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Here is a PHP code sample of a vulnerable application: $', $input) However, we found this type of bug you can easily find by fuzzing! These vulnerabilities are very easy to overlook both by developers and hackers. Nested condition is when one payload is processed by two different parsers, which, with some manipulations, allows us to inject arbitrary JavaScript into the page. HTML related encoding could be an urlencode ' – (%22) or HTML entities transformation ' – ("e /'/') Input Vulnerability when a parser converts user input to HTML, sanitizes HTML characters, but after it decodes user input from known encoding.

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They could be used for the “pass the hash” or offline password brute force attacks against Windows users: Inputįile://localhost/C:/windows/system32/calc.exe For example, arbitrary client-side file reading using JavaScript, arbitrary client-side file execution using plain HTML, leakage of NTLM hashes.

This vulnerability could lead to critical attacks against desktop applications. This time the cause is insufficient “file://” URL scheme sanitization. This is another vulnerability when a parser converts user input that contains URLs. If such parsers do not sanitize the “javascript:” URL scheme, it will allow the attacker to execute arbitrary JavaScript and perform XSS attacks: Input This vulnerability can be exploited when a parser converts user input that contains URLs. Missing “javascript:” URL scheme sanitization It could affect characters such as angle brackets There is a vulnerability when a parser converts user input to HTML and at the same time does not sanitize HTML characters. If you google “markdown XSS”, you will find examples with missing sanitization of HTML characters and URL schemes. Also, you can find emoticons and emojis that become pictures from text, links to the user profile and hashtags that become clickable: do we know about bugs in this functionality? In addition to text markup parsers in messages and comments, you can also find URL and email parsers, smart URL parsers, which understand and transform to HTML not only HTTP links but also images or YouTube links. ReStructuredText (RST, ReST, or reST) is a file format for textual data used primarily in the Python programming language community for technical documentation. A few other examples: InputĪsciiDoc is a human-readable document format semantically equivalent to DocBook XML but uses plain-text markup conventions introduced in 2002: Input Markdown is a lightweight markup language for creating formatted text using a plain-text editor. There’re a few examples of the BBCode and the corresponding HTML code: Input BBcodeīBcode is a lightweight markup language used to format messages in many Internet forums, first introduced in 1998. That is why we think that allowing even a limited list of tags is one of the developers’ worst choices. It is very easy for developers without the hacker’s mentality to overlook some possibilities whilst sanitizing these tags. Some known applications allow using whitelisted HTML tags like, , (WordPress, Vanilla forums, etc.). Well known parsers in messages HTML as message markup When parsing messages, they can find a substring and convert it to the correct HTML code.

Parsers are applications that find a substring in a text. Parsers What are parsers, and what are they for in messages? The technique was presented at Power Of Community 2021. This technique allowed us to find a bunch of vulnerabilities in the popular products that no one had noticed before.

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We show how to fuzz and detect issues in the HTML parsers with nested conditions. This article reveals our novel technique for finding sanitization issues that could lead to XSS attacks. And as we know, wherever there is HTML, there can be XSS. Some of us encounter markdown on Telegram or GitHub, while forum-dwellers might be more familiar with BBCode.Īll this is made possible by parsers, which find a special string (code/tag/character) in messages and convert it into beautiful text using HTML. When communicating online, we constantly use emoticons and put text in bold.