Validating parsers

To ensure that the application is robust against all forms of input data, whether obtained from the user, infrastructure, external entities or database systems. This weakness leads to almost all of the major vulnerabilities in applications, such as Interpreter Injection, locale/Unicode attacks, file system attacks and buffer overflows.

All sections should be reviewed The most common web application security weakness is the failure to properly validate input from the client or environment.

Say you want to set up a site where users can upload arbitrary files so they can share them or download them again from another location.

In this case validation is impossible because there is no valid or invalid content.

For example, the web / presentation tier should validate for web related issues, persistence layers should validate for persistence issues such as SQL / HQL injection, directory lookups should check for LDAP injection, and so on.

Business rules are known during design, and they influence implementation.

int payee Lst Id = Parameter('payeelstid'); account From = Acct Number By Index(payee Lst Id); Not only is this easier to render in HTML, it makes validation and business rule validation trivial. To provide defense in depth and to prevent attack payloads from trust boundaries, such as backend hosts, which are probably incapable of handling arbitrary input data, business rule validation is to be performed (preferably in workflow or command patterns), even if it is known that the back end code performs business rule validation.

This is not to say that the entire set of business rules need be applied - it means that the fundamentals are performed to prevent unnecessary round trips to the backend and to prevent the backend from receiving most tampered data.

This strategy is directly akin to anti-virus pattern updates.Integrity checks must be included wherever data passes from a trusted to a less trusted boundary, such as from the application to the user's browser in a hidden field, or to a third party payment gateway, such as a transaction ID used internally upon return.The type of integrity control (checksum, HMAC, encryption, digital signature) should be directly related to the risk of the data transiting the trust boundary. However, validation should be performed as per the function of the server executing the code.Essentially, if you don't expect to see characters such as ?or Java Script or similar, reject strings containing them.For example, if you use HTML entity encoding on user input before it is sent to a browser, it will prevent most XSS attacks.

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