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The escaping is necessary to prevent the general attribute-based script-injection attacks described in the previous section. Enclosing the URL in quotes is necessary to prevent attribute-injection attacks (see Section 10.5.3). We note that it may be safe to omit the quotes around the attribute s value if you are sure that the URL does not contain any whitespace characters. However, we advise against such cutting of corners; the savings of two characters of page size per attribute rarely justifies the additional risk due to the special casing in code and/or coding conventions. To prevent javascript: injection attacks, one might be tempted to just disallow URLs that start with javascript:. However, there are many rather obscure variations of this injection, and it is much safer to apply a positive filter. For example, some versions of Internet Explorer will ignore a 0x08 (\010 octal) character at the beginning of the string. Similarly, if the : character is escaped as an HTML character reference such as javascript:evil-script; both Internet Explorer and Firefox still execute the script. Internet Explorer executes vbscript: URLs. The data: scheme can also be used to cause script execution for example, data:text/ html,<script>evil-script;</script>. It is quite possible (or rather, likely) that there are additional browser behaviors that similarly can be used to cause script execution. This example vividly illustrates the virtues of the general security paradigm of preferring whitelisting over blacklisting. Validating a parameter with anything but the most trivial semantics is almost

Foo source = new Foo { Name = "Fred" }; var result = source.Where(f => f.Name == "Fred").Select(f => f.Name);

var apiKey = "{{Key:key;http://api.technorati.com/}}";

Since the Foo class provides the Where and Select operators that C# expects, this will compile and run. It won t be particularly useful, because our Where implementation completely ignores the predicate. And it s also a slightly bizarre thing to do our Foo class doesn t appear to represent any kind of collection, so it s rather misleading to use syntax that s intended to be used with collections. In fact, Example 8-7 has the same effect as:

var result = source.Name;

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So you d never write code like Example 8-6 and Example 8-7 for a type as simple as Foo in practice the purpose of these examples is to illustrate that the C# compiler blindly translates query expressions into method calls, and has no understanding or expectation of what those calls might do. The real functionality of LINQ lives entirely in the class library. Query expressions are just a convenient syntax.

Query expressions can contain let clauses. This is an interesting kind of clause in that unlike most of the rest of a query, it doesn t correspond directly to any particular LINQ operator. It s just a way of making it easier to structure your query. You would use a let clause when you need to use the same information in more than one place in a query. For example, suppose we want to modify the query in Example 8-2 to return a FileInfo object, rather than a filename. We could do this:

always safer by testing inclusion in a known-safe subset of possible values, rather than trying to exclude the set of values you think are problematic it is often very difficult to reliably characterize the set of bad values.

var bigFiles = from file in GetAllFilesInDirectory(@"c:\") where new FileInfo(file).Length > 10000000 select new FileInfo(file);

But this code repeats itself it creates a FileInfo object in the where clause and then creates another one in the select clause. We can avoid this repetition with a let clause:

var bigFiles = from file in GetAllFilesInDirectory(@"c:\") let info = new FileInfo(file) where info.Length > 10000000 select info;

The C# compiler jumps through some significant hoops to make this work. There s no need to know the details to make use of a let clause, but if you re curious to know how it works, here s what happens. Under the covers it generates a class containing two properties called file and info, and ends up generating two queries:

Style attributes can be dangerous if an attacker can control the value of the attribute, since CSS styles can cause script to execute in various ways.

var temp = from file in GetAllFilesInDirectory(@"c:\") select new CompilerGeneratedType(file, new FileInfo(file)); var bigFiles = from item in temp where item.info.Length > 10000000 select item.info;

The purpose of the first query is to produce a sequence in which the range variable is wrapped in the compiler-generated type, alongside any variables declared with a let clause. (It s not actually called CompilerGeneratedType, of course the compiler generates a unique, meaningless name.) This allows all these variables to be available in all the clauses of the query.

The string assigned to the apiKey variable indicates that it is to look up the value of the API key in the key attribute defined in the keys section of the definition file.

Before we look in detail at the services LINQ offers, there are some features that apply across all of LINQ that you should be aware of.

LINQ query syntax makes implicit use of lambdas. The expressions that appear in where, select, or most other clauses are written as ordinary expressions, but as you ve seen, the C# compiler turns queries into a series of method calls, and the expressions become lambda expressions.

Most of the time, you can just write the expressions you need and they work. But you need to be wary of code that has side effects. For example, it would be a bad idea to write the sort of query shown in Example 8-8.

For example, consider the following template fragment: <div style="background: %(color)s;"> I like colors. </div> If the attacker can cause color to contain green; background-image: url(javascript:evil-script;) after substitution, the HTML evaluated by the browser would be <div style="background: green; background-image: url(javascript:evil-script;);"> I like colors. </div> This does result in evil-script; being executed (at least in Internet Explorer 6.0; Firefox version 1.5, for example, apparently does not de-reference javascript: URLs in this context).

int x = 10000; var bigFiles = from file in GetAllFilesInDirectory(@"c:\") where new FileInfo(file).Length > x++ select file;

This is allowed (although it s a bad idea) in LINQ to Objects. Some LINQ providers, such as the ones you would use with databases, will reject such a query at runtime.

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