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Automated static analysis, commonly referred to as Static Application Security Testing (SAST),
can find some instances of this weakness by analyzing source code (or binary/compiled code)
without having to execute it. Typically, this is done by building a model of data flow and control
flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input)
with "sinks" (destinations where the data interacts with external components, a lower layer such
as the OS, etc.)
Effectiveness = High
Potential Mitigations
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 this example, a web application uses the value of a hidden form field (accountID) without having
done any input validation because it was assumed to be immutable.
Example Language: Java (Bad)
String accountID = request.getParameter("accountID");
User user = getUserFromID(Long.parseLong(accountID));
Example 2:
Hidden fields should not be trusted as secure parameters.
An attacker can intercept and alter hidden fields in a post to the server as easily as user input
fields. An attacker can simply parse the HTML for the substring:
Example Language: HTML (Bad)
<input type="hidden"
or even just "hidden". Hidden field values displayed later in the session, such as on the following
page, can open a site up to cross-site scripting attacks.
Observed Examples
CWE Version 4.14
CWE-472: External Control of Assumed-Immutable Web Parameter
CWE-472: External Control of Assumed-Immutable Web Parameter