IDS03-J. Sanitize non-character code points before performing other sanitization

According to the Unicode Technical Report #36, Unicode Security Considerations [[Davis 2008b]], Section 3.5, "Deletion of Noncharacters"

Conformance clause C7 reads

C7. When a process purports not to modify the interpretation of a valid coded character sequence, it shall make no change to that coded character sequence other than the possible replacement of character sequences by their canonical-equivalent sequences or the deletion of noncharacter code points.

Although the last phrase permits the deletion of noncharacter code points, for security reasons, they only should be removed with caution.

Whenever a character is invisibly deleted (instead of replaced), it may cause a security problem. The issue is the following: A gateway might be checking for a sensitive sequence of characters, say "delete". If what is passed in is "deXlete", where X is a noncharacter, the gateway lets it through: the sequence "deXlete" may be in and of itself harmless. But suppose that later on, past the gateway, an internal process invisibly deletes the X. In that case, the sensitive sequence of characters is formed, and can lead to a security breach.

Since character-level modifications of a string can nullify substring-level checks, it is important to perform the character-level modifications before substring-level checks.

Noncompliant Code Example

This noncompliant code example accepts only valid ASCII characters and deletes any non conforming characters. It also checks for the existence of a <script> tag.

Input validation is being performed before the character checks. As such, this code also violates [IDS02-J. Normalize strings before validating them]. Consequently, an attacker can disguise a <script> tag and fool the filter.

String s = "<scr" + "\uFEFF" + "ipt>"; // "\uFEFF" is a non-character code point
s = Normalizer.normalize(s, Form.NFKC);

// Input validation
Pattern pattern = Pattern.compile("<script>");
Matcher matcher = pattern.matcher(s);
if(matcher.find()) {
  System.out.println("Found black listed tag");
} else {
  // ... 
}

s = s.replaceAll("^\\p{ASCII}]", ""); // Deletes all non-valid characters
// s now contains "<script>"		

Compliant Solution

This compliant solution replaces the unknown or unrepresentable character with unicode sequence \uFFFD which is reserved to denote this condition. It also does this replacement before doing any other sanitization, in particular, checking for <script>. This ensures that malicious input cannot bypass filters.

String s = "<scr" + "\uFEFF" + "ipt>";

s = Normalizer.normalize(s, Form.NFKC);
s = s.replaceAll("^\\p{ASCII}]", "\uFFFD"); // Replaces all non-valid characters with unicode U+FFFD

Pattern pattern = Pattern.compile("<script>");
Matcher matcher = pattern.matcher(s);
if(matcher.find()) {
  System.out.println("Found black listed tag");
} else {
  // ... 
}

"U+FFFD is usually unproblematic, because it is designed expressly for this kind of purpose. That is, because it doesn't have syntactic meaning in programming languages or structured data, it will typically just cause a failure in parsing. Where the output character set is not Unicode, though, this character may not be available" [[Davis 2008b]].

Risk Assessment

Deleting non-character code points can allow malicious input to bypass validation checks.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Bibliography

[[API 2006]]
[[Davis 2008b]] 3.5 Deletion of Noncharacters
[[MITRE 2009]] CWE ID "Collapse of Data Into Unsafe Value"
[[Weber 2009]] Handling the Unexpected: Character-deletion

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