From 4b48783c3e209d900a459a2bd76aa314cb62f7e1 Mon Sep 17 00:00:00 2001
From: Rasmus Andersson <rasmus@notion.se>
Date: Wed, 30 Jun 2021 20:01:49 -0700
Subject: [PATCH] Disallow "javascript:" URIs in links. Adds option allowJSURIs
 to explicitly allow it. Closes #14

---
 example/example.html |  1 +
 example/example.md   |  2 ++
 markdown.d.ts        |  5 ++++-
 src/common.h         |  7 +++++++
 src/fmt_html.c       | 27 ++++++++++++++++++++-------
 src/fmt_html.h       |  8 +++-----
 src/md.c             | 13 ++-----------
 src/md.js            | 10 ++++++----
 8 files changed, 45 insertions(+), 28 deletions(-)

diff --git a/example/example.html b/example/example.html
index e46b151..7ecb278 100644
--- a/example/example.html
+++ b/example/example.html
@@ -79,3 +79,4 @@ <h2><a id="tables" class="anchor" aria-hidden="true" href="#tables"></a>Tables</
 <h2><a id="anot-her" class="anchor" aria-hidden="true" href="#anot-her"></a>Anöt######her!</h2>
 <h2><a id="anot-her" class="anchor" aria-hidden="true" href="#anot-her"></a>?!Anöt//her!!</h2>
 <h2><a id="" class="anchor" aria-hidden="true" href="#"></a>?!!</h2>
+<p><a href="">XSS test</a></p>
diff --git a/example/example.md b/example/example.md
index 5d5ce8d..5beb1ea 100644
--- a/example/example.md
+++ b/example/example.md
@@ -82,3 +82,5 @@ function codeBlocks() {
 ## ?!Anöt//her!!
 
 ## ?!!
+
+[XSS test](javAscRipt:alert("xss"))
diff --git a/markdown.d.ts b/markdown.d.ts
index 6e7d1a5..638fd82 100644
--- a/markdown.d.ts
+++ b/markdown.d.ts
@@ -28,8 +28,11 @@ export interface ParseOptions {
    */
   bytes? :boolean
 
+  /** Allow "javascript:" in links */
+  allowJSURIs? :boolean
+
   /**
-   * onCodeBlock is an optional callback which if provided is called for each code block.
+   * Optional callback which if provided is called for each code block.
    * langname holds the "language tag", if any, of the block.
    *
    * The returned value is inserted into the resulting HTML verbatim, without HTML escaping.
diff --git a/src/common.h b/src/common.h
index 836b2a5..5a0c8f3 100644
--- a/src/common.h
+++ b/src/common.h
@@ -61,6 +61,13 @@ typedef int32_t  i32;
 
 #include "wbuf.h"
 
+// these should be in sync with "OutputFlags" in md.js
+typedef enum OutputFlags {
+  OutputFlagHTML       = 1 << 0,
+  OutputFlagXHTML      = 1 << 1,
+  OutputFlagAllowJSURI = 1 << 2, // allow "javascript:" URIs in links
+} OutputFlags;
+
 typedef int(*JSTextFilterFun)(
   const char*  metaptr, u32 metalen,
   const char*  inptr,   u32 inlen,
diff --git a/src/fmt_html.c b/src/fmt_html.c
index 2e71b04..6894ef4 100644
--- a/src/fmt_html.c
+++ b/src/fmt_html.c
@@ -25,6 +25,7 @@
 
 #include <string.h>
 #include <ctype.h>
+#include <strings.h>
 
 #include "common.h"
 #include "fmt_html.h"
@@ -257,9 +258,21 @@ static void render_open_td_block(FmtHTML* r, bool isTH, const MD_BLOCK_TD_DETAIL
   }
 }
 
+static bool is_javascript_uri(const MD_CHAR* text, size_t len) {
+  return (
+    len >= strlen("javascript:") &&
+    strncasecmp(text, "javascript:", strlen("javascript:")) == 0
+  );
+}
+
 static void render_open_a_span(FmtHTML* r, const MD_SPAN_A_DETAIL* det) {
   render_literal(r, "<a href=\"");
-  render_attribute(r, &det->href);
+  // skip "javascript:" URIs unless explicitly allowed
+  if ((r->flags & OutputFlagAllowJSURI) != 0 ||
+      !is_javascript_uri(det->href.text, det->href.size))
+  {
+    render_attribute(r, &det->href);
+  }
   if (det->title.text != NULL) {
     render_literal(r, "\" title=\"");
     render_attribute(r, &det->title);
@@ -279,7 +292,7 @@ static void render_close_img_span(FmtHTML* r, const MD_SPAN_IMG_DETAIL* det) {
     render_literal(r, "\" title=\"");
     render_attribute(r, &det->title);
   }
-  render_literal(r, (r->flags & MD_HTML_FLAG_XHTML) ? "\"/>" : "\">");
+  render_literal(r, (r->flags & OutputFlagXHTML) ? "\"/>" : "\">");
   r->imgnest--;
 }
 
@@ -306,7 +319,7 @@ static int enter_block_callback(MD_BLOCKTYPE type, void* detail, void* userdata)
     case MD_BLOCK_UL:    render_literal(r, "<ul>\n"); break;
     case MD_BLOCK_OL:    render_open_ol_block(r, (const MD_BLOCK_OL_DETAIL*)detail); break;
     case MD_BLOCK_LI:    render_open_li_block(r, (const MD_BLOCK_LI_DETAIL*)detail); break;
-    case MD_BLOCK_HR:    render_literal(r, (r->flags & MD_HTML_FLAG_XHTML) ? "<hr/>\n" : "<hr>\n"); break;
+    case MD_BLOCK_HR:    render_literal(r, (r->flags & OutputFlagXHTML) ? "<hr/>\n" : "<hr>\n"); break;
     case MD_BLOCK_H:
     {
       render_literal(r, head[((MD_BLOCK_H_DETAIL*)detail)->level - 1]);
@@ -379,8 +392,8 @@ static int enter_span_callback(MD_SPANTYPE type, void* detail, void* userdata) {
     case MD_SPAN_EM:                render_literal(r, "<em>"); break;
     case MD_SPAN_STRONG:            render_literal(r, "<b>"); break;
     case MD_SPAN_U:                 render_literal(r, "<u>"); break;
-    case MD_SPAN_A:                 render_open_a_span(r, (MD_SPAN_A_DETAIL*) detail); break;
-    case MD_SPAN_IMG:               render_open_img_span(r, (MD_SPAN_IMG_DETAIL*) detail); break;
+    case MD_SPAN_A:                 render_open_a_span(r, (MD_SPAN_A_DETAIL*)detail); break;
+    case MD_SPAN_IMG:               render_open_img_span(r, (MD_SPAN_IMG_DETAIL*)detail); break;
     case MD_SPAN_CODE:              render_literal(r, "<code>"); break;
     case MD_SPAN_DEL:               render_literal(r, "<del>"); break;
     case MD_SPAN_LATEXMATH:         render_literal(r, "<x-equation>"); break;
@@ -452,12 +465,12 @@ static int text_callback(MD_TEXTTYPE type, const MD_CHAR* text, MD_SIZE size, vo
       render_literal(
         r,
         r->imgnest == 0 ?
-          ((r->flags & MD_HTML_FLAG_XHTML) ? "<br/>\n" : "<br>\n") :
+          ((r->flags & OutputFlagXHTML) ? "<br/>\n" : "<br>\n") :
           " "
       );
       break;
 
-    render_literal(r, (r->flags & MD_HTML_FLAG_XHTML) ? "<hr/>\n" : "<hr>\n"); break;
+    render_literal(r, (r->flags & OutputFlagXHTML) ? "<hr/>\n" : "<hr>\n"); break;
 
     case MD_TEXT_SOFTBR:    render_literal(r, (r->imgnest == 0 ? "\n" : " ")); break;
     case MD_TEXT_HTML:      render_text(r, text, size); break;
diff --git a/src/fmt_html.h b/src/fmt_html.h
index a903981..cfe8215 100644
--- a/src/fmt_html.h
+++ b/src/fmt_html.h
@@ -1,11 +1,9 @@
 #pragma once
 
-#define MD_HTML_FLAG_XHTML 0x0008 // instead of e.g. <br>, generate <br/>
-
 typedef struct FmtHTML {
-  u32   flags;       // MD_HTML_FLAG_*
-  u32   parserFlags; // passed along to md_parse
-  WBuf* outbuf;
+  OutputFlags flags;
+  u32         parserFlags; // passed along to md_parse
+  WBuf*       outbuf;
 
   // optional callbacks
   JSTextFilterFun onCodeBlock;
diff --git a/src/md.c b/src/md.c
index b0c6225..f17f8cc 100644
--- a/src/md.c
+++ b/src/md.c
@@ -4,12 +4,6 @@
 #include "fmt_html.h"
 // #include "fmt_json.h"
 
-// these should be in sync with "OutputFlags" in md.js
-typedef enum OutputFlags {
-  OutputFlagHTML  = 1 << 0,
-  OutputFlagXHTML = 1 << 1,
-} OutputFlags;
-
 typedef enum ErrorCode {
   ERR_NONE,
   ERR_MD_PARSE,
@@ -41,19 +35,16 @@ export size_t parseUTF8(
 
   WBufReset(&outbuf);
 
-  if (outflags & OutputFlagHTML) {
+  if ((outflags & OutputFlagHTML) || (outflags & OutputFlagXHTML)) {
     WBufReserve(&outbuf, inbuflen * 2);  // approximate output size to minimize reallocations
 
     FmtHTML fmt = {
-      .flags = 0,
+      .flags = outflags,
       .parserFlags = parser_flags,
       .outbuf = &outbuf,
       .onCodeBlock = onCodeBlock,
     };
 
-    if (outflags & OutputFlagXHTML)
-      fmt.flags |= MD_HTML_FLAG_XHTML;
-
     if (fmt_html(inbufptr, inbuflen, &fmt) != 0) {
       // fmt_html returns status of md_parse which only fails in extreme cases
       // like when out of memory. md4c does not provide error codes or error messages.
diff --git a/src/md.js b/src/md.js
index c5d05cd..c85a52f 100644
--- a/src/md.js
+++ b/src/md.js
@@ -41,10 +41,11 @@ export const ParseFlags = {
   NO_HTML: 0x0020 | 0x0040, // NO_HTML_BLOCKS | NO_HTML_SPANS
 }
 
-// these should be in sync with "OutputFlags" in md.c
+// these should be in sync with "OutputFlags" in common.h
 const OutputFlags = {
-  HTML:  1 << 0, // Output HTML
-  XHTML: 1 << 1, // Output XHTML (only has effect with HTML flag set)
+  HTML:       1 << 0, // Output HTML
+  XHTML:      1 << 1, // Output XHTML (only has effect with HTML flag set)
+  AllowJSURI: 1 << 2, // Allow "javascript:" URIs
 }
 
 
@@ -56,7 +57,8 @@ export function parse(source, options) {
     options.parseFlags
   )
 
-  let outputFlags = 0
+  let outputFlags = options.allowJSURIs ? OutputFlags.AllowJSURI : 0
+
   switch (options.format) {
     case "xhtml":
       outputFlags |= OutputFlags.HTML | OutputFlags.XHTML