markdown-rs

CommonMark compliant markdown parser in Rust with ASTs and extensions.

Feature highlights

• compliant (100% to CommonMark)
• extensions (100% GFM, 100% MDX, frontmatter, math)
• safe (100% safe Rust, also 100% safe HTML by default)
• robust (2300+ tests, 100% coverage, fuzz testing)
• ast (mdast)

Links

• GitHub: wooorm/markdown-rs
• crates.io: markdown
• docs.rs: markdown

When should I use this?

• if you just want to turn markdown into HTML (with maybe a few extensions)
• if you want to do really complex things with markdown

What is this?

markdown-rs is an open source markdown parser written in Rust. It’s implemented as a state machine (#![no_std] + alloc) that emits concrete tokens, so that every byte is accounted for, with positional info. The API then exposes this information as an AST, which is easier to work with, or it compiles directly to HTML.

While most markdown parsers work towards compliancy with CommonMark (or GFM), this project goes further by following how the reference parsers (cmark, cmark-gfm) work, which is confirmed with thousands of extra tests.

Other than CommonMark and GFM, this project also supports common extensions to markdown such as MDX, math, and frontmatter.

This Rust crate has a sibling project in JavaScript: micromark (and mdast-util-from-markdown for the AST).

P.S. if you want to compile MDX, use mdxjs-rs.

Questions

• to learn markdown, see this cheatsheet and tutorial
• for the API, see the crate docs
• for questions, see Discussions
• to help, see contribute or sponsor below

Contents

• Install
• Use
• API
• Extensions
• Project
  • Overview
  • File structure
  • Test
  • Version
  • Security
  • Contribute
  • Sponsor
  • Thanks
• Related
• License

Install

With Rust (rust edition 2018+, ±version 1.56+), install with cargo:

cargo add markdown

Use

fn main() {
    println!("{}", markdown::to_html("## Hi, *Saturn*! 🪐"));
}

Yields:

<h2>Hi, <em>Saturn</em>! 🪐</h2>

Extensions (in this case GFM):

fn main() -> Result<(), markdown::message::Message> {
    println!(
        "{}",
        markdown::to_html_with_options(
            "* [x] contact ~Mercury~Venus at hi@venus.com!",
            &markdown::Options::gfm()
        )?
    );

    Ok(())
}

Yields:

<ul>
  <li>
    <input checked="" disabled="" type="checkbox" />
    contact <del>Mercury</del>Venus at <a href="mailto:hi@venus.com">hi@venus.com</a>!
  </li>
</ul>

Syntax tree (mdast):

fn main() -> Result<(), markdown::message::Message> {
    println!(
        "{:?}",
        markdown::to_mdast("# Hi *Earth*!", &markdown::ParseOptions::default())?
    );

    Ok(())
}

Yields:

Root { children: [Heading { children: [Text { value: "Hi ", position: Some(1:3-1:6 (2-5)) }, Emphasis { children: [Text { value: "Earth", position: Some(1:7-1:12 (6-11)) }], position: Some(1:6-1:13 (5-12)) }, Text { value: "!", position: Some(1:13-1:14 (12-13)) }], position: Some(1:1-1:14 (0-13)), depth: 1 }], position: Some(1:1-1:14 (0-13)) }

API

markdown-rs exposes to_html, to_html_with_options, to_mdast, Options, and a few other structs and enums.

See the crate docs for more info.

Extensions

markdown-rs supports extensions to CommonMark. These extensions are maintained in this project. They are not enabled by default but can be turned on with options.

• GFM
  • autolink literal
  • footnote
  • strikethrough
  • table
  • tagfilter
  • task list item
• MDX
  • ESM
  • expressions
  • JSX
• frontmatter
• math

It is not a goal of this project to support lots of different extensions. It’s instead a goal to support very common and mostly standardized extensions.

Project

markdown-rs is maintained as a single monolithic crate.

Overview

The process to parse markdown looks like this:

                    markdown-rs
+-------------------------------------------------+
|            +-------+         +---------+--html- |
| -markdown->+ parse +-events->+ compile +        |
|            +-------+         +---------+-mdast- |
+-------------------------------------------------+

File structure

The files in src/ are as follows:

• construct/*.rs — CommonMark, GFM, and other extension constructs used in markdown
• util/*.rs — helpers often needed when parsing markdown
• event.rs — things with meaning happening somewhere
• lib.rs — public API
• mdast.rs — syntax tree
• parser.rs — turn a string of markdown into events
• resolve.rs — steps to process events
• state.rs — steps of the state machine
• subtokenize.rs — handle content in other content
• to_html.rs — turns events into a string of HTML
• to_mdast.rs — turns events into a syntax tree
• tokenizer.rs — glue the states of the state machine together
• unist.rs — point and position, used in mdast

Test

markdown-rs is tested with the ~650 CommonMark tests and more than 1k extra tests confirmed with CM reference parsers. Then there’s even more tests for GFM and other extensions. These tests reach all branches in the code, which means that this project has 100% code coverage. Fuzz testing is used to check for things that might fall through coverage.

The following bash scripts are useful when working on this project:

• generate code (latest CM tests and Unicode info):

  cargo run --manifest-path generate/Cargo.toml
  

• run examples:

  RUST_BACKTRACE=1 RUST_LOG=trace cargo run --example lib --features log
  

• format:

  cargo fmt && cargo fix --all-features --all-targets --workspace
  

• lint:

  cargo fmt --check && cargo clippy --all-features --all-targets --workspace
  

• test:

  RUST_BACKTRACE=1 cargo test --all-features --workspace
  

• docs:

  cargo doc --document-private-items --examples --workspace
  

• fuzz:

  cargo install cargo-fuzz
  cargo install honggfuzz
  cargo +nightly fuzz run markdown_libfuzz
  cargo hfuzz run markdown_honggfuzz
  

Version

markdown-rs follows SemVer.

Security

The typical security aspect discussed for markdown is cross-site scripting (XSS) attacks. Markdown itself is safe if it does not include embedded HTML or dangerous protocols in links/images (such as javascript:). markdown-rs makes any markdown safe by default, even if HTML is embedded or dangerous protocols are used, as it encodes or drops them.

Turning on the allow_dangerous_html or allow_dangerous_protocol options for user-provided markdown opens you up to XSS attacks.

Additionnally, you should be able to set allow_any_img_src safely. The default is to allow only http:, https:, and relative images, which is what GitHub does. But it should be safe to allow any value on src.

The HTML specification prohibits dangerous scripts in images and all modern browsers respect this and are thus safe. Opera 12 (from 2012) is a notable browser that did not respect this.

An aspect related to XSS for security is syntax errors: markdown itself has no syntax errors. Some syntax extensions (specifically, only MDX) do include syntax errors. For that reason, to_html_with_options returns Result<String, Message>, of which the error is a struct indicating where the problem happened, what occurred, and what was expected instead. Make sure to handle your errors when using MDX.

Another security aspect is DDoS attacks. For example, an attacker could throw a 100mb file at markdown-rs, in which case it’s going to take a long while to finish. It is also possible to crash markdown-rs with smaller payloads, notably when thousands of links, images, emphasis, or strong are opened but not closed. It is wise to cap the accepted size of input (500kb can hold a big book) and to process content in a different thread so that it can be stopped when needed.

For more information on markdown sanitation, see improper-markup-sanitization.md by @chalker.

Contribute

See contributing.md for ways to help. See support.md for ways to get help. See code-of-conduct.md for how to communicate in and around this project.

Sponsor

Support this effort and give back by sponsoring:

• GitHub Sponsors (personal; monthly or one-time)
• OpenCollective or GitHub Sponsors (unified; monthly or one-time)

Thanks

Special thanks go out to:

• Vercel for funding the initial development
• @Murderlon for the design of the logo
• @johannhof for the crate name

Related

• micromark — same as markdown-rs but in JavaScript
• mdxjs-rs — wraps markdown-rs to compile MDX to JavaScript

License

MIT © Titus Wormer