diff --git a/doc/plugins/aggregate.mdwn b/doc/plugins/aggregate.mdwn
index 75123d923..b1db828d1 100644
--- a/doc/plugins/aggregate.mdwn
+++ b/doc/plugins/aggregate.mdwn
@@ -11,6 +11,10 @@ The [[meta]] and [[tag]] plugins are also recommended to be used with this
 one. Either the [[htmltidy]] or [[htmlbalance]] plugin is suggested, since
 feeds can easily contain html problems, some of which these plugins can fix.
 
+Installing the [[!cpan LWPx::ParanoidAgent]] Perl module is strongly
+recommended. The [[!cpan LWP]] module can also be used, but is susceptible
+to server-side request forgery.
+
 ## triggering aggregation
 
 You will need to run ikiwiki periodically from a cron job, passing it the
diff --git a/doc/plugins/blogspam.mdwn b/doc/plugins/blogspam.mdwn
index 745fc48e2..0ebae7d84 100644
--- a/doc/plugins/blogspam.mdwn
+++ b/doc/plugins/blogspam.mdwn
@@ -11,6 +11,8 @@ To check for and moderate comments, log in to the wiki as an admin,
 go to your Preferences page, and click the "Comment Moderation" button.
 
 The plugin requires the [[!cpan JSON]] perl module.
+The [[!cpan LWPx::ParanoidAgent]] Perl module is recommended,
+although this plugin can also fall back to [[!cpan LWP]].
 
 You can control how content is tested via the `blogspam_options` setting.
 The list of options is [here](http://blogspam.net/api/2.0/testComment.html#options).
diff --git a/doc/plugins/openid.mdwn b/doc/plugins/openid.mdwn
index 4c8e0d381..a061cb43f 100644
--- a/doc/plugins/openid.mdwn
+++ b/doc/plugins/openid.mdwn
@@ -7,8 +7,11 @@ into the wiki.
 The plugin needs the [[!cpan Net::OpenID::Consumer]] perl module.
 Version 1.x is needed in order for OpenID v2 to work.
 
-The [[!cpan LWPx::ParanoidAgent]] perl module is used if available, for
-added security. Finally, the [[!cpan Crypt::SSLeay]] perl module is needed
+The [[!cpan LWPx::ParanoidAgent]] Perl module is strongly recommended.
+The [[!cpan LWP]] module can also be used, but is susceptible to
+server-side request forgery.
+
+The [[!cpan Crypt::SSLeay]] Perl module is needed
 to support users entering "https" OpenID urls.
 
 This plugin is enabled by default, but can be turned off if you want to
diff --git a/doc/plugins/pinger.mdwn b/doc/plugins/pinger.mdwn
index 00d83e1bb..f37979ac6 100644
--- a/doc/plugins/pinger.mdwn
+++ b/doc/plugins/pinger.mdwn
@@ -10,9 +10,11 @@ can be kept up-to-date.
 To configure what URLs to ping, use the [[ikiwiki/directive/ping]]
 [[ikiwiki/directive]].
 
-The [[!cpan LWP]] perl module is used for pinging. Or the [[!cpan
-LWPx::ParanoidAgent]] perl module is used if available, for added security.
-Finally, the [[!cpan Crypt::SSLeay]] perl module is needed to support pinging
+The [[!cpan LWPx::ParanoidAgent]] Perl module is strongly recommended.
+The [[!cpan LWP]] module can also be used, but is susceptible
+to server-side request forgery.
+
+The [[!cpan Crypt::SSLeay]] perl module is needed to support pinging
 "https" urls.
 
 By default the pinger will try to ping a site for 15 seconds before timing
diff --git a/doc/security.mdwn b/doc/security.mdwn
index e7770dd27..378a2e4bc 100644
--- a/doc/security.mdwn
+++ b/doc/security.mdwn
@@ -611,3 +611,52 @@ This was fixed in ikiwiki 3.20170111, with fixes backported to Debian 8
 in version 3.20141016.4.
 
 ([[!debcve CVE-2017-0356]]/OVE-20170111-0001)
+
+## Server-side request forgery via aggregate plugin
+
+The ikiwiki maintainers discovered that the [[plugins/aggregate]] plugin
+did not use [[!cpan LWPx::ParanoidAgent]]. On sites where the
+aggregate plugin is enabled, authorized wiki editors could tell ikiwiki
+to fetch potentially undesired URIs even if LWPx::ParanoidAgent was
+installed:
+
+* local files via `file:` URIs
+* other URI schemes that might be misused by attackers, such as `gopher:`
+* hosts that resolve to loopback IP addresses (127.x.x.x)
+* hosts that resolve to RFC 1918 IP addresses (192.168.x.x etc.)
+
+This could be used by an attacker to publish information that should not have
+been accessible, cause denial of service by requesting "tarpit" URIs that are
+slow to respond, or cause undesired side-effects if local web servers implement
+["unsafe"](https://tools.ietf.org/html/rfc7231#section-4.2.1) GET requests.
+([[!debcve CVE-2019-9187]])
+
+Additionally, if the LWPx::ParanoidAgent module was not installed, the
+[[plugins/blogspam]], [[plugins/openid]] and [[plugins/pinger]] plugins
+would fall back to [[!cpan LWP]], which is susceptible to similar attacks.
+This is unlikely to be a practical problem for the blogspam plugin because
+the URL it requests is under the control of the wiki administrator, but
+the openid plugin can request URLs controlled by unauthenticated remote
+users, and the pinger plugin can request URLs controlled by authorized
+wiki editors.
+
+This is addressed in ikiwiki 3.20190228 as follows, with the same fixes
+backported to Debian 9 in version 3.20170111.1:
+
+* URI schemes other than `http:` and `https:` are not accepted, preventing
+  access to `file:`, `gopher:`, etc.
+
+* If a proxy is [[configured in the ikiwiki setup file|tips/using_a_proxy]],
+  it is used for all outgoing `http:` and `https:` requests. In this case
+  the proxy is responsible for blocking any requests that are undesired,
+  including loopback or RFC 1918 addresses.
+
+* If a proxy is not configured, and LWPx::ParanoidAgent is installed,
+  it will be used. This prevents loopback and RFC 1918 IP addresses, and
+  sets a timeout to avoid denial of service via "tarpit" URIs.
+
+* Otherwise, the ordinary LWP user-agent will be used. This allows requests
+  to loopback and RFC 1918 IP addresses, and has less robust timeout
+  behaviour. We are not treating this as a vulnerability: if this
+  behaviour is not acceptable for your site, please make sure to install
+  LWPx::ParanoidAgent or disable the affected plugins.
diff --git a/doc/tips/using_a_proxy.mdwn b/doc/tips/using_a_proxy.mdwn
new file mode 100644
index 000000000..39df3c42a
--- /dev/null
+++ b/doc/tips/using_a_proxy.mdwn
@@ -0,0 +1,22 @@
+Some ikiwiki plugins make outgoing HTTP requests from the web server:
+
+* [[plugins/aggregate]] (to download Atom and RSS feeds)
+* [[plugins/blogspam]] (to check whether a comment or edit is spam)
+* [[plugins/openid]] (to authenticate users)
+* [[plugins/pinger]] (to ping other ikiwiki installations)
+
+If your ikiwiki installation cannot contact the Internet without going
+through a proxy, you can configure this in the [[setup file|setup]] by
+setting environment variables:
+
+    ENV:
+        http_proxy: "http://proxy.example.com:8080"
+        https_proxy: "http://proxy.example.com:8080"
+        # optional
+        no_proxy: ".example.com,www.example.org"
+
+Note that some plugins will use the configured proxy for all destinations,
+even if they are listed in `no_proxy`.
+
+To avoid server-side request forgery attacks, ensure that your proxy does
+not allow requests to addresses that are considered to be internal.