html &apos: Reserved Characters and Entity References

In HTML, certain characters have special meanings and can't be used as is in your code. The apostrophe, or single quote, is one of these reserved characters.

To use an apostrophe in your HTML, you need to replace it with an entity reference, which is a special code that represents a character. This is done using the `'` syntax.

This is because the apostrophe is used to indicate attribute values, such as in `alt='image description'`. If you were to use a regular apostrophe, it would be interpreted as part of the attribute value, rather than as a special character.

In HTML, the `'` entity reference represents a single quotation mark, which is the same as an apostrophe.

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Character Entity References are a fundamental part of HTML, and understanding how they work can save you a lot of headaches down the line.

In HTML5, there are many named entities that act as mnemonic aliases for certain Unicode characters. You can use these entities by entering an ampersand (&) followed by the entity name, and a semicolon (e.g., © for the copyright symbol).

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To use one of these character entity references in an HTML or XML document, you must include the semicolon, as it is mandatory in XML and strongly recommended in HTML.

The HTML5 specification does not allow users to define additional entities, as it no longer accepts any DTD to be referenced or extended inside HTML documents.

There are no predefined character entities in HTML for characters or sequences of most scripts encoded in the UCS, except for a common subset of whitespace, punctuation, mathematical or technical symbols, currency symbols, and a few Hebrew symbols used in mathematical notations.

Most entities are predefined in XML and HTML to reference just one character in the UCS, but there are no predefined entities for isolated combining characters, variation selectors, or characters for private use assignments.

Here are the 5 built-in XML character entities that can safely be used in all XHTML processing situations:

Note that only ", &, <, and > will work in all XHTML processing situations.

Entities representing special characters in HTML and XHTML are a crucial aspect of coding. These entities are used to represent characters that have special meanings in HTML and XHTML documents.

The HTML5 specification defines many named entities, references to which act as mnemonic aliases for certain Unicode characters. To use one of these character entity references, you need to enter an ampersand (&) followed by the entity name, and a semicolon (mandatory in XML, and strongly recommended in HTML).

There are 253 entities in XHTML DTDs that explicitly declare entities whose expansion is a single character. These entities have the same names and represent the same characters as the 252 character entities in HTML 4.0.

Some special character entities are predefined in XML and HTML, such as © for the copyright symbol. However, not all bidirectional controls defined in UCS/Unicode are represented as standard character entities in HTML.

Here are the special character entities that will work in all XHTML processing situations: ", &, <, and >.

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Reserved characters in HTML, specifically the ' entity, can be tricky to work with. In XHTML, these characters are represented by entities, and there are 253 of them, including the 5 predefined entities of XML 1.0.

The ' entity is a special case, and only four entities will work in all XHTML processing situations: ", &, <, and >. This is because the ' entity is not one of the 252 character entities in HTML 4.0.

If you're working with XHTML documents, it's essential to understand how entity references are processed. Here are the key takeaways:

HTML5 defines many named entities, references to which act as mnemonic aliases for certain Unicode characters.

In HTML5, you can use an ampersand (&) followed by the entity name, and a semicolon (mandatory in XML, and strongly recommended in HTML) to reference a character entity, e.g., © for the copyright symbol.

Not all bidirectional controls defined in UCS/Unicode are represented as standard character entities in HTML, not even in HTML5.

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Most entities are predefined in XML and HTML to reference just one character in the UCS, but there are no predefined entities for isolated combining characters, variation selectors, or characters for private use assignments.

The following character entities are usable and valid in HTML, XML, XHTML, and MathML, either in plain-text values of attributes or in text elements: all valid characters and sequences in the UCS, including all bidirectional controls or private-use assignments (but with the exception of non-whitespace C0 and C1 controls, non-characters, and surrogates).

Here are the five built-in XML character entities that can safely be used: ', &, <, >, and ".

Intriguing read: Html Special Characters

Crafting the Payload is a crucial step in exploiting XSS vulnerabilities.

Developers often manually input each individual character they want to escape, which can sometimes lead to mistakes.

A good starting point is to focus on the 'onclick' attribute, as it's where the stored XSS will be.

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Let's test and get a feel for how the site works by replacing single quotes with their HTML encoded equivalent (').

This tells the browser to treat the single quote as part of its code, allowing the JavaScript payload to be interpreted correctly.

The 'href' attribute is another spot to check for 'breaking out', with a website wrapped in double quotes.

However, we're starting with the 'onclick' attribute, so let's focus on that for now.

Here are the two spots to check for 'breaking out':