From e0b15fcba3125d7f5f981f565a65f6bae770ace6 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Jonas=20Sch=C3=A4fer?= Date: Mon, 10 Jan 2022 18:23:54 +0100 Subject: util.xml: Do not allow doctypes, comments or processing instructions Yes. This is as bad as it sounds. CVE pending. In Prosody itself, this only affects mod_websocket, which uses util.xml to parse the frame, thus allowing unauthenticated remote DoS using Billion Laughs. However, third-party modules using util.xml may also be affected by this. This commit installs handlers which disallow the use of doctype declarations and processing instructions without any escape hatch. It, by default, also introduces such a handler for comments, however, there is a way to enable comments nontheless. This is because util.xml is used to parse human-facing data, where comments are generally a desirable feature, and also because comments are generally harmless. --- util/xml.lua | 25 +++++++++++++++++++++++-- 1 file changed, 23 insertions(+), 2 deletions(-) (limited to 'util') diff --git a/util/xml.lua b/util/xml.lua index 38411bfb..549ab104 100644 --- a/util/xml.lua +++ b/util/xml.lua @@ -3,6 +3,7 @@ local st = require "util.stanza"; local lxp = require "lxp"; local t_insert = table.insert; local t_remove = table.remove; +local error = error; local _ENV = nil; -- luacheck: std none @@ -13,7 +14,7 @@ local parse_xml = (function() }; local ns_separator = "\1"; local ns_pattern = "^([^"..ns_separator.."]*)"..ns_separator.."?(.*)$"; - return function(xml) + return function(xml, options) --luacheck: ignore 212/self local handler = {}; local stanza = st.stanza("root"); @@ -64,7 +65,27 @@ local parse_xml = (function() function handler:EndElement() stanza:up(); end - local parser = lxp.new(handler, ns_separator); + local parser; + -- SECURITY: These two handlers, especially the Doctype one, are required to prevent exploits such as Billion Laughs. + function handler:StartDoctypeDecl() + if not parser.stop or not parser:stop() then + error("Failed to abort parsing"); + end + end + function handler:ProcessingInstruction() + if not parser.stop or not parser:stop() then + error("Failed to abort parsing"); + end + end + if not options or not options.allow_comments then + -- NOTE: comments are generally harmless and can be useful when parsing configuration files or other data, even user-provided data + function handler:Comment() + if not parser.stop or not parser:stop() then + error("Failed to abort parsing"); + end + end + end + parser = lxp.new(handler, ns_separator); local ok, err, line, col = parser:parse(xml); if ok then ok, err, line, col = parser:parse(); end --parser:close(); -- cgit v1.2.3