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Character information in Java is based on the Unicode Standard. The following table shows the version of Unicode supported by the latest three releases of Java SE.

Applications that accept untrusted input should normalize the input before validating it.  Normalization is important because in Unicode, the same string can have many different representations.  According to the Unicode Standard [Davis 2008], annex #15, Unicode Normalization Forms:

When implementations keep strings in a normalized form, they can be assured that equivalent strings have a unique binary representation.

Noncompliant Code Example

The Normalizer.normalize() method transforms Unicode text into the standard normalization forms described in Unicode Standard Annex #15 Unicode Normalization Forms. Frequently, the most suitable normalization form for performing input validation on arbitrarily encoded strings is KC (NFKC) .

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// String s may be user controllable
// \uFE64 is normalized to < and \uFE65 is normalized to > using the NFKC normalization form
String s = "\uFE64" + "script" + "\uFE65";

// Validate
Pattern pattern = Pattern.compile("[<>]"); // Check for angle brackets
Matcher matcher = pattern.matcher(s);
if (matcher.find()) {
  // Found black listed tag
  throw new IllegalStateException();
} else {
  // ...
}

// Normalize
s = Normalizer.normalize(s, Form.NFKC);

The validation logic fails to detect the <script> tag because it is not normalized at the time. Therefore the system accepts the invalid input.

Compliant Solution

This compliant solution normalizes the string before validating it. Alternative representations of the string are normalized to the canonical angle brackets. Consequently, input validation correctly detects the malicious input and throws an IllegalStateException.

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Validating input before normalization affords attackers the opportunity to bypass filters and other security mechanisms. It can result in the execution of arbitrary code.

Automated Detection

Related Guidelines

Android Implementation Details

Android apps can receive string data from the outside and normalize it.

Bibliography

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