Merge pull request 'Completely disable xml entity resolution' (#614) from MaeIsBad/akkoma:completely-disable-xml-entity-resolution into develop

Reviewed-on: https://akkoma.dev/AkkomaGang/akkoma/pulls/614

CHANGELOG.md

@@ -39,7 +39,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 ## Security
 
 - Add `no_new_privs` hardening to OpenRC and systemd service files
-- Ensured that XML parsers cannot load external entities (thanks @Mae@is.badat.dev!)
+- XML parsers cannot load any entities (thanks @Mae@is.badat.dev!)
 - Reduced permissions of config files and directories, distros requiring greater permissions like group-read need to pre-create the directories
 
 ## Removed

lib/pleroma/web/xml.ex

@@ -31,7 +31,7 @@ defmodule Pleroma.Web.XML do
         |> :binary.bin_to_list()
         |> :xmerl_scan.string(
           quiet: true,
-          fetch_fun: fn _, _ -> raise "Resolving external entities not supported" end
+          allow_entities: false
         )
 
       {:ok, doc}

test/fixtures/xml_billion_laughs.xml

@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<!DOCTYPE lolz [
+ <!ENTITY lol "lol">
+ <!ELEMENT lolz (#PCDATA)>
+ <!ENTITY lol1 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
+ <!ENTITY lol2 "&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;">
+ <!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
+ <!ENTITY lol4 "&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;">
+ <!ENTITY lol5 "&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;">
+ <!ENTITY lol6 "&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;">
+ <!ENTITY lol7 "&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;">
+ <!ENTITY lol8 "&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;">
+ <!ENTITY lol9 "&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;">
+]>
+<lolz>&lol9;</lolz>

test/pleroma/web/xml_test.exs

@@ -3,6 +3,11 @@ defmodule Pleroma.Web.XMLTest do
 
   alias Pleroma.Web.XML
 
+  test "refuses to parse any entities from XML" do
+    data = File.read!("test/fixtures/xml_billion_laughs.xml")
+    assert(:error == XML.parse_document(data))
+  end
+
   test "refuses to load external entities from XML" do
     data = File.read!("test/fixtures/xml_external_entities.xml")
     assert(:error == XML.parse_document(data))