diff --git a/gnu/local.mk b/gnu/local.mk
index a35e5ae7e..cf1dcc2cc 100644
--- a/gnu/local.mk
+++ b/gnu/local.mk
@@ -1063,6 +1063,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/plink-endian-detection.patch		\
   %D%/packages/patches/plotutils-libpng-jmpbuf.patch		\
   %D%/packages/patches/podofo-cmake-3.12.patch			\
+  %D%/packages/patches/polkit-CVE-2018-19788.patch		\
   %D%/packages/patches/poppler-CVE-2018-19149.patch		\
   %D%/packages/patches/portaudio-audacity-compat.patch		\
   %D%/packages/patches/portmidi-modular-build.patch		\
diff --git a/gnu/packages/patches/polkit-CVE-2018-19788.patch b/gnu/packages/patches/polkit-CVE-2018-19788.patch
new file mode 100644
index 000000000..58cde6c5d
--- /dev/null
+++ b/gnu/packages/patches/polkit-CVE-2018-19788.patch
@@ -0,0 +1,197 @@
+Fix CVE-2018-19788:
+
+https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-19788
+https://gitlab.freedesktop.org/polkit/polkit/issues/74
+
+Patch copied from upstream source repository:
+
+https://gitlab.freedesktop.org/polkit/polkit/commit/2cb40c4d5feeaa09325522bd7d97910f1b59e379
+
+From 2cb40c4d5feeaa09325522bd7d97910f1b59e379 Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?Zbigniew=20J=C4=99drzejewski-Szmek?= <zbyszek@in.waw.pl>
+Date: Mon, 3 Dec 2018 10:28:58 +0100
+Subject: [PATCH] Allow negative uids/gids in PolkitUnixUser and Group objects
+
+(uid_t) -1 is still used as placeholder to mean "unset". This is OK, since
+there should be no users with such number, see
+https://systemd.io/UIDS-GIDS#special-linux-uids.
+
+(uid_t) -1 is used as the default value in class initialization.
+
+When a user or group above INT32_MAX is created, the numeric uid or
+gid wraps around to negative when the value is assigned to gint, and
+polkit gets confused. Let's accept such gids, except for -1.
+
+A nicer fix would be to change the underlying type to e.g. uint32 to
+not have negative values. But this cannot be done without breaking the
+API, so likely new functions will have to be added (a
+polkit_unix_user_new variant that takes a unsigned, and the same for
+_group_new, _set_uid, _get_uid, _set_gid, _get_gid, etc.). This will
+require a bigger patch.
+
+Fixes https://gitlab.freedesktop.org/polkit/polkit/issues/74.
+---
+ src/polkit/polkitunixgroup.c   | 15 +++++++++++----
+ src/polkit/polkitunixprocess.c | 12 ++++++++----
+ src/polkit/polkitunixuser.c    | 13 ++++++++++---
+ 3 files changed, 29 insertions(+), 11 deletions(-)
+
+diff --git a/src/polkit/polkitunixgroup.c b/src/polkit/polkitunixgroup.c
+index c57a1aa..309f689 100644
+--- a/src/polkit/polkitunixgroup.c
++++ b/src/polkit/polkitunixgroup.c
+@@ -71,6 +71,7 @@ G_DEFINE_TYPE_WITH_CODE (PolkitUnixGroup, polkit_unix_group, G_TYPE_OBJECT,
+ static void
+ polkit_unix_group_init (PolkitUnixGroup *unix_group)
+ {
++  unix_group->gid = -1; /* (git_t) -1 is not a valid GID under Linux */
+ }
+ 
+ static void
+@@ -100,11 +101,14 @@ polkit_unix_group_set_property (GObject      *object,
+                                GParamSpec   *pspec)
+ {
+   PolkitUnixGroup *unix_group = POLKIT_UNIX_GROUP (object);
++  gint val;
+ 
+   switch (prop_id)
+     {
+     case PROP_GID:
+-      unix_group->gid = g_value_get_int (value);
++      val = g_value_get_int (value);
++      g_return_if_fail (val != -1);
++      unix_group->gid = val;
+       break;
+ 
+     default:
+@@ -131,9 +135,9 @@ polkit_unix_group_class_init (PolkitUnixGroupClass *klass)
+                                    g_param_spec_int ("gid",
+                                                      "Group ID",
+                                                      "The UNIX group ID",
+-                                                     0,
++                                                     G_MININT,
+                                                      G_MAXINT,
+-                                                     0,
++                                                     -1,
+                                                      G_PARAM_CONSTRUCT |
+                                                      G_PARAM_READWRITE |
+                                                      G_PARAM_STATIC_NAME |
+@@ -166,9 +170,10 @@ polkit_unix_group_get_gid (PolkitUnixGroup *group)
+  */
+ void
+ polkit_unix_group_set_gid (PolkitUnixGroup *group,
+-                          gint gid)
++                           gint gid)
+ {
+   g_return_if_fail (POLKIT_IS_UNIX_GROUP (group));
++  g_return_if_fail (gid != -1);
+   group->gid = gid;
+ }
+ 
+@@ -183,6 +188,8 @@ polkit_unix_group_set_gid (PolkitUnixGroup *group,
+ PolkitIdentity *
+ polkit_unix_group_new (gint gid)
+ {
++  g_return_val_if_fail (gid != -1, NULL);
++
+   return POLKIT_IDENTITY (g_object_new (POLKIT_TYPE_UNIX_GROUP,
+                                        "gid", gid,
+                                        NULL));
+diff --git a/src/polkit/polkitunixprocess.c b/src/polkit/polkitunixprocess.c
+index 972b777..b02b258 100644
+--- a/src/polkit/polkitunixprocess.c
++++ b/src/polkit/polkitunixprocess.c
+@@ -159,9 +159,14 @@ polkit_unix_process_set_property (GObject      *object,
+       polkit_unix_process_set_pid (unix_process, g_value_get_int (value));
+       break;
+ 
+-    case PROP_UID:
+-      polkit_unix_process_set_uid (unix_process, g_value_get_int (value));
++    case PROP_UID: {
++      gint val;
++
++      val = g_value_get_int (value);
++      g_return_if_fail (val != -1);
++      polkit_unix_process_set_uid (unix_process, val);
+       break;
++    }
+ 
+     case PROP_START_TIME:
+       polkit_unix_process_set_start_time (unix_process, g_value_get_uint64 (value));
+@@ -239,7 +244,7 @@ polkit_unix_process_class_init (PolkitUnixProcessClass *klass)
+                                    g_param_spec_int ("uid",
+                                                      "User ID",
+                                                      "The UNIX user ID",
+-                                                     -1,
++                                                     G_MININT,
+                                                      G_MAXINT,
+                                                      -1,
+                                                      G_PARAM_CONSTRUCT |
+@@ -303,7 +308,6 @@ polkit_unix_process_set_uid (PolkitUnixProcess *process,
+                              gint               uid)
+ {
+   g_return_if_fail (POLKIT_IS_UNIX_PROCESS (process));
+-  g_return_if_fail (uid >= -1);
+   process->uid = uid;
+ }
+ 
+diff --git a/src/polkit/polkitunixuser.c b/src/polkit/polkitunixuser.c
+index 8bfd3a1..234a697 100644
+--- a/src/polkit/polkitunixuser.c
++++ b/src/polkit/polkitunixuser.c
+@@ -72,6 +72,7 @@ G_DEFINE_TYPE_WITH_CODE (PolkitUnixUser, polkit_unix_user, G_TYPE_OBJECT,
+ static void
+ polkit_unix_user_init (PolkitUnixUser *unix_user)
+ {
++  unix_user->uid = -1;  /* (uid_t) -1 is not a valid UID under Linux */
+   unix_user->name = NULL;
+ }
+ 
+@@ -112,11 +113,14 @@ polkit_unix_user_set_property (GObject      *object,
+                                GParamSpec   *pspec)
+ {
+   PolkitUnixUser *unix_user = POLKIT_UNIX_USER (object);
++  gint val;
+ 
+   switch (prop_id)
+     {
+     case PROP_UID:
+-      unix_user->uid = g_value_get_int (value);
++      val = g_value_get_int (value);
++      g_return_if_fail (val != -1);
++      unix_user->uid = val;
+       break;
+ 
+     default:
+@@ -144,9 +148,9 @@ polkit_unix_user_class_init (PolkitUnixUserClass *klass)
+                                    g_param_spec_int ("uid",
+                                                      "User ID",
+                                                      "The UNIX user ID",
+-                                                     0,
++                                                     G_MININT,
+                                                      G_MAXINT,
+-                                                     0,
++                                                     -1,
+                                                      G_PARAM_CONSTRUCT |
+                                                      G_PARAM_READWRITE |
+                                                      G_PARAM_STATIC_NAME |
+@@ -182,6 +186,7 @@ polkit_unix_user_set_uid (PolkitUnixUser *user,
+                           gint uid)
+ {
+   g_return_if_fail (POLKIT_IS_UNIX_USER (user));
++  g_return_if_fail (uid != -1);
+   user->uid = uid;
+ }
+ 
+@@ -196,6 +201,8 @@ polkit_unix_user_set_uid (PolkitUnixUser *user,
+ PolkitIdentity *
+ polkit_unix_user_new (gint uid)
+ {
++  g_return_val_if_fail (uid != -1, NULL);
++
+   return POLKIT_IDENTITY (g_object_new (POLKIT_TYPE_UNIX_USER,
+                                         "uid", uid,
+                                         NULL));
+-- 
+2.18.1
+
diff --git a/gnu/packages/polkit.scm b/gnu/packages/polkit.scm
index 7cc5205fa..830006d43 100644
--- a/gnu/packages/polkit.scm
+++ b/gnu/packages/polkit.scm
@@ -49,6 +49,7 @@
              (uri (string-append
                    "https://www.freedesktop.org/software/polkit/releases/"
                    name "-" version ".tar.gz"))
+             (patches (search-patches "polkit-CVE-2018-19788.patch"))
              (sha256
               (base32
                "0c91y61y4gy6p91cwbzg32dhavw4b7fflg370rimqhdxpzdfr1rg"))
