Web application security
In the past, a breach in security was often just an annoyance, but today that is seldom the case. For example, attacks in which a hacker gained access to a web server and made unauthorized modifications to (defaced) a web site used to be common. They were usually launched by hackers who had no motivation beyond demonstrating their skills to fellow hackers or embarrassing the targeted person or company. Most current security breaches, however, are motivated by a desire for money. The majority attempt to accomplish one or both of the following goals: to obtain sensitive and potentially valuable private information, or to obtain unauthorized access to and control of a web site or web server.
Certain forms of web attacks focus on obtaining private information. These attacks are often possible even against web sites that are secure enough to prevent an attacker from taking full control. The information that an attacker can obtain from a web site can include customer names, addresses, phone numbers, social security numbers, credit card numbers, medical records, and other private information. The attacker can then use this information or sell it to others. Much of the information obtained by such attacks is protected by law, and all of it by custom and expectation. A breach of this type can have extremely serious consequences for customers whose private information is compromised. At best, these customers will have to exercise vigilance to prevent others from abusing their credit cards, opening unauthorized credit accounts in their name, or appropriating their identities outright (identity theft). At worst, the customers may face ruined credit ratings or even be blamed for criminal activities in which they had no part.
Other web attacks are aimed at obtaining control of (or compromising) a web site or the server on which it operates, or both. A hacker who gains control of a web site or server can use it to host unauthorized content, act as a proxy for content hosted on another web server, provide SMTP services to send unsolicited bulk email, or provide DNS services to support such activities on other compromised web servers. Most web sites that are hosted on compromised web servers promote questionable or outright fraudulent businesses. For example, the majority of phishing web sites and child exploitation web sites are hosted on compromised web servers.
Protecting your web sites and web services against these attacks requires a multilayered defense capable of both blocking known attacks with identifiable characteristics and protecting against unknown attacks, which can often be detected because they look different from the normal traffic to your web sites and web services.
Known web attacks
The first line of defense for your web sites is protection against the large number of attacks that are known to exist and have been observed and analyzed by web security experts. Common types of attacks against HTML-based web sites include:
- Buffer overflow attacks. Sending an extremely long URL, extremely long cookie, or other extremely long bit of information to a web server in hopes of causing it or the underlying operating system to hang, crash, or provide the attacker with access to the underlying operating system. A buffer overflow attack can be used to gain access to unauthorized information, to compromise a web server, or both.
- Cookie security attacks. Sending a modified cookie to a web server, usually in hopes of obtaining access to unauthorized content by using falsified credentials.
- Forceful browsing. Accessing URLs on a web site directly, without navigating to the URLs by means of hyperlinks on the home page or other common start URLs on the web site. Individual instances of forceful browsing may simply indicate a user who bookmarked a page on your web site, but repeated attempts to access nonexistent content, or content that users should never access directly, often represent an attack on web site security. Forceful browsing is normally used to gain access to unauthorized information, but can also be combined with a buffer overflow attack in an attempt to compromise your server.
- Web form security attacks. Sending inappropriate content to your web site in a web form. Inappropriate content can include modified hidden fields, HTML or code in a field intended for alphanumeric data only, an overly long string in a field that accepts only a short string, an alphanumeric string in a field that accepts only an integer, and a wide variety of other data that your web site does not expect to receive in that web form. A web form security attack can be used either to obtain unauthorized information from your web site or to compromise the web site outright, usually when combined with a buffer overflow attack.
Two specialized types of attacks on web form security deserve special mention:
- SQL injection attacks. Sending an active SQL command or commands in a web form or as part of a URL, with the goal of causing an SQL database to execute the command or commands. SQL injection attacks are normally used to obtain unauthorized information.
- Cross-site scripting attacks. Using a URL or a script on a web page to violate the same-origin policy, which forbids any script from obtaining properties from or modifying any content on a different web site. Since scripts can obtain information and modify files on your web site, allowing a script access to content on a different web site can provide an attacker the means to obtain unauthorized information, to compromise a web server, or both.
Attacks against XML-based web services normally fall into at least one of the following two categories: attempts to send inappropriate content to a web service, or attempts to breach security on a web service. Common types of attacks against XML-based web services include:
- Malicious code or objects. XML requests that contain code or objects that can either directly obtain sensitive information or can give an attacker control of the web service or underlying server.
- Badly-formed XML requests. XML requests that do not conform to the W3C XML specification, and that can therefore breach security on an insecure web service
- Denial of service (DoS) attacks. XML requests that are sent repeatedly and in high volume, with the intent of overwhelming the targeted web service and denying legitimate users access to the web service.
In addition to standard XML-based attacks, XML web services and Web 2.0 sites are also vulnerable to SQL injection and cross-site scripting attacks, as described below:
- SQL injection attacks. Sending an active SQL command or commands in an XML-based request, with the goal of causing an SQL database to execute that command or commands. As with HTML SQL injection attacks, XML SQL injection attacks are normally used to obtain unauthorized information.
- Cross-site scripting attacks. Using a script included in an XML based application to violate the same-origin policy, which does not allow any script to obtain properties from or modify any content on a different application. Since scripts can obtain information and modify files by using your XML application, allowing a script access to content belonging to a different application can give an attacker the means to obtain unauthorized information, to compromise the application, or both
Known web attacks can usually be stopped by filtering web site traffic for specific characteristics (signatures) that always appear for a specific attack and should never appear in legitimate traffic. This approach has the advantages of requiring relatively few resources and posing relatively little risk of false positives. It is therefore a valuable tool in fighting attacks on web sites and web services, and configuring basic signature protections that intercept most known web attacks is easy to do.
Unknown web attacks
The greatest threat against web sites and applications does not come from known attacks, but from unknown attacks. Most unknown attacks fall into one of two categories: newly-launched attacks for which security firms have not yet developed an effective defense (zero-day attacks), and carefully-targeted attacks on a specific web site or web service rather than many web sites or web services (spear attacks). These attacks, like known attacks, are usually intended to obtain sensitive private information, compromise the web site or web service and allow it to be used for further attacks, or both of those goals.
Zero-day attacks are a major threat to all users. These attacks are usually of the same types as known attacks; zero-day attacks often involve injected SQL, a cross-site script, a cross-site request forgery, or another type of attack similar to known attacks. In most cases, they target vulnerabilities that the developers of the targeted software, web site, or web service either are unaware of or have just learned about. Security firms have therefore usually not developed defenses against these attacks, and even if they have, users have usually not obtained and installed the patches or performed the workarounds necessary to protect against these attacks. The time between discovery of a zero-day attack and availability of a defense (the vulnerability window) is shrinking, but perpetrators can still count on hours or even days in which many web sites and web services lack any specific protection against the attack.
Spear attacks are a major threat, but to a more select group of users. A common type of spear attack, a spear phish, is usually targeted at customers of a specific bank or financial institution, or (less commonly) at employees of a specific company or organization. Unlike other phishes, which are often crudely written forgeries that a user with any familiarity with the actual communications of that bank or financial institution can recognize, spear phishes are letter perfect and extremely convincing. They can contain information specific to the individual that, at first look, no stranger should know or be able to obtain. The spear phisher is therefore able to convince his or her target to provide the requested information, which the phisher can then use to loot accounts, to process illegitimately obtained money from other sources, or to gain access to other, even more sensitive information.
Both of these types of attack have certain characteristics that can usually be detected, although not by using static patterns that look for specific characteristics, as do standard signatures. Detecting these types of attacks requires more sophisticated and more resource-intensive approaches, such as heuristic filtering and positive security model systems. Heuristic filtering looks, not for specific patterns, but for patterns of behaviors. Positive security model systems model the normal behavior of the web site or web service that they are protecting, and then block connections that do not fit within that model of normal use. URL based and web-form based security checks profile normal use of your web sites, and then control how users interact with your web sites, using both heuristics and positive security to block anomalous or unexpected traffic. Both heuristic and positive security, properly designed and deployed, can catch most attacks that signatures miss. However, they require considerably more resources than do signatures, and you must spend some time configuring them properly to avoid false positives. They are therefore usually used, not as the primary line of defense, but as backups to signatures or other less resource-intensive approaches.
By configuring these advanced protections in addition to signatures, you create a hybrid security model, which enables the Web App Firewall to provide comprehensive protection against both known and unknown attacks.