CVE-2020-4054 (published 2020-06-16T22:15Z)

In Sanitize (RubyGem sanitize) greater than or equal to 3.0.0 and less than 5.2.1, there is a cross-site scripting vulnerability. When HTML is sanitized using Sanitize\'s "relaxed" config, or a custom config that allows certain elements, some content in a math or svg element may not be sanitized correctly even if math and svg are not in the allowlist. You are likely to be vulnerable to this issue if you use Sanitize\'s relaxed config or a custom config that allows one or more of the following HTML elements: iframe, math, noembed, noframes, noscript, plaintext, script, style, svg, xmp. Using carefully crafted input, an attacker may be able to sneak arbitrary HTML through Sanitize, potentially resulting in XSS (cross-site scripting) or other undesired behavior when that HTML is rendered in a browser. This has been fixed in 5.2.1.

Severity: MEDIUM
CVSS v2 base score: 6.8 (AV:N/AC:M/Au:N/C:P/I:P/A:P)
CVSS v3 base score: 7.3, HIGH (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L)

Weakness information:
cwe_id: CWE-79
cwe_name: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
description: The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.
extended_description: 
            Cross-site scripting (XSS) vulnerabilities occur when:
            
               Untrusted data enters a web application, typically from a web request.
               The web application dynamically generates a web page that contains this untrusted data.
               During page generation, the application does not prevent the data from containing content that is executable by a web browser, such as JavaScript, HTML tags, HTML attributes, mouse events, Flash, ActiveX, etc.
               A victim visits the generated web page through a web browser, which contains malicious script that was injected using the untrusted data.
               Since the script comes from a web page that was sent by the web server, the victim's web browser executes the malicious script in the context of the web server's domain.
               This effectively violates the intention of the web browser's same-origin policy, which states that scripts in one domain should not be able to access resources or run code in a different domain.
            
            There are three main kinds of XSS:
            
               
                  Type 1: Reflected XSS (or Non-Persistent) - 
         			The server reads data directly from the HTTP request and reflects it back in the HTTP response. Reflected XSS exploits occur when an attacker causes a victim to supply dangerous content to a vulnerable web application, which is then reflected back to the victim and executed by the web browser. The most common mechanism for delivering malicious content is to include it as a parameter in a URL that is posted publicly or e-mailed directly to the victim. URLs constructed in this manner constitute the core of many phishing schemes, whereby an attacker convinces a victim to visit a URL that refers to a vulnerable site. After the site reflects the attacker's content back to the victim, the content is executed by the victim's browser.
               
                  Type 2: Stored XSS (or Persistent) - 
               The application stores dangerous data in a database, message forum, visitor log, or other trusted data store. At a later time, the dangerous data is subsequently read back into the application and included in dynamic content. From an attacker's perspective, the optimal place to inject malicious content is in an area that is displayed to either many users or particularly interesting users. Interesting users typically have elevated privileges in the application or interact with sensitive data that is valuable to the attacker. If one of these users executes malicious content, the attacker may be able to perform privileged operations on behalf of the user or gain access to sensitive data belonging to the user. For example, the attacker might inject XSS into a log message, which might not be handled properly when an administrator views the logs.
            
               
                  Type 0: DOM-Based XSS - 
               In DOM-based XSS, the client performs the injection of XSS into the page; in the other types, the server performs the injection. DOM-based XSS generally involves server-controlled, trusted script that is sent to the client, such as Javascript that performs sanity checks on a form before the user submits it. If the server-supplied script processes user-supplied data and then injects it back into the web page (such as with dynamic HTML), then DOM-based XSS is possible.
            
            
            Once the malicious script is injected, the attacker can perform a variety of malicious activities. The attacker could transfer private information, such as cookies that may include session information, from the victim's machine to the attacker. The attacker could send malicious requests to a web site on behalf of the victim, which could be especially dangerous to the site if the victim has administrator privileges to manage that site. Phishing attacks could be used to emulate trusted web sites and trick the victim into entering a password, allowing the attacker to compromise the victim's account on that web site. Finally, the script could exploit a vulnerability in the web browser itself possibly taking over the victim's machine, sometimes referred to as "drive-by hacking."
            In many cases, the attack can be launched without the victim even being aware of it. Even with careful users, attackers frequently use a variety of methods to encode the malicious portion of the attack, such as URL encoding or Unicode, so the request looks less suspicious.
         
         

Fix information:
repo_url: https://github.com/rgrove/sanitize
commit_details: repo_url: https://github.com/rgrove/sanitize
author: Ryan Grove
author_date: 2020-06-15 14:27:07-07:00
msg: Fix sanitization bypass in HTML foreign content

https://github.com/rgrove/sanitize/security/advisories/GHSA-p4x4-rw2p-8j8m
repository: repo_url: https://github.com/rgrove/sanitize
repo_name: rgrove/sanitize
description: Ruby HTML and CSS sanitizer.
repo_language: HTML
owner: rgrove
file_changes: filename: README.md
change_type: ModificationType.MODIFY
diff: @@ -72,6 +72,11 @@ Sanitize can sanitize the following types of input:
 * Standalone CSS stylesheets
 * Standalone CSS properties
 
+However, please note that Sanitize _cannot_ fully sanitize the contents of
+`<math>` or `<svg>` elements, since these elements don't follow the same parsing
+rules as the rest of HTML. If this is something you need, you may want to look
+for another solution.
+
 ### HTML Fragments
 
 A fragment is a snippet of HTML that doesn't contain a root-level `<html>`
@@ -415,6 +420,12 @@ elements not in this array will be removed.
 ]
 ```
 
+**Warning:** Sanitize cannot fully sanitize the contents of `<math>` or `<svg>`
+elements, since these elements don't follow the same parsing rules as the rest
+of HTML. If you add `math` or `svg` to the allowlist, you must assume that any
+content inside them will be allowed, even if that content would otherwise be
+removed by Sanitize.
+
 #### :parser_options (Hash)
 
 [Parsing options](https://github.com/rubys/nokogumbo/tree/v2.0.1#parsing-options) supplied to `nokogumbo`.

programming_language: Markdown
filename: default.rb
change_type: ModificationType.MODIFY
diff: @@ -74,7 +74,7 @@ class Sanitize
       # the specified elements (when filtered) will be removed, and the contents
       # of all other filtered elements will be left behind.
       :remove_contents => %w[
-        iframe noembed noframes noscript script style
+        iframe math noembed noframes noscript plaintext script style svg xmp
       ],
 
       # Transformers allow you to filter or alter nodes using custom logic. See

programming_language: Ruby
filename: test_clean_element.rb
change_type: ModificationType.MODIFY
diff: @@ -192,21 +192,16 @@ describe 'Sanitize::Transformers::CleanElement' do
         .must_equal ''
     end
 
-    it 'should escape the content of removed `plaintext` elements' do
-      Sanitize.fragment('<plaintext>hello! <script>alert(0)</script>')
-        .must_equal 'hello! &lt;script&gt;alert(0)&lt;/script&gt;'
-    end
-
-    it 'should escape the content of removed `xmp` elements' do
-      Sanitize.fragment('<xmp>hello! <script>alert(0)</script></xmp>')
-        .must_equal 'hello! &lt;script&gt;alert(0)&lt;/script&gt;'
-    end
-
     it 'should not preserve the content of removed `iframe` elements' do
       Sanitize.fragment('<iframe>hello! <script>alert(0)</script></iframe>')
         .must_equal ''
     end
 
+    it 'should not preserve the content of removed `math` elements' do
+      Sanitize.fragment('<math>hello! <script>alert(0)</script></math>')
+        .must_equal ''
+    end
+
     it 'should not preserve the content of removed `noembed` elements' do
       Sanitize.fragment('<noembed>hello! <script>alert(0)</script></noembed>')
         .must_equal ''
@@ -222,6 +217,11 @@ describe 'Sanitize::Transformers::CleanElement' do
         .must_equal ''
     end
 
+    it 'should not preserve the content of removed `plaintext` elements' do
+      Sanitize.fragment('<plaintext>hello! <script>alert(0)</script>')
+        .must_equal ''
+    end
+
     it 'should not preserve the content of removed `script` elements' do
       Sanitize.fragment('<script>hello! <script>alert(0)</script></script>')
         .must_equal ''
@@ -232,6 +232,16 @@ describe 'Sanitize::Transformers::CleanElement' do
         .must_equal ''
     end
 
+    it 'should not preserve the content of removed `svg` elements' do
+      Sanitize.fragment('<svg>hello! <script>alert(0)</script></svg>')
+        .must_equal ''
+    end
+
+    it 'should not preserve the content of removed `xmp` elements' do
+      Sanitize.fragment('<xmp>hello! <script>alert(0)</script></xmp>')
+        .must_equal ''
+    end
+
     strings.each do |name, data|
       it "should clean #{name} HTML" do
         Sanitize.fragment(data[:html]).must_equal(data[:default])

programming_language: Ruby
filename: test_malicious_html.rb
change_type: ModificationType.MODIFY
diff: @@ -219,4 +219,17 @@ describe 'Malicious HTML' do
       end
     end
   end
+
+  # https://github.com/rgrove/sanitize/security/advisories/GHSA-p4x4-rw2p-8j8m
+  describe 'foreign content bypass in relaxed config' do
+    it 'prevents a sanitization bypass via carefully crafted foreign content' do
+      %w[iframe noembed noframes noscript plaintext script style xmp].each do |tag_name|
+        @s.fragment(%[<math><#{tag_name}>/*&lt;/#{tag_name}&gt;&lt;img src onerror=alert(1)>*/]).
+          must_equal ''
+
+        @s.fragment(%[<svg><#{tag_name}>/*&lt;/#{tag_name}&gt;&lt;img src onerror=alert(1)>*/]).
+          must_equal ''
+      end
+    end
+  end
 end

programming_language: Ruby