Common Weakness Enumeration

Example 1

Debug code can be used to bypass authentication. For example, suppose an application has a login script that receives a username and a password. Assume also that a third, optional, parameter, called "debug", is interpreted by the script as requesting a switch to debug mode, and that when this parameter is given the username and password are not checked. In such a case, it is very simple to bypass the authentication process if the special behavior of the application regarding the debug parameter is known. In a case where the form is:

Then a conforming link will look like:

An attacker can change this to:

Which will grant the attacker access to the site, bypassing the authentication process.

Example 1

The file web.config contains the debug mode setting. Setting debug to "true" will let the browser display debugging information.

Change the debug mode to false when the application is deployed into production.

Example 1

The mode attribute of the <customErrors> tag in the Web.config file defines whether custom or default error pages are used.

In the following insecure ASP.NET application setting, custom error message mode is turned off. An ASP.NET error message with detailed stack trace and platform versions will be returned.

A more secure setting is to set the custom error message mode for remote users only. No defaultRedirect error page is specified. The local user on the web server will see a detailed stack trace. For remote users, an ASP.NET error message with the server customError configuration setting and the platform version will be returned.

Another secure option is to set the mode attribute of the <customErrors> tag to use a custom page as follows:

Example 1

The following example shows a portion of a configuration file for an ASP.Net application. This configuration file includes username and password information for a connection to a database, but the pair is stored in plaintext.

Username and password information should not be included in a configuration file or a properties file in plaintext as this will allow anyone who can read the file access to the resource. If possible, encrypt this information.

Example 1

This code listens on a port for DNS requests and sends the result to the requesting address.

This code sends a DNS record to a requesting IP address. UDP allows the source IP address to be easily changed ('spoofed'), thus allowing an attacker to redirect responses to a target, which may be then be overwhelmed by the network traffic.

Example 2

This function prints the contents of a specified file requested by a user.

This code first reads a specified file into memory, then prints the file if the user is authorized to see its contents. The read of the file into memory may be resource intensive and is unnecessary if the user is not allowed to see the file anyway.

Example 3

The DTD and the very brief XML below illustrate what is meant by an XML bomb. The ZERO entity contains one character, the letter A. The choice of entity name ZERO is being used to indicate length equivalent to that exponent on two, that is, the length of ZERO is 2^0. Similarly, ONE refers to ZERO twice, therefore the XML parser will expand ONE to a length of 2, or 2^1. Ultimately, we reach entity THIRTYTWO, which will expand to 2^32 characters in length, or 4 GB, probably consuming far more data than expected.

Example 4

This example attempts to check if an input string is a "sentence" [REF-1164].

The regular expression has a vulnerable backtracking clause inside (\w+\s?)*$ which can be triggered to cause a Denial of Service by processing particular phrases.

To fix the backtracking problem, backtracking is removed with the ?= portion of the expression which changes it to a lookahead and the \2 which prevents the backtracking. The modified example is:

Note that [REF-1164] has a more thorough (and lengthy) explanation of everything going on within the RegEx.

Example 5

An adversary can cause significant resource consumption on a server by filtering the cryptographic algorithms offered by the client to the ones that are the most resource-intensive on the server side. After discovering which cryptographic algorithms are supported by the server, a malicious client can send the initial cryptographic handshake messages that contains only the resource-intensive algorithms. For some cryptographic protocols, these messages can be completely prefabricated, as the resource-intensive part of the handshake happens on the server-side first (such as TLS), rather than on the client side. In the case of cryptographic protocols where the resource-intensive part should happen on the client-side first (such as SSH), a malicious client can send a forged/precalculated computation result, which seems correct to the server, so the resource-intensive part of the handshake is going to happen on the server side. A malicious client is required to send only the initial messages of a cryptographic handshake to initiate the resource-consuming part of the cryptographic handshake. These messages are usually small, and generating them requires minimal computational effort, enabling a denial-of-service attack. An additional risk is the fact that higher key size increases the effectiveness of the attack. Cryptographic protocols where the clients have influence over the size of the used key (such as TLS 1.3 or SSH) are most at risk, as the client can enforce the highest key size supported by the server.