446
Phase: Architecture and Design
Strategy = Input Validation
Avoid making decisions based on names of resources (e.g. files) if those resources can have
alternate names.
Phase: Implementation
Strategy = Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use
a list of acceptable inputs that strictly conform to specifications. Reject any input that does not
strictly conform to specifications, or transform it into something that does. When performing
input validation, consider all potentially relevant properties, including length, type of input, the
full range of acceptable values, missing or extra inputs, syntax, consistency across related
fields, and conformance to business rules. As an example of business rule logic, "boat" may be
syntactically valid because it only contains alphanumeric characters, but it is not valid if the input
is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for
malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if
the code's environment changes. This can give attackers enough room to bypass the intended
validation. However, denylists can be useful for detecting potential attacks or determining which
inputs are so malformed that they should be rejected outright.
Phase: Implementation
Strategy = Input Validation
Inputs should be decoded and canonicalized to the application's current internal representation
before being validated (CWE-180). Make sure that the application does not decode the same
input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by
introducing dangerous inputs after they have been checked.
Demonstrative Examples
Example 1:
In the following example, an XSS neutralization method intends to replace script tags in usersupplied
input with a safe equivalent:
Example Language: Java (Bad)
public String preventXSS(String input, String mask) {
return input.replaceAll("script", mask);
}
The code only works when the "script" tag is in all lower-case, forming an incomplete denylist
(CWE-184). Equivalent tags such as "SCRIPT" or "ScRiPt" will not be neutralized by this method,
allowing an XSS attack.
Observed Examples
Reference Description
CVE-2000-0499 Application server allows attackers to bypass execution of a jsp page and read
the source code using an upper case JSP extension in the request.
https://www.cve.org/CVERecord?id=CVE-2000-0499
CVE-2000-0497 The server is case sensitive, so filetype handlers treat .jsp and .JSP as
different extensions. JSP source code may be read because .JSP defaults to
the filetype "text".
https://www.cve.org/CVERecord?id=CVE-2000-0497
CVE-2000-0498 The server is case sensitive, so filetype handlers treat .jsp and .JSP as
different extensions. JSP source code may be read because .JSP defaults to
the filetype "text".
CWE Version 4.14
CWE-178: Improper Handling of Case Sensitivity
CWE-178: Improper Handling of Case Sensitivity