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 <open/> 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.
module:set_global();
local server = require "net.http.server";
local codes = require "net.http.codes";
local xml_escape = require "util.stanza".xml_escape;
local render = require "util.interpolation".new("%b{}", xml_escape);
local show_private = module:get_option_boolean("http_errors_detailed", false);
local always_serve = module:get_option_boolean("http_errors_always_show", true);
local default_message = { module:get_option_string("http_errors_default_message", "That's all I know.") };
local default_messages = {
[400] = { "What kind of request do you call that??" };
[403] = { "You're not allowed to do that." };
[404] = { "Whatever you were looking for is not here. %";
"Where did you put it?", "It's behind you.", "Keep looking." };
[500] = { "% Check your error log for more info.";
"Gremlins.", "It broke.", "Don't look at me." };
};
local messages = setmetatable(module:get_option("http_errors_messages", {}), { __index = default_messages });
local html = [[
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>{title}</title>
<style>
body{
margin-top:14%;
text-align:center;
background-color:#F8F8F8;
font-family:sans-serif;
}
h1{
font-size:xx-large;
}
p{
font-size:x-large;
}
p+p {
font-size:large;
font-family:courier;
}
</style>
</head>
<body>
<h1>{title}</h1>
<p>{message}</p>
<p>{extra?}</p>
</body>
</html>
]];
local function get_page(code, extra)
local message = messages[code];
if always_serve or message then
message = message or default_message;
return render(html, {
title = rawget(codes, code) or ("Code "..tostring(code));
message = message[1]:gsub("%%", function ()
return message[math.random(2, math.max(#message,2))];
end);
extra = extra;
});
end
end
module:hook_object_event(server, "http-error", function (event)
if event.response then
event.response.headers.content_type = "text/html; charset=utf-8";
end
return get_page(event.code, (show_private and event.private_message) or event.message);
end);