Unicode normalisers are used to transform Unicode strings into a consistent form so that equivalent characters can be reliably compared. Without normalisation, string matching may fail because the same character can be represented in multiple ways (e.g., a precomposed character vs. a base character + diacritic). Adversaries exploit this ambiguity to bypass security validation rules, such as input filters, authentication checks, or access controls.

Unicode provides four normalisation forms:

• **Canonical** (NFC, NFD) — preserve character semantics but unify equivalent representations.
• **Compatibility** (NFKC, NFKD) — apply broader transformations, potentially converting characters into visually similar but semantically different ones.

Improper use of compatibility normalisers can unintentionally change the meaning of data and enable spoofing attacks.

Remediation

• Follow W3C recommendations: use NFC normalisation to enforce strong equivalence without converting characters into visually similar but different characters.
• Normalise all inputs before applying validation or comparison logic, ensuring consistency across the entire application stack.
• Avoid using compatibility normalisers (NFKC/NFKD) in security-sensitive contexts, as they may collapse distinct characters into unsafe equivalents.
• Apply allow-lists or restricted character sets for identifiers (usernames, domains, resource names) to reduce the attack surface.
• Learn more about Unicode normalisation issues in our https://learn.secdim.com/course/spotify-bigbird[Unsafe Normaliser] short course.