
   if (g_hostname_is_non_ascii (decoded))
-    host = g_hostname_to_ascii (decoded);
+    {
+      host = g_hostname_to_ascii (decoded);
+      if (host == NULL)
+        {
+          g_free (decoded);
+          g_set_error (error, G_URI_ERROR, G_URI_ERROR_BAD_HOST,
+                       _("Illegal internationalized hostname ‘%.*s’ in URI"),
+                       (gint) length, start);
+          return FALSE;
+        }
+    }
   else
-    host = g_steal_pointer (&decoded);
+    {
+      host = g_steal_pointer (&decoded);
+    }
 
  ok:
   if (out)
diff --git a/glib/tests/hostutils.c b/glib/tests/hostutils.c
index d694e626c..1d6f8550d 100644
--- a/glib/tests/hostutils.c
+++ b/glib/tests/hostutils.c
@@ -69,7 +69,23 @@ static const gint num_non_round_trip_names = G_N_ELEMENTS (non_round_trip_names)
 static const gchar *bad_names[] = {
   "disallowed\xef\xbf\xbd" "character",
   "non-utf\x88",
-  "xn--mixed-\xc3\xbcp"
+  "xn--mixed-\xc3\xbcp",
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong"
+  "verylongverylongverylongverylongverylongverylongverylongverylongverylong",
 };
 static const gint num_bad_names = G_N_ELEMENTS (bad_names);
 
@@ -81,7 +97,7 @@ test_to_ascii (void)
 
   for (i = 0; i < num_idn_test_domains; i++)
     {
-      g_assert (g_hostname_is_non_ascii (idn_test_domains[i].unicode_name));
+      g_assert_true (g_hostname_is_non_ascii (idn_test_domains[i].unicode_name));
       ascii = g_hostname_to_ascii (idn_test_domains[i].unicode_name);
       g_assert_cmpstr (idn_test_domains[i].ascii_name, ==, ascii);
       g_free (ascii);
@@ -94,14 +110,14 @@ test_to_ascii (void)
   for (i = 0; i < num_non_round_trip_names; i++)
     {
       if (non_round_trip_names[i].orig_is_unicode)
-	g_assert (g_hostname_is_non_ascii (non_round_trip_names[i].orig_name));
+        g_assert_true (g_hostname_is_non_ascii (non_round_trip_names[i].orig_name));
       else
-	g_assert (!g_hostname_is_non_ascii (non_round_trip_names[i].orig_name));
+        g_assert_true (!g_hostname_is_non_ascii (non_round_trip_names[i].orig_name));
 
       if (non_round_trip_names[i].ascii_is_encoded)
-	g_assert (g_hostname_is_ascii_encoded (non_round_trip_names[i].ascii_name));
+        g_assert_true (g_hostname_is_ascii_encoded (non_round_trip_names[i].ascii_name));
       else
-	g_assert (!g_hostname_is_ascii_encoded (non_round_trip_names[i].ascii_name));
+        g_assert_true (!g_hostname_is_ascii_encoded (non_round_trip_names[i].ascii_name));
 
       ascii = g_hostname_to_ascii (non_round_trip_names[i].orig_name);
       g_assert_cmpstr (non_round_trip_names[i].ascii_name, ==, ascii);
@@ -127,7 +143,7 @@ test_to_unicode (void)
 
   for (i = 0; i < num_idn_test_domains; i++)
     {
-      g_assert (g_hostname_is_ascii_encoded (idn_test_domains[i].ascii_name));
+      g_assert_true (g_hostname_is_ascii_encoded (idn_test_domains[i].ascii_name));
       unicode = g_hostname_to_unicode (idn_test_domains[i].ascii_name);
       g_assert_cmpstr (idn_test_domains[i].unicode_name, ==, unicode);
       g_free (unicode);
diff --git a/glib/tests/uri.c b/glib/tests/uri.c
index 72698b73c..2c610382b 100644
--- a/glib/tests/uri.c
+++ b/glib/tests/uri.c
@@ -758,6 +758,10 @@ static const UriAbsoluteTest absolute_tests[] = {
     { NULL, NULL, NULL, -1, NULL, NULL, NULL } },
   { "http://[fe80::dead:beef%25em1%00]/", G_URI_FLAGS_NONE, FALSE, G_URI_ERROR_BAD_HOST,
     { NULL, NULL, NULL, -1, NULL, NULL, NULL } },
+
+  /* Invalid IDN hostname */
+  { "http://xn--mixed-\xc3\xbcp/", G_URI_FLAGS_NONE, FALSE, G_URI_ERROR_BAD_HOST,
+    { NULL, NULL, NULL, -1, NULL, NULL, NULL } },
 };
 
 static void