CVE-2023-29528 (published 2023-04-20T18:15Z)

XWiki Commons are technical libraries common to several other top level XWiki projects. The "restricted" mode of the HTML cleaner in XWiki, introduced in version 4.2-milestone-1 and massively improved in version 14.6-rc-1, allowed the injection of arbitrary HTML code and thus cross-site scripting via invalid HTML comments. As a consequence, any code relying on this "restricted" mode for security is vulnerable to JavaScript injection ("cross-site scripting"/XSS). When a privileged user with programming rights visits such a comment in XWiki, the malicious JavaScript code is executed in the context of the user session. This allows server-side code execution with programming rights, impacting the confidentiality, integrity and availability of the XWiki instance. This problem has been patched in XWiki 14.10, HTML comments are now removed in restricted mode and a check has been introduced that ensures that comments don\'t start with `>`. There are no known workarounds apart from upgrading to a version including the fix.\n

CVSS v3 base score: 9.0, CRITICAL (CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:H)

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/xwiki/xwiki-commons
commit_details: repo_url: https://github.com/xwiki/xwiki-commons
author: Michael Hamann
author_date: 2022-11-16 18:20:53+01:00
msg: XCOMMONS-2568: Improve comment handling in HTMLCleaner (#306)
repository: repo_url: https://github.com/xwiki/xwiki-commons
repo_name: xwiki/xwiki-commons
description: Common Modules used by other XWiki top level projects
repo_language: Java
owner: xwiki
file_changes: filename: HTMLUtils.java
change_type: ModificationType.MODIFY
diff: @@ -26,6 +26,7 @@
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
+import org.jdom.Comment;
 import org.jdom.DocType;
 import org.jdom.Element;
 import org.jdom.input.DOMBuilder;
@@ -179,6 +180,22 @@ protected void printElement(Writer out, Element element, int level, NamespaceSta
                 currentFormat.setExpandEmptyElements(currentFormatPolicy);
             }
         }
+
+        @Override
+        protected void printComment(Writer out, Comment comment) throws IOException
+        {
+            String commentText = comment.getText();
+
+            // TODO: remove this again when https://sourceforge.net/p/htmlcleaner/bugs/234/ has been fixed.
+            // Make sure that the comment text conforms to the HTML specification, in particular: "Optionally, text,
+            // with the additional restriction that the text must not start with the string ">", nor start with the
+            // string "->", nor contain the strings "<!--", "-->", or "--!>", nor end with the string "<!-"."
+            while (commentText.startsWith(">") || commentText.startsWith("->")) {
+                commentText = commentText.substring(1);
+            }
+
+            super.printComment(out, new Comment(commentText));
+        }
     }
 
     /**

programming_language: Java
filename: DefaultHTMLCleaner.java
change_type: ModificationType.MODIFY
diff: @@ -267,6 +267,13 @@ private CleanerProperties getDefaultCleanerProperties(HTMLCleanerConfiguration c
 
         defaultProperties.setDeserializeEntities(true);
 
+        // Omit comments in restricted mode to avoid any potential parser confusion.
+        // Any part of the filtered HTML that contains unfiltered input is potentially dangerous/a candidate for
+        // parser confusion. Comments, style and script elements seem to be frequently found ingredients in successful
+        // attacks against good sanitizers. We're already removing style and script elements, so removing comments
+        // seems like a good defense against future attacks.
+        defaultProperties.setOmitComments(isRestricted(configuration));
+
         return defaultProperties;
     }
 
@@ -314,8 +321,7 @@ private TrimAttributeCleanerTransformations getDefaultCleanerTransformations(HTM
             defaultTransformations.addTransformation(tt);
         }
 
-        String restricted = configuration.getParameters().get(HTMLCleanerConfiguration.RESTRICTED);
-        if ("true".equalsIgnoreCase(restricted)) {
+        if (isRestricted(configuration)) {
 
             tt = new TagTransformation(HTMLConstants.TAG_SCRIPT, HTMLConstants.TAG_PRE, false);
             defaultTransformations.addTransformation(tt);
@@ -336,6 +342,16 @@ private boolean isHTML5(HTMLCleanerConfiguration configuration)
         return getHTMLVersion(configuration) == 5;
     }
 
+    /**
+     * @param configuration the configuration to parse
+     * @return if the parsing should happen in restricted mode
+     */
+    private boolean isRestricted(HTMLCleanerConfiguration configuration)
+    {
+        String restricted = configuration.getParameters().get(HTMLCleanerConfiguration.RESTRICTED);
+        return "true".equalsIgnoreCase(restricted);
+    }
+
     /**
      * @param configuration The configuration to parse.
      * @return The HTML version specified in the configuration.

programming_language: Java
filename: DefaultHTMLCleanerTest.java
change_type: ModificationType.MODIFY
diff: @@ -33,6 +33,8 @@
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
 import org.w3c.dom.Document;
 import org.w3c.dom.NodeList;
 import org.xwiki.component.manager.ComponentManager;
@@ -327,6 +329,44 @@ void restrictedAttributesAndTags() throws Exception
                 + "</mi></math>");
     }
 
+    /**
+     * Verify comment handling in restricted mode.
+     */
+    @ParameterizedTest
+    @CsvSource({
+        "<p><strong>Hello  World</strong></p>,<strong>Hello <!-- a comment --> World</strong>",
+        "'', <!--My favorite operators are > and <!-->",
+        // FIXME: Actually, just the comment should be removed but due to erroneous parsing in HTMLCleaner, the whole
+        // string is treated as a comment.
+        "'', <!--> <a href=\"#\">no comment</a>",
+        "'', <!---> <a href=\"#\">no comment</a>"
+    })
+    void restrictedComments(String expected, String actual)
+    {
+        Map<String, String> parameters = new HashMap<>(this.cleanerConfiguration.getParameters());
+        parameters.put("restricted", "true");
+        this.cleanerConfiguration.setParameters(parameters);
+
+        assertHTML(expected, actual);
+    }
+
+    @ParameterizedTest
+    @CsvSource({
+        "<!--My favorite operators are > and <!-->, <!--My favorite operators are > and <!-->",
+        "<!-- a comment ==!> not a comment-->, <!-- a comment --!> not a comment",
+        // FIXME: this is wrongly parsed as a full comment.
+        "<!-- <a foo=`bar`>not a comment</a>-->, <!--> <a foo=`bar`>not a comment</a>",
+        "<!--=>-->, <!--->",
+        // FIXME: according to the HTML specification, this should be a comment.
+        "'', <! fake comment >",
+        "<!-- <!== comment -->, <!-- <!-- comment -->",
+        "<!--My favorite operators are > and <!=-->, <!--My favorite operators are > and <!--->"
+    })
+    void comments(String expected, String actual)
+    {
+        assertHTML(expected, actual);
+    }
+
     /**
      * Verify that passing a fully-formed XHTML header works fine.
      */

programming_language: Java