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Difference "Secure Programming with CppCMS" ver. 20 versus ver. 21

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## Introduction

Security is one of the major concerns in development of web applications as they are instantly exposed to the world that is full of potential attackers.

Writing web applications today is very similar to walking

over a minefield. If you don't have proper tools and knowledge you are most likely going to loose a "virtual lag".

## State of Mind

There are several golden rules in writing secure web applications:

1. Never trust users input.

2. Think - "How can I attack my application"

3. Learn common vulnerabilities and methods of their prevention so you at least would not do the mistakes others did.

So when it comes to writing web applications always remember that there *is* somebody who would actually try

to something bad to your site and you are lucky if you

will discover this.

What is even more important that at some point, your

_will_ make a mistake and your web site

_will_ be cracked and you'll have to do a hard work

to restore it. So keep backups and even more important

check your [restore procedures](http://www.joelonsoftware.com/items/2009/12/14.html) work.

## SQL Injections

This is one of the most basic problems that

web developer should be familiar with.

Have you ever written code like:

std::string query=

"SELECT 1 from users "

"WHERE username='" + username +"' AND "

" password='" + password "';

mysql_query(conn,query.c_str());

...

What is the problem?

Assume that password value is

' OR ''='

So the query would look like

SELECT 1 from users

WHERE username='user' AND

password='' OR ''=''

Which would always return 1...

So you get authenticated for knowing SQL `:-)` and not

having proper password

If you had ever did it then you should never do

web development till you read this article:

[SQL Injection](http://en.wikipedia.org/wiki/SQL_injection)

How do you prevent SQL injections?

Use prepared statements. [CppDB library](http://art-blog.no-ip.info/sql/cppdb/) provides all tools you need.

Just rewrite the example above"

sql << "SELECT 1 FROM users "

"WHERE username=? AND password=?"

<< username << password;

Now the values of ? would be _properly_ substituted

with correct values with their real equivalent.

The query as value would be physically separated.

## Sessions

HTTP protocol is stateless so in order to keep

a track of the users on our web site we store

an information in the session. For example:

session().set("username",username);

Now we can know who is the user. Now if we have

the `username` in the session we know what he can

do for example.

There are two ways to store the data in the session:

1. Store all the data at the server side saving a unique

session id at the client side in cookie.

2. Store all the data at the client side in a cookie and

we ensure its validity by digitally signing it.

See: [CppCMS Configuration: session](http://art-blog.no-ip.info/wikipp/en/page/cppcms_1x_config#session)

This cookie is very valuable information as if attacker

gets this cookie he can impersonate himself to this

particular user.

### What to store in the session?

It is ok to store some not-so valuable information

like user name user id and may be some properties

then can affect user itself.

However there are several things you should not

keep in session: user permissions and rights.

Remember that each user can have more then one active

session. Consider you store his rights in the session,

now we want to drop his right and we can't as we don't

know which sessions belong to the user.

### Session handling is cookie based

In CppCMS session handing is cookie based and

only cookie based, so trust CppCMS and don't

reinvent your own methods.

There are many popular methods to store session

id that have design flaws. For example, consider

a popular method of keeping session id in the query string:

http://www.mybank.com/?sid=1234564435

Now publish a link `http://attckers_site.com/` on the

`www.mybank.com` web site. The innocent user

clicks on this link the attacker get user's session id

in the Referrer HTTP header.

Now attacker can go to `www.mybank.com` add `?sid=1234564435` to the URL and transfer all

users money to his account.

So don't reinvent the wheel, use CppCMS session

mechanism that handles this in safely for you.

### Client side storage considerations (Replay attack)

Client side storage is very useful as it allows to

keep all the data at the client side without need

of keeping tack of file system or using DB for

the session storage.

This is especially critical for high performance

applications.

However client side storage has one important

security problem - we have no way to invalidate it

by anything else then timeout.

Consider a situation were we playing a game where

on each level we need to select a one answer from 4.

Now consider user had made a mistake. Now he is not

pleased and he restores his previous cookie and now

goes to the same level again - returns back in time.

There is no way to reset his session to level 1 because

he always can go back in time.

So client side storage in very good thing to use but

you should be very careful with it.

### Hiding session data.

Sometimes it is useful to store some data in the

session the way the user would not know what it contains.

There is no problem when you are using server side storage,

however when you use client side storage you need

to encrypt the data.

CppCMS provides an option to use encryption such that

the data stored on the client side would be [automatically

encrypted](http://art-blog.no-ip.info/wikipp/en/page/cppcms_1x_config#session.client.cbc)

Use it if you don't want user to see the data you

store in its session.

### Session expiration

There are two basic methods used for session expiration: fixed and renew. See [session.expire](http://art-blog.no-ip.info/wikipp/en/page/cppcms_1x_config#session.expire)

1. `fixed` - the session would expire in fixed period of time since its creation. Use this for all critical parts. Because if the session is hijacked it would limit the amount

of time the attacker can spend on the web site.

2. `renew` - the session is automatically renewed on every visit. It is very user friendly policy, but it allows

to keep the session virtually forever.

Now when bad things happen and we want to invalidate

all existing sessions.

- Server side storage: just clear the directory that

keeps the sessions. By default it is `/tmp/cppcms_sessions/`

- Client side storage: change the secret key of the

the hmac signature.

### Session Fixation

This is a kind of attack that is used for impersonating

other users.

Consider example:

1. Attacker goes to a bank side and receives a session id - special cookie.

2. Attacker injects this cookie to the innocent user using

some kind of XSS attack (see below).

3. User logs in into bank, because the had received the

cookie he already has the session id and this session id

receives some additional rights like to do operations

on the bank account.

4. Attacker goes to the bank site using the same cookie

stored in the user's browser and performs any kind

of operations user can do.

How to prevent this?

cppcms::session_interface::reset_session();

By calling this function during the critical operations

like user log-in the session id would be replaced with

a new one:

if(authentication_ok()) {

session().reset_session();

session().set("user_id",id);

}

This simple operation would prevent session fixation attacks.

### Client side session private keys

In order to use client side session storage

you need to create a secret private keys for this

purpose.

They can be easily generated using `cppcms_make_key` tool.

Don't forget that these keys are top secret, if

attacker discovers them he would be able to escalate

his rights to any level and forge any session data.

## Cross Site Scripting (XSS)

[Cross Site Scripting](http://en.wikipedia.org/wiki/Cross-site_scripting) in one of the most dangerous problems

web developers should deal with.

Most web application deal with content, sometimes it

may be very rich HTML content. Allowing user to

set his own HTML content as-is provides lots of

tool for attacking web site via injecting JavaScript

and for example storing uses session cookies,

and escalating user privileges to much higher ones,

defacing web sites and much more

Many dangerous things can be done even without

writing Java Script code but only using simple

CSS: [Clickjacking](http://en.wikipedia.org/wiki/Clickjacking), [defacement](http://en.wikipedia.org/wiki/Website_defacement) and more.

So if you want to display a rich content you are

most likely need to apply some XSS filters to prevent

bad things.

### Do not allow HTML

First and most basic rule... Do not allow user to

write HTML content on the web site.

This is done by template system by default. Any value

that is displayed on the web site is escaped by default

such that HTTP attributes like `<` are

converted to the save values like `<`.

However you should notice two things:

1. This does not makes it safe if you,

for example write the value inside `href` attribute.

You should use `urlencode` filter (done automatically

with `<% url %>` url-mapper.

Consider:

<a href='/blog/post/<%= title %>'>

Now user creates a title

' onmouseover='alert(XSS)' '

And even after escaping he gets:

<a href='/blog/post/' onmouseover='alert(XSS)' >

Which allow code execution.

Using `<% url %>` tag or urlencode filter would solve the problem:

<a href='<% url "/post/" using tile %>'>

or

<a href='/blog/post/<%= title | urlencode %>'>

2. The escape filter is applied only when no other

filters are used so if you use stuff like this:

<h1><%= title | to_title %></h1>

The content would not be escaped as one filter is already used. So you need to add `escape` filter explicitly

<h1><%= title | to_title | escape %></h1>

### XSS Filter

Sometimes we do need rich HTML content. In such cases

we must ensure that the generated HTML do not include

malicious code.

CppCMS provides XSS filters that allow to filter HTML

content using strict white-list rules.

Then can be found under [`cppcms::xss`](http://art-blog.no-ip.info/cppcms_ref_v0_99/namespacecppcms_1_1xss.html) namespace.

The filter uses a predefined set of rules that define

what HTML tags and attributes can be used.

The web developer should carefully select the tags

ones allow to use for content. Special care should be

given for url and style attributes.

Starting from CppCMS 0.99.10 it is possible to create

policy file using simple JSON format.

The policy can be loaded from file by simple calling:

cppcms::xss::rules rules(path_to_file);

And then content can be filtered as:

safe = cppcms::xss:filter(unsafe,rules);

It would validate the input and remove all tags and

properties that do not fit this policy.

For example, this JSON file defiles the simple rule that

can be used with basic TinyMCE WYSIWYG editor:

{

"encoding" : "UTF-8",

"entities" : [ "nbsp" ],

"tags" : {

"opening_and_closing" : [

"p",

"ul","ol","li",

"strong", "em",

"span",

"a"

],

"stand_alone" : [ "br" , "hr" ]

},

"attributes" : [

{

"tags" : [ "a" ],

"attributes" : [ "href" ],

"type" : "absolute_uri"

},

{

"tags" : [ "span" ],

"attributes" : [ "style" ],

"type" : "regex",

"expression" :

"(\\s*text-decoration\\s*:\\s*(underline|line-through)\\s*;)*"

}

]

}

Of course more complex input requires more complex rules. So

when you write such rules make sure:

1. You use only special `uri` attributes for URL tags like `href` or `src` - the primary candidate for Javascipt code injection.

2. Be very careful and strict with allowed CSS styles - best - do not allow them or use very strict set of

used styles. Remember CSS allows XSS injection as well using `url()` tags and is very "helpful" in creation of

click-jacking attacks.

### CSS Styles

CSS styles can be very dangerous. Best practice - do

not allow them at all!

The simplest example would be a click-jacking.

<a href="http://example.com/attack.html"

style="display: block; z-index: 100000;

opacity: 0.5; position: fixed;

top: 0px; left: 0;

width: 1000000px; height: 100000px;

background-color: red;"> </a>

Such style that does not include any URL or unsafe

JavaScript code allows to steal clicks for users.

So if you do allow CSS styles specify the exact

attributes that you allow as it is shown in the example

abive.

### Remember!

Vast majority of web attacks like session hijacking,

session fixation, privileges escalation, impersonating,

start with a simple XSS hole...

## Cross Site Request Forgery (CSRF)

### The attack

Is yet another dangerous and frequently used attack.

Consider a situation. On your web site you have a

simple web form, that allows for example to update some

content:

<form method="post" action="/update">

<input type="hidden" name="id" value="1434">

<textarea name="content">...</textarea>

<input type="submit" value="Update">

</form>

It is placed on www.mysite.com.

Now the controller code validates that the user

that perform this operation allows to do it (has

valid user in session with valid privileges)

Now the attacker creates a small form on his site

www.foe-cats.com that rates cute kittens and writes a

simple form:

<form method="post" action="http://mysite.com/update">

<input type="hidden" name="id" value="1434">

<input type="hidden" name="conent" value="My Site Sucks">

<input type="checkbox" ... > ...

<input type="submit" value="Rate Kittens">

</form>

The only task that remains for the attacker is to

make sure that somebody with correct rights

registered on the mysite.com would rate the

kittens.

Of course much more dangerous things could be done

this way.

### Prevention

There are many different ways to do this, like

never updating data on GET requests or

checking referrer had been proven to have some

security faults.

Currently the best known technique is to use

session specific secret token set withing the

form request.

In order to use CSRF prevention feature you need

to do two things:

1. Set [`security.csrf.enable`](/wikipp/en/page/cppcms_1x_config#security.csrf.enable) option to true, such that every session would have such token.

2. Add hidden token to every POST form using `<% csrf %>` tag.

For example:

<form action="/url" method="post"><% csrf %>

<% form as_p my_form %>

</form>

It would automatically inject the hidden input that would

include session specific CSRF token:

<input type="hidden" name="_csrf" value="df_d482">

And this token would be validated upon `cppcms::form::load`

call automatically.

This efficient and simple technique would prevent

CSRF attacks.

_Remember:_ it should be done for "POST" forms only as

"GET" forms may disclose the secret token in Referrer

header.

## Random numbers generation

At some point you'll need to deal with random

numbers generation. And [this sample](http://xkcd.com/221/)

would be very good example how not to do things.

However even if you thing that the code like:

int getRandomNumber()

{

return rand();

}

Is better then your are wrong.

Built in C and C++ library pseudo-random number generators

are good for mathematical purposes but they are predictable.

If you want truly random numbers (as for generating some

UUIDs or generating some secret unguessable keys) use

`cppcms::urandom_device` that uses OS services for

generation of proper, cryptographically safe numbers

that can be used for these purposes.

## Character Set Considerations

One of the things that can be used for exploits is

messing up-with character set, providing invalid

character set.

There are many exploits that use invalid character

set to do bad things, for example XSS. This is

especially frequent with non-ASCII compatible

encodings like Shift-JIS.

So the recommendation is to use Unicode UTF-8 encoding

anywhere and validate the input.

CppCMS text widgets forms, JSON parser do this by default.

However if for some reason you prefer to use POST/GET

data directly like:

msg = request().post("message");

Do yourself a favor and validate or filter text encoding using even very simple:

msg = request().post("message");

msg = booster::locale::conv::utf_to_utf<char>(

msg,

booster::locale::conv::skip);

That would remove all invalid UTF-8 sequences.

## C and C++ Security

I put this section C++ secure programming the

last one mostly because most of C++ programmers

know about these issues forgetting that there

are lots of danger in web programming.

### General Notes

C++ language is considered unsafe - it does not provide

array boundary checks by default, uses pointer and many

other "dangerous stuff"

However unlike C, C++ is high level language that

provides abstraction for many things. So using

modern language techniques programming in C++ is

not more dangerous then programming in Java, Python or Ruby.

### Buffer Overflow

Is one of the classic problems:

sprintf(small_buff,"%d",huge_number);

strcpy(small_buffer,huge_string)

These are probably the classic examples of buffer

overflows...

However, in C++ you have `std::string` and `std::stringstream` and allow you to prevent using

these "dangerous" functions and write safer code.

You can use tools like `booster::locale::format` and

of course many tools that come with Boost libraries.

Use them and not "old-school" tools, they will

help you to write safe code and forget about

this problem.

### Dangling pointers

Consider situation:

foo *p=new foo();

...

delete p;

And accidentally on some later point.

p->bar();

The problem that there is some unknown data located

at the pointer. There are techniques can be used

to execute miscellaneous during access to dangling pointer.

The standard solution:

delete p;

p=0;

Is good but it should be done manually and so somebody

at some point will forget it.

How to deal with it right? Use smart pointers.

- `booster::shared_ptr` - smart reference counting pointer (or `std::shared_ptr` for C++0x compilers)

- `std::auto_ptr` or `std::unique_ptr` for C++0x compilers

- Other smart pointers with different copy policies in

booster namespace `hold_ptr`, `clone_ptr`, `copy_ptr` and

more.

They will solve you actually 3 problems not 1.

- Dangling pointers - they will reset them to 0 automatically.

- Memory leaks - manage memory automatically for you

- Make code exception safe such that, if the exception

is thrown between `new` and `delete` the object will

be properly destroyed by smart pointer.

### Integer overflows

Consider you are accepting from the user some number like

a size of data chunk he is going to send and then

add some data you add and ensure that it fits the

required limits:

unsigned user_data_size = // input from user

char *user_data = ...; // some input

unsigned my_data_size = sizeof(my_data);

char buffer[SOME_LIMIT];

if(user_data_size + sizeof(my_data) <= sizeof(buffer)) {

memcpy(buffer,some_data,user_data_size);

memcpy(buffer+user_data_size,

my_data,sizeof(my_data));

}

What is the problem? The code looks perfectly correct

but... lets set `user_data_size=0xFFFFFFFF` and lets

assume that `sizeof(my_data)` is 4 then `user_data_size+sizeof(my_data)` would be 3 and deferentially

smaller then `sizeof(buffer)` but the next `memcpy`

would cause severe buffer overflow...

How to prevent it? Always put the user supplied

data on its own the comparison side.

if(user_data_size <= sizeof(buffer)-sizeof(my_data)) {

...

}

Note signess of the numbers compare signed with signed

and unsigned with unsigned.

## What happens if we still fail?

Even if we take all precautions, we still, sooner

or later will fail.

The question is how to minimize the possible damage.

### After Failure

The most important thing you should care about is backups.

You need to do _backups_ of the software, data

files and of course the data base.

But what is even more important is to check that

you can restore your web site easily from the last

backup. So make regular checks that your restore procedures

work properly. So on the day the bad thing happens you

will be prepared.

It is important to keep logs - logs from your web

server your application logs. In any case of suspicious

operation... Report it to the log. It would be

very helpful to understand what happened.

If the application crashes in production environment

it is very hard to understand what exactly happened.

So when you deploy the application compile it with

debug information. Use CMake's flag `-DCMAKE_BUILD_TYPE=RelWithDebInfo` or just use GCC's

flags `-O2 -g`. Enable core dumps - if the program

crashes you'll be able to open it with debugger

and see what exactly had happened.

### Running as limited user

### Pre-`fork`ing

### `chroot`ing

### After the Damage

There are several important thigns

## Links

- [Ruby On Rails Security Guide](http://guides.rubyonrails.org/security.html)

- [Open Web Application Security Project](https://www.owasp.org/index.php/Main_Page)

- [XSS Cheat Sheet](http://ha.ckers.org/xss.html)

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