diff --git a/changelog.d/5134.bugfix b/changelog.d/5134.bugfix
new file mode 100644
index 000000000..684d48c53
--- /dev/null
+++ b/changelog.d/5134.bugfix
@@ -0,0 +1 @@
+Blacklist 0.0.0.0 and :: by default for URL previews. Thanks to @opnsec for identifying and responsibly disclosing this issue too!
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index 4ada0fba0..6ed75ff76 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -506,11 +506,12 @@ uploads_path: "DATADIR/uploads"
 #    height: 600
 #    method: scale
 
-# Is the preview URL API enabled?  If enabled, you *must* specify
-# an explicit url_preview_ip_range_blacklist of IPs that the spider is
-# denied from accessing.
+# Is the preview URL API enabled?
 #
-#url_preview_enabled: false
+# 'false' by default: uncomment the following to enable it (and specify a
+# url_preview_ip_range_blacklist blacklist).
+#
+#url_preview_enabled: true
 
 # List of IP address CIDR ranges that the URL preview spider is denied
 # from accessing.  There are no defaults: you must explicitly
@@ -520,6 +521,12 @@ uploads_path: "DATADIR/uploads"
 # synapse to issue arbitrary GET requests to your internal services,
 # causing serious security issues.
 #
+# (0.0.0.0 and :: are always blacklisted, whether or not they are explicitly
+# listed here, since they correspond to unroutable addresses.)
+#
+# This must be specified if url_preview_enabled is set. It is recommended that 
+# you uncomment the following list as a starting point.
+#
 #url_preview_ip_range_blacklist:
 #  - '127.0.0.0/8'
 #  - '10.0.0.0/8'
@@ -530,7 +537,7 @@ uploads_path: "DATADIR/uploads"
 #  - '::1/128'
 #  - 'fe80::/64'
 #  - 'fc00::/7'
-#
+
 # List of IP address CIDR ranges that the URL preview spider is allowed
 # to access even if they are specified in url_preview_ip_range_blacklist.
 # This is useful for specifying exceptions to wide-ranging blacklisted
diff --git a/synapse/config/repository.py b/synapse/config/repository.py
index 3f34ad9b2..fbfcecc24 100644
--- a/synapse/config/repository.py
+++ b/synapse/config/repository.py
@@ -186,17 +186,21 @@ class ContentRepositoryConfig(Config):
             except ImportError:
                 raise ConfigError(MISSING_NETADDR)
 
-            if "url_preview_ip_range_blacklist" in config:
-                self.url_preview_ip_range_blacklist = IPSet(
-                    config["url_preview_ip_range_blacklist"]
-                )
-            else:
+            if "url_preview_ip_range_blacklist" not in config:
                 raise ConfigError(
                     "For security, you must specify an explicit target IP address "
                     "blacklist in url_preview_ip_range_blacklist for url previewing "
                     "to work"
                 )
 
+            self.url_preview_ip_range_blacklist = IPSet(
+                config["url_preview_ip_range_blacklist"]
+            )
+
+            # we always blacklist '0.0.0.0' and '::', which are supposed to be
+            # unroutable addresses.
+            self.url_preview_ip_range_blacklist.update(['0.0.0.0', '::'])
+
             self.url_preview_ip_range_whitelist = IPSet(
                 config.get("url_preview_ip_range_whitelist", ())
             )
@@ -260,11 +264,12 @@ class ContentRepositoryConfig(Config):
         #thumbnail_sizes:
 %(formatted_thumbnail_sizes)s
 
-        # Is the preview URL API enabled?  If enabled, you *must* specify
-        # an explicit url_preview_ip_range_blacklist of IPs that the spider is
-        # denied from accessing.
+        # Is the preview URL API enabled?
         #
-        #url_preview_enabled: false
+        # 'false' by default: uncomment the following to enable it (and specify a
+        # url_preview_ip_range_blacklist blacklist).
+        #
+        #url_preview_enabled: true
 
         # List of IP address CIDR ranges that the URL preview spider is denied
         # from accessing.  There are no defaults: you must explicitly
@@ -274,6 +279,12 @@ class ContentRepositoryConfig(Config):
         # synapse to issue arbitrary GET requests to your internal services,
         # causing serious security issues.
         #
+        # (0.0.0.0 and :: are always blacklisted, whether or not they are explicitly
+        # listed here, since they correspond to unroutable addresses.)
+        #
+        # This must be specified if url_preview_enabled is set. It is recommended that
+        # you uncomment the following list as a starting point.
+        #
         #url_preview_ip_range_blacklist:
         #  - '127.0.0.0/8'
         #  - '10.0.0.0/8'
@@ -284,7 +295,7 @@ class ContentRepositoryConfig(Config):
         #  - '::1/128'
         #  - 'fe80::/64'
         #  - 'fc00::/7'
-        #
+
         # List of IP address CIDR ranges that the URL preview spider is allowed
         # to access even if they are specified in url_preview_ip_range_blacklist.
         # This is useful for specifying exceptions to wide-ranging blacklisted