My users are allowed to insert anything into my database.
So using a whitelist / blacklist of characters is not an option.
I'm not worried (covered it) about the database end (SQL injection), but rather code injection in my pages.
Are there any situations where htmlspecialchars() wouldn't be sufficient to prevent code injection?
Plain htmlspecialchars is not sufficient when inserting user text into single quoted attributes. You need to add ENT_QUOTES in that case and you need to pass the encoding.
<tag attr='<?php echo htmlspecialchars($usertext);?>'> //dangerous if ENT_QUOTES is not used
When inserting user text into javascript/json as string you'll need additional escaping.
I think it fails for stange character sets too. But if you use one of the usual charsets UTF-8, Latin1,... it will work as expected.
No, it's not sufficient in all situations. It highly depends on your codebase. For example, if you use JavaScript to make certain AJAX requests to a database, htmlspecialchars() will sometimes not be enough (depending where you use it). If you want to protect cookies from JavaScript XSS, htmlspecialchars() will also not be good enough.
Here are some examples of when htmlspecialchars() may fail: https://www.owasp.org/index.php/Interpreter_Injection#Why_htmlspecialchars_is_not_always_enough. Your question is also highly dependent on what database you're using (not everyone uses MySQL). If you're writing a complex applicaton I highly suggest using one of the many frameworks out there that abstract these annoying little idiosyncrasies and let you worry about the application code.
Using htmlspecialchars is sufficient when inserting inside HTML code. The way it encodes the characters makes it impossible for the resulting text to “break out” of the current element. That way it can neither create other elements, nor script segments etc.
However in all other situations, htmlspecialchars it not automatically enough. For example when you use it to insert code within some JavaScript area, for example when you fill a JavaScript string with it, you will need additional methods to make it safe. In that case addslashes could help.
So depending on where you insert the resulting text, htmlspecialchars gives you either enough security or not. As the function name already suggests, it just promises security for HTML.
htmlspecialchars will suffice. With < and > being converted to < and > you cannot include scripts anymore.
Related
I have a textarea which will be available to users as comment box so any sort of inputs are acceptable but that should be accepted only as text and not code. Basically I want to protect my database. I don't want to strip tags or such thing, I just want that if any users even inputs a code that should be stored in database as text and shouldn't be causing any harm to database. So came across these two php functions now I am not sure which one ofthese I should use as I am not able to understand difference in them.
According to official PHP docs, htmlspecialchars() and FILTER_SANITIZE_FULL_SPECIAL_CHARS should be equivalent:
Equivalent to calling htmlspecialchars() with ENT_QUOTES set. Encoding quotes can be disabled by setting FILTER_FLAG_NO_ENCODE_QUOTES. Like htmlspecialchars(), this filter is aware of the default_charset and if a sequence of bytes is detected that makes up an invalid character in the current character set then the entire string is rejected resulting in a 0-length string. When using this filter as a default filter, see the warning below about setting the default flags to 0.
Taken from here - https://www.php.net/manual/en/filter.filters.sanitize.php
Going from here, I think it would be a matter of personal preference as to which function you prefer more.
From this : http://forums.phpfreaks.com/topic/275315-htmlspecialchars-vs-filter-sanitize-special-chars/
They are quite similar yes, but as the PHP manual states
htmlspecialchars escapes a bit more than just
FILTER_SANITIZE_SPECIAL_CHARS.
That brings us to the next point, SQL injection prevention. As stated
htmlspecialchars is for escaping output to a HTML-parser, not a
database engine. The DB engine doesn't understand HTML, and doesn't
care about it either. What it does understand, is SQL queries. SQL
queries and HTML use quite different meta-characters, with only a few
in common: Quotes being the most obvious, and even that is somewhat
conditional for HTML. However, due to the other meta-characters (which
HTML does not share) using HTML escaping methods for SQL queries will
not protect you. Those meta-characters will go through
htmlspecialchars unscathed, and thus be able to cause SQL injections.
Same the other way around, if you use SQL escaping methods to escape
output going to a browser. It will not escape the < and > signs,
meaning an attacker can easily perform HTML injection attacks (XSS
etc). Not only that, but you'll suddenly have a lot of slashes in
places where there shouldn't be any. Which is quite annoying, at best.
This is why it's so important to know, and use, the proper method for
the third party system you're sending the data to. If you don't, you
are still vulnerable
I'm adding some xss protection to the website I'm working on, the platform is zendFrameWork 2 and therefor I'm using Zend\escaper. from zend documentation i knew that:
Zend\Escaper is meant to be used only for escaping data that is to be
output, and as such should not be misused for filtering input data.
For such tasks, the Zend\Filter component, HTMLPurifier.
but what are the riskes if i escaped the data before inserting it into the database, am i so wrong to do that? please explane to me as im somehow new to this topic.
thanks
When encoding data before storing it you will have to decode it before you can do anything sensible with it before outputting it. That's why I'd not do it.
Let's say you have an international application and you want to store the escaped value of a form field which might contain any NON-ASCII characters those might become escaped into HTML-Entities. So what if you have to quantify the content of that field? Like counting the characters? You will always have to de-escape the content before counting it. and then you have to re-escape it again. Much work done but nothing gained.
The same applies to search-operations in your database. You will have to escape the search-phrase the same way then your input for the database to understand what you are looking for.
I'd use one character-set throughout the application and database (I prefer UTF-8, beware of the MySQL-Connection....) and only escape content on output. Thant way I can then do whatever I like with the data and are on the safe side on output. And escaping is done in my view-layer automaticaly so I don't even have to think about it every time I handle data as it works automaticaly. That way you can't forget it.
That does not prevent me from filtering and sanitizing the input. And it doesn't prevent me from escaping the database-content using the appropriate database-escaping mechanisms like mysqli_real_escape_string or similar or using prepared statements!
But that's just my opinion, others might think otherwise!
"Output" here refers to the web page. A form field ( HTML tag) is an INPUT (from the webpage), any text is an OUTPUT (to the webpage). You need to ensure any output (to the webpage) does not contain dangerous characters that could be used to forge XSS attack vectors.
This said, if you have DANGEROUS_INPUT_X given by the user and then
$NOT_DANGEROUS_ANYMORE = ZED.HtmlPurifier(DANGEROUS_INPUT_X)
DBSave($NOT_DANGEROUS_ANYMORE)
and somewhere else
$OUTPUT = DBLoad($NOT_DANGEROUS_ANYMORE)
echo $OUTPUT
you should be fine, as long as you do not apply any additional encoding/decoding to this output. It will be displayed in the way it is saved, that was safe.
I would suggest to look at output encoding more than validation: HtmlPurifier cleans the HTML, while you could accept any kind of bad characters if you ensure your output is encoded in the page.
Here https://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet some general rules, here the PHP example
echo htmlspecialchars($DANGEROUS_INPUT_X_NOW_OUTPUT, ENT_QUOTES, "UTF-8");
Remember to set the Character Set and be consistent with the same one throughout your pages/scripts/binaries and in the database as well.
Note: I take care of SQL injection and output escaping elsewhere - this question is about input filtering only, thanks.
I'm in the middle of refactoring my user input filtering functions. Before passing the GET/POST parameter to a type-specific filter with filter_var() I do the following:
check the parameter encoding with mb_detect_encoding()
convert to UTF-8 with iconv() (with //IGNORE) if it's not ASCII or UTF-8
clean white-spaces with a function found on GnuCitizen.org
pass the result thru strip_tags() - no tags allowed at all, Markdown only
Now the question: does it still make sense to pass the parameter to a filter like htmLawed or HTML Purifier, or can I think of the input as safe? It seems to me that these two differ mostly on the granularity of allowed HTML elements and attributes (which I'm not interested into, as I remove everything), but htmLawed docs have a section about 'dangerous characters' that suggests there might be a reason to use it. In this case, what would be a sane configuration for it?
There are many different approaches to XSS that are secure. The only why to know if your approach holds water is to test though exploitation. I recommend using a Free XSS vulnerability Scanner*, or the open source wapiti.
To be honest I'll never use strip_tags() becuase you don't always need html tags to execute javascript! I like htmlspecialchars($var,ENT_QUOTES); .
For instance this is vulnerable to xss:
print('link');
You don't need <> to execute javascript in this case because you can use
onmouseover, here is an example attack:
$_REQUEST[xss]='" onMouseOver="alert(/xss/)"';
The ENT_QUOTES will take care of the double quotes which will patch this XSS vulnerability.
*I am affiliated with this site/service.
i think what you're doing is safe, at least from my point of view no html code should get through your filter
I am creating a forum software using php and mysql backend, and want to know what is the most secure way to escape user input for forum posts.
I know about htmlentities() and strip_tags() and htmlspecialchars() and mysql_real_escape_string(), and even javascript's escape() but I don't know which to use and where.
What would be the safest way to process these three different types of input (by process, I mean get, save in a database, and display):
A title of a post (which will also be the basis of the URL permalink).
The content of a forum post limited to basic text input.
The content of a forum post which allows html.
I would appreciate an answer that tells me how many of these escape functions I need to use in combination and why.
Thanks!
When generating HTLM output (like you're doing to get data into the form's fields when someone is trying to edit a post, or if you need to re-display the form because the user forgot one field, for instance), you'd probably use htmlspecialchars() : it will escape <, >, ", ', and & -- depending on the options you give it.
strip_tags will remove tags if user has entered some -- and you generally don't want something the user typed to just disappear ;-)
At least, not for the "content" field :-)
Once you've got what the user did input in the form (ie, when the form has been submitted), you need to escape it before sending it to the DB.
That's where functions like mysqli_real_escape_string become useful : they escape data for SQL
You might also want to take a look at prepared statements, which might help you a bit ;-)
with mysqli - and with PDO
You should not use anything like addslashes : the escaping it does doesn't depend on the Database engine ; it is better/safer to use a function that fits the engine (MySQL, PostGreSQL, ...) you are working with : it'll know precisely what to escape, and how.
Finally, to display the data inside a page :
for fields that must not contain HTML, you should use htmlspecialchars() : if the user did input HTML tags, those will be displayed as-is, and not injected as HTML.
for fields that can contain HTML... This is a bit trickier : you will probably only want to allow a few tags, and strip_tags (which can do that) is not really up to the task (it will let attributes of the allowed tags)
You might want to take a look at a tool called HTMLPUrifier : it will allow you to specify which tags and attributes should be allowed -- and it generates valid HTML, which is always nice ^^
This might take some time to compute, and you probably don't want to re-generate that HTML each time is has to be displayed ; so you can think about storing it in the database (either only keeping that clean HTML, or keeping both it and the not-clean one, in two separate fields -- might be useful to allow people editing their posts ? )
Those are only a few pointers... hope they help you :-)
Don't hesitate to ask if you have more precise questions !
mysql_real_escape_string() escapes everything you need to put in a mysql database. But you should use prepared statements (in mysqli) instead, because they're cleaner and do any escaping automatically.
Anything else can be done with htmlspecialchars() to remove HTML from the input and urlencode() to put things in a format for URL's.
There are two completely different types of attack you have to defend against:
SQL injection: input that tries to manipulate your DB. mysql_real_escape_string() and addslashes() are meant to defend against this. The former is better, but parameterized queries are better still
Cross-Site scripting (XSS): input that, when displayed on your page, tries to execute JavaScript in a visitor's browser to do all kinds of things (like steal the user's account data). htmlspecialchars() is the definite way to defend against this.
Allowing "some HTML" while avoiding XSS attacks is very, very hard. This is because there are endless possibilities of smuggling JavaScript into HTML. If you decided to do this, the safe way is to use BBCode or Markdown, i.e. a limited set of non-HTML markup that you then convert to HTML, while removing all real HTML with htmlspecialchars(). Even then you have to be careful not to allow javascript: URLs in links. Actually allowing users to input HTML is something you should only do if it's absolutely crucial for your site. And then you should spend a lot of time making sure you understand HTML and JavaScript and CSS completely.
The answer to this post is a good answer
Basically, using the pdo interface to parameterize your queries is much safer and less error prone than escaping your inputs manually.
I have a tendency to escape all characters that would be problematic in page display, Javascript and SQL all at the same time. It leaves it readable on the web and in HTML eMail and at the same time removes any problems with the code.
A vb.NET Line Of Code Would Be:
SafeComment = Replace( _
Replace(Replace(Replace( _
Replace(Replace(Replace( _
Replace(Replace(Replace( _
Replace(Replace(Replace( _
HttpUtility.HtmlEncode(Trim(strInput)), _
":", ":"), "-", "-"), "|", "|"), _
"`", "`"), "(", "("), ")", ")"), _
"%", "%"), "^", "^"), """", """), _
"/", "/"), "*", "*"), "\", "\"), _
"'", "'")
First of all, general advice: don't escape variables literally when inserting in the database. There are plenty of solutions that let you use prepared statements with variable binding. The reason to not do this explicitly is because it is only a matter of time then before you forget it just once.
If you're inserting plain text in the database, don't try to clean it on insert, but instead clean it on display. That is to say, use htmlentities to encode it as HTML (and pass the correct charset argument). You want to encode on display because then you're no longer trusting that the database contents are correct, which isn't necessarily a given.
If you're dealing with rich text (html), things get more complicated. Removing the "evil" bits from HTML without destroying the message is a difficult problem. Realistically speaking, you'll have to resort to a standardized solution, like HTMLPurifier. However, this is generally too slow to run on every page view, so you'll be forced to do this when writing to the database. You'll also have to ensure that the user can see their "cleaned up" html and correct the cleaned up version.
Definitely try to avoid "rolling your own" filter or encoding solution at any step. These problems are notoriously tricky, and you run a large risk of overlooking some minor detail that has big security implications.
I second Joeri, do not roll your own, go here to see some of the the many possible XSS attacks
http://ha.ckers.org/xss.html
htmlentities() -> turns text into html, converting characters to entities. If using UTF-8 encoding then use htmlspecialchars() instead as the other entities are not needed. This is the best defence against XSS. I use it on every variable I output regardless of type or origin unless I intend it to be html. There is only a tiny performance cost and it is easier than trying to work out what needs escaping and what doesn't.
strip_tags() - turns html into text by removing all html tags. Use this to ensure that there is nothing nasty in your input as a adjunct to escaping your output.
mysql_real_escape_string() - escapes a string for mysql and is your defence against SQL injections from little Bobby tables (better to use mysqli and prepare/bind as escaping is then done for you and you can avoid lots of messy string concatenations)
The advice given obve re avoiding HTML input unless it is essential and opting for BBCode or similar (make your own up if needs be) is very sound indeed.
I want to prevent XSS attacks in my web application. I found that HTML Encoding the output can really prevent XSS attacks. Now the problem is that how do I HTML encode every single output in my application? I there a way to automate this?
I appreciate answers for JSP, ASP.net and PHP.
One thing that you shouldn't do is filter the input data as it comes in. People often suggest this, since it's the easiest solution, but it leads to problems.
Input data can be sent to multiple places, besides being output as HTML. It might be stored in a database, for example. The rules for filtering data sent to a database are very different from the rules for filtering HTML output. If you HTML-encode everything on input, you'll end up with HTML in your database. (This is also why PHP's "magic quotes" feature is a bad idea.)
You can't anticipate all the places your input data will travel. The safe approach is to prepare the data just before it's sent somewhere. If you're sending it to a database, escape the single quotes. If you're outputting HTML, escape the HTML entities. And once it's sent somewhere, if you still need to work with the data, use the original un-escaped version.
This is more work, but you can reduce it by using template engines or libraries.
You don't want to encode all HTML, you only want to HTML-encode any user input that you're outputting.
For PHP: htmlentities and htmlspecialchars
For JSPs, you can have your cake and eat it too, with the c:out tag, which escapes XML by default. This means you can bind to your properties as raw elements:
<input name="someName.someProperty" value="<c:out value='${someName.someProperty}' />" />
When bound to a string, someName.someProperty will contain the XML input, but when being output to the page, it will be automatically escaped to provide the XML entities. This is particularly useful for links for page validation.
A nice way I used to escape all user input is by writing a modifier for smarty wich escapes all variables passed to the template; except for the ones that have |unescape attached to it. That way you only give HTML access to the elements you explicitly give access to.
I don't have that modifier any more; but about the same version can be found here:
http://www.madcat.nl/martijn/archives/16-Using-smarty-to-prevent-HTML-injection..html
In the new Django 1.0 release this works exactly the same way, jay :)
My personal preference is to diligently encode anything that's coming from the database, business layer or from the user.
In ASP.Net this is done by using Server.HtmlEncode(string) .
The reason so encode anything is that even properties which you might assume to be boolean or numeric could contain malicious code (For example, checkbox values, if they're done improperly could be coming back as strings. If you're not encoding them before sending the output to the user, then you've got a vulnerability).
You could wrap echo / print etc. in your own methods which you can then use to escape output. i.e. instead of
echo "blah";
use
myecho('blah');
you could even have a second param that turns off escaping if you need it.
In one project we had a debug mode in our output functions which made all the output text going through our method invisible. Then we knew that anything left on the screen HADN'T been escaped! Was very useful tracking down those naughty unescaped bits :)
If you do actually HTML encode every single output, the user will see plain text of <html> instead of a functioning web app.
EDIT: If you HTML encode every single input, you'll have problem accepting external password containing < etc..
The only way to truly protect yourself against this sort of attack is to rigorously filter all of the input that you accept, specifically (although not exclusively) from the public areas of your application. I would recommend that you take a look at Daniel Morris's PHP Filtering Class (a complete solution) and also the Zend_Filter package (a collection of classes you can use to build your own filter).
PHP is my language of choice when it comes to web development, so apologies for the bias in my answer.
Kieran.
OWASP has a nice API to encode HTML output, either to use as HTML text (e.g. paragraph or <textarea> content) or as an attribute's value (e.g. for <input> tags after rejecting a form):
encodeForHTML($input) // Encode data for use in HTML using HTML entity encoding
encodeForHTMLAttribute($input) // Encode data for use in HTML attributes.
The project (the PHP version) is hosted under http://code.google.com/p/owasp-esapi-php/ and is also available for some other languages, e.g. .NET.
Remember that you should encode everything (not only user input), and as late as possible (not when storing in DB but when outputting the HTTP response).
Output encoding is by far the best defense. Validating input is great for many reasons, but not 100% defense. If a database becomes infected with XSS via attack (i.e. ASPROX), mistake, or maliciousness input validation does nothing. Output encoding will still work.
there was a good essay from Joel on software (making wrong code look wrong I think, I'm on my phone otherwise I'd have a URL for you) that covered the correct use of Hungarian notation. The short version would be something like:
Var dsFirstName, uhsFirstName : String;
Begin
uhsFirstName := request.queryfields.value['firstname'];
dsFirstName := dsHtmlToDB(uhsFirstName);
Basically prefix your variables with something like "us" for unsafe string, "ds" for database safe, "hs" for HTML safe. You only want to encode and decode where you actually need it, not everything. But by using they prefixes that infer a useful meaning looking at your code you'll see real quick if something isn't right. And you're going to need different encode/decode functions anyways.