Add support for sanitizing urls

*   Add support for properly encoding characters in urls
*   Add support for sanitizing potentially dangerous urls
*   Add safe defaults, optionally live dangerously

4 files changed, 130 insertions, 15 deletions

diff --git a/src/compiler.rs b/src/compiler.rs
index df26f1b..c451887 100644
--- a/src/compiler.rs
+++ b/src/compiler.rs
@@ -3,7 +3,7 @@ use crate::construct::character_reference::Kind as CharacterReferenceKind;
 use crate::tokenizer::{Code, Event, EventType, TokenType};
 use crate::util::{
     decode_named_character_reference, decode_numeric_character_reference, encode, get_span,
-    slice_serialize,
+    sanitize_uri, slice_serialize,
 };
 
 /// Configuration (optional).
@@ -13,6 +13,11 @@ pub struct CompileOptions {
     /// The default is `false`, you can turn it on to `true` for trusted
     /// content.
     pub allow_dangerous_html: bool,
+
+    /// Whether to allow (dangerous) protocols in links and images.
+    /// The default is `false`, you can turn it on to `true` for trusted
+    /// content.
+    pub allow_dangerous_protocol: bool,
 }
 
 /// Turn events and codes into a string of HTML.
@@ -28,6 +33,17 @@ pub fn compile(events: &[Event], codes: &[Code], options: &CompileOptions) -> St
     let mut slurp_one_line_ending = false;
     let mut ignore_encode = false;
     let mut character_reference_kind: Option<CharacterReferenceKind> = None;
+    let protocol_href = if options.allow_dangerous_protocol {
+        None
+    } else {
+        Some(vec!["http", "https", "irc", "ircs", "mailto", "xmpp"])
+    };
+    // let protocol_src = if options.allow_dangerous_protocol {
+    //     None
+    // } else {
+    //     Some(vec!["http", "https"])
+    // };
+
     // let mut slurp_all_line_endings = false;
 
     println!("events: {:#?}", events);
@@ -238,20 +254,20 @@ pub fn compile(events: &[Event], codes: &[Code], options: &CompileOptions) -> St
                 TokenType::AutolinkProtocol => {
                     let slice = slice_serialize(codes, &get_span(events, index), false);
                     let buf = buf_tail_mut(buffers);
-                    // To do: options.allowDangerousProtocol ? undefined : protocolHref
-                    // let url = sanitize_uri(slice);
-                    let url = encode(&slice);
-                    buf.push(format!("<a href=\"{}\">", url));
+                    buf.push(format!(
+                        "<a href=\"{}\">",
+                        sanitize_uri(slice.as_str(), &protocol_href)
+                    ));
                     buf.push(encode(&slice));
                     buf.push("</a>".to_string());
                 }
                 TokenType::AutolinkEmail => {
                     let slice = slice_serialize(codes, &get_span(events, index), false);
                     let buf = buf_tail_mut(buffers);
-                    // To do: options.allowDangerousProtocol ? undefined : protocolHref
-                    // let url = sanitize_uri(slice);
-                    let url = encode(&slice);
-                    buf.push(format!("<a href=\"mailto:{}\">", url));
+                    buf.push(format!(
+                        "<a href=\"mailto:{}\">",
+                        sanitize_uri(slice.as_str(), &protocol_href)
+                    ));
                     buf.push(encode(&slice));
                     buf.push("</a>".to_string());
                 }
diff --git a/src/construct/autolink.rs b/src/construct/autolink.rs
index 24f2c20..c190d40 100644
--- a/src/construct/autolink.rs
+++ b/src/construct/autolink.rs
@@ -41,12 +41,12 @@
 //! Interestingly, there are a couple of things that are valid autolinks in
 //! markdown but in HTML would be valid tags, such as `<svg:rect>` and
 //! `<xml:lang/>`.
-//! However, because CommonMark employs a naïve HTML parsing algorithm, those
+//! However, because `CommonMark` employs a naïve HTML parsing algorithm, those
 //! are not considered HTML.
 //!
-//! While CommonMark restricts links from occurring in other links in the case
-//! of bracketed links, this restriction is not in place for autolinks inside
-//! autolinks:
+//! While `CommonMark` restricts links from occurring in other links in the
+//! case of bracketed links, this restriction is not in place for autolinks
+//! inside autolinks:
 //!
 //! ```markdown
 //! [<https://example.com>](#)
@@ -74,8 +74,6 @@
 //! [autolink_scheme_size_max]: crate::constant::AUTOLINK_SCHEME_SIZE_MAX
 //! [autolink_domain_size_max]: crate::constant::AUTOLINK_DOMAIN_SIZE_MAX
 //! [html-a]: https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-a-element
-//!
-//! <!-- To do: link to `encode` -->
 
 use crate::constant::{AUTOLINK_DOMAIN_SIZE_MAX, AUTOLINK_SCHEME_SIZE_MAX};
 use crate::tokenizer::{Code, State, StateFnResult, TokenType, Tokenizer};
diff --git a/src/lib.rs b/src/lib.rs
index cf0b05b..906cd4b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -42,6 +42,7 @@ pub fn micromark(value: &str) -> String {
 ///
 /// let result = micromark_with_options("<div>\n\n# Hello, world!\n\n</div>", &CompileOptions {
 ///     allow_dangerous_html: true,
+///     allow_dangerous_protocol: true,
 /// });
 ///
 /// assert_eq!(result, "<div>\n<h1>Hello, world!</h1>\n</div>");
diff --git a/src/util.rs b/src/util.rs
index 5a916cd..accc48e 100644
--- a/src/util.rs
+++ b/src/util.rs
@@ -31,6 +31,106 @@ pub fn encode(value: &str) -> String {
         .replace('>', "&gt;")
 }
 
+/// Make a value safe for injection as a URL.
+///
+/// This encodes unsafe characters with percent-encoding and skips already
+/// encoded sequences (see `normalize_uri` below).
+/// Further unsafe characters are encoded as character references (see
+/// `encode`).
+///
+/// Then, a vec of (lowercase) allowed protocols can be given, in which case
+/// the URL is sanitized.
+///
+/// For example, `Some(vec!["http", "https", "irc", "ircs", "mailto", "xmpp"])`
+/// can be used for `a[href]`, or `Some(vec!["http", "https"])` for `img[src]`.
+/// If the URL includes an unknown protocol (one not matched by `protocol`, such
+/// as a dangerous example, `javascript:`), the value is ignored.
+pub fn sanitize_uri(value: &str, protocols: &Option<Vec<&str>>) -> String {
+    let value = encode(&normalize_uri(value));
+
+    if let Some(protocols) = protocols {
+        let chars: Vec<char> = value.chars().collect();
+        let mut index = 0;
+        let mut colon: Option<usize> = None;
+
+        while index < chars.len() {
+            let char = chars[index];
+
+            match char {
+                ':' => {
+                    colon = Some(index);
+                    break;
+                }
+                '?' | '#' | '/' => break,
+                _ => {}
+            }
+
+            index += 1;
+        }
+
+        // If there is no protocol, or the first colon is after `?`, `#`, or `/`, it’s relative.
+        // It is a protocol, it should be allowed.
+        if let Some(colon) = colon {
+            let protocol = chars[0..colon].iter().collect::<String>().to_lowercase();
+            if !protocols.contains(&protocol.as_str()) {
+                return "".to_string();
+            }
+        }
+    }
+
+    value
+}
+
+/// Normalize a URL (such as used in definitions).
+///
+/// Encode unsafe characters with percent-encoding, skipping already encoded
+/// sequences.
+fn normalize_uri(value: &str) -> String {
+    let chars: Vec<char> = value.chars().collect();
+    let mut result: Vec<String> = vec![];
+    let mut index = 0;
+    let mut start = 0;
+    let mut buff = [0; 4];
+
+    while index < chars.len() {
+        let char = chars[index];
+
+        // A correct percent encoded value.
+        if char == '%'
+            && index + 2 < chars.len()
+            && chars[index + 1].is_ascii_alphanumeric()
+            && chars[index + 2].is_ascii_alphanumeric()
+        {
+            index += 3;
+            continue;
+        }
+
+        // Note: Rust already takes care of lone astral surrogates.
+        // Non-ascii or not allowed ascii.
+        if char >= '\u{0080}'
+            || !matches!(char, '!' | '#' | '$' | '&'..=';' | '=' | '?'..='Z' | '_' | 'a'..='z' | '~')
+        {
+            result.push(chars[start..index].iter().collect::<String>());
+
+            char.encode_utf8(&mut buff);
+            result.push(
+                buff[0..char.len_utf8()]
+                    .iter()
+                    .map(|&byte| format!("%{:X}", byte))
+                    .collect::<String>(),
+            );
+
+            start = index + 1;
+        }
+
+        index += 1;
+    }
+
+    result.push(chars[start..].iter().collect::<String>());
+
+    result.join("")
+}
+
 /// Decode numeric character references.
 ///
 /// Turn the number (in string form as either hexadecimal or decimal) coming