Normalization ensures two strings that may use a different binary representation for their characters have the same binary value after normalization.
There are two overall types of equivalence between characters, “**Canonical Equivalence**” and “**Compatibility Equivalence**”:
**Canonical Equivalent** characters are assumed to have the same appearance and meaning when printed or displayed. **Compatibility Equivalence** is a weaker equivalence, in that two values may represent the same abstract character but can be displayed differently. There are **4 Normalization algorithms** defined by the **Unicode** standard; **NFC, NFD, NFKD and NFKD**, each applies Canonical and Compatibility normalization techniques in a different way. You can read more on the different techniques at Unicode.org.
### Unicode Encoding
Although Unicode was in part designed to solve interoperability issues, the evolution of the standard, the need to support legacy systems and different encoding methods can still pose a challenge.
Before we delve into Unicode attacks, the following are the main points to understand about Unicode:
* Each character or symbol is mapped to a numerical value which is referred to as a “code point”.
* The code point value \(and therefore the character itself\) is represented by 1 or more bytes in memory. LATIN-1 characters like those used in English speaking countries can be represented using 1 byte. Other languages have more characters and need more bytes to represent all the different code points \(also since they can’t use the ones already taken by LATIN-1\).
* The term “encoding” means the method in which characters are represented as a series of bytes. The most common encoding standard is UTF-8, using this encoding scheme ASCII characters can be represented using 1 byte or up to 4 bytes for other characters.
* When a system processes data it needs to know the encoding used to convert the stream of bytes to characters.
* Though UTF-8 is the most common, there are similar encoding standards named UTF-16 and UTF-32, the difference between each is the number of bytes used to represent each character. i.e. UTF-16 uses a minimum of 2 bytes \(but up to 4\) and UTF-32 using 4 bytes for all characters.
An example of how Unicode normalise two different bytes representing the same character:
**A list of Unicode equivalent characters can be found here:** [https://appcheck-ng.com/wp-content/uploads/unicode\_normalization.html](https://appcheck-ng.com/wp-content/uploads/unicode_normalization.html)
### Discovering
If you can find inside a webapp a value that is being echoed back, you could try to send **‘KELVIN SIGN’ \(U+0212A\)** which **normalises to "K"** \(you can send it as `%e2%84%aa`\). **If a "K" is echoed back**, then, some kind of **Unicode normalisation** is being performed.
Other **example**: `%F0%9D%95%83%E2%85%87%F0%9D%99%A4%F0%9D%93%83%E2%85%88%F0%9D%94%B0%F0%9D%94%A5%F0%9D%99%96%F0%9D%93%83` after **unicode** is `Leonishan`.
## **Vulnerable Examples**
### **SQL Injection filter bypass**
Imagine a web page that is using the character `'` to create SQL queries with the user input. This web, as a security measure, **deletes** all occurrences of the character **`'`** from the user input, but **after that deletion** and **before the creation** of the query, it **normalises** using **Unicode** the input of the user.
Then, a malicious user could insert a different Unicode character equivalent to `' (0x27)` like `%ef%bc%87` , when the input gets normalised, a single quote is created and a **SQLInjection vulnerability** appears:
**All the information of this page was taken from:** [**https://appcheck-ng.com/unicode-normalization-vulnerabilities-the-special-k-polyglot/\#**](https://appcheck-ng.com/unicode-normalization-vulnerabilities-the-special-k-polyglot/#)
