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Many applications that accept untrusted input strings employ input filtering and validation mechanisms based on the strings' character data. For example, an application's strategy for avoiding cross-site scripting (XSS) vulnerabilities may include forbidding <script> tags in inputs. Such blacklisting mechanisms are a useful part of a security strategy, even though they are insufficient for complete input validation and sanitization. When implemented, this form of validation must be performed only after normalizing the input.

Character information in Java SE 6 is based on the Unicode Standard. The following table shows the version of Unicode supported by the latest three releases of Java SE.

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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.

Normalization Forms KC and KD must not be blindly applied to arbitrary text. Because they erase many formatting distinctions, they will prevent round-trip conversion to and from many legacy character sets, and unless supplanted by formatting markup, they may remove distinctions that are important to the semantics of the text. It is best to think of these Normalization Forms as being like uppercase or lowercase mappings: useful in certain contexts for identifying core meanings, but also performing modifications to the text that may not always be appropriate. They can be applied more freely to domains with restricted character sets.

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) because normalizing to KC transforms the input into an equivalent canonical form that can be safely compared with the required input form .

Noncompliant Code Example

This noncompliant code example attempts to validate the the String before  before performing normalization. Consequently, the validation logic fails to detect inputs that should be rejected because the check for angle brackets fails to detect alternative Unicode representations.

// 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);

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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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Automated Detection

Related Guidelines

Android Implementation Details

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

Bibliography

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Image AddedIDS00-J. Sanitize untrusted data passed across a trust boundary      Image Modified