CVE-2021-4231 (published 2022-05-26T14:15Z)

A vulnerability was found in Angular up to 11.0.4/11.1.0-next.2. It has been classified as problematic. Affected is the handling of comments. The manipulation leads to cross site scripting. It is possible to launch the attack remotely but it might require an authentication first. Upgrading to version 11.0.5 and 11.1.0-next.3 is able to address this issue. The name of the patch is ba8da742e3b243e8f43d4c63aa842b44e14f2b09. It is recommended to upgrade the affected component.

Severity: LOW
CVSS v2 base score: 3.5 (AV:N/AC:M/Au:S/C:N/I:P/A:N)
CVSS v3 base score: 5.4, MEDIUM (CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N)

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/angular/angular
commit_details: repo_url: https://github.com/angular/angular
author: Misko Hevery
author_date: 2021-01-22 10:05:17-08:00
msg: fix(core): fix possible XSS attack in development through SSR (#40525)

This is a follow up fix for
https://github.com/angular/angular/pull/40136/commits/894286dd0c92b5af223364237e63798e18b14f58.

It turns out that comments can be closed in several ways:
- `<!-->`
- `<!-- -->`
- `<!-- --!>`

All of the above are valid ways to close comment per:
https://html.spec.whatwg.org/multipage/syntax.html#comments

The new fix surrounds `<` and `>` with zero width space so that it
renders in the same way, but it prevents the comment to be closed eagerly.

PR Close #40525
repository: repo_url: https://github.com/angular/angular
repo_name: angular/angular
description: Deliver web apps with confidence 🚀
repo_language: TypeScript
owner: angular
file_changes: filename: dom.ts
change_type: ModificationType.MODIFY
diff: @@ -6,15 +6,27 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-const END_COMMENT = /-->/g;
-const END_COMMENT_ESCAPED = '-\u200B-\u200B>';
+/**
+ * Disallowed strings in the comment.
+ *
+ * see: https://html.spec.whatwg.org/multipage/syntax.html#comments
+ */
+const COMMENT_DISALLOWED = /^>|^->|<!--|-->|--!>|<!-$/g;
+/**
+ * Delimiter in the disallowed strings which needs to be wrapped with zero with character.
+ */
+const COMMENT_DELIMITER = /(<|>)/;
+const COMMENT_DELIMITER_ESCAPED = '\u200B$1\u200B';
 
 /**
- * Escape the content of the strings so that it can be safely inserted into a comment node.
+ * Escape the content of comment strings so that it can be safely inserted into a comment node.
  *
  * The issue is that HTML does not specify any way to escape comment end text inside the comment.
- * `<!-- The way you close a comment is with "-->". -->`. Above the `"-->"` is meant to be text not
- * an end to the comment. This can be created programmatically through DOM APIs.
+ * Consider: `<!-- The way you close a comment is with ">", and "->" at the beginning or by "-->" or
+ * "--!>" at the end. -->`. Above the `"-->"` is meant to be text not an end to the comment. This
+ * can be created programmatically through DOM APIs. (`<!--` are also disallowed.)
+ *
+ * see: https://html.spec.whatwg.org/multipage/syntax.html#comments
  *
  * ```
  * div.innerHTML = div.innerHTML
@@ -25,13 +37,15 @@ const END_COMMENT_ESCAPED = '-\u200B-\u200B>';
  * opening up the application for XSS attack. (In SSR we programmatically create comment nodes which
  * may contain such text and expect them to be safe.)
  *
- * This function escapes the comment text by looking for the closing char sequence `-->` and replace
- * it with `-_-_>` where the `_` is a zero width space `\u200B`. The result is that if a comment
- * contains `-->` text it will render normally but it will not cause the HTML parser to close the
- * comment.
+ * This function escapes the comment text by looking for comment delimiters (`<` and `>`) and
+ * surrounding them with `_>_` where the `_` is a zero width space `\u200B`. The result is that if a
+ * comment contains any of the comment start/end delimiters (such as `<!--`, `-->` or `--!>`) the
+ * text it will render normally but it will not cause the HTML parser to close/open the comment.
  *
- * @param value text to make safe for comment node by escaping the comment close character sequence
+ * @param value text to make safe for comment node by escaping the comment open/close character
+ *     sequence.
  */
 export function escapeCommentText(value: string): string {
-  return value.replace(END_COMMENT, END_COMMENT_ESCAPED);
+  return value.replace(
+      COMMENT_DISALLOWED, (text) => text.replace(COMMENT_DELIMITER, COMMENT_DELIMITER_ESCAPED));
 }
\ No newline at end of file

programming_language: TypeScript
filename: security_spec.ts
change_type: ModificationType.MODIFY
diff: @@ -11,24 +11,33 @@ import {TestBed} from '@angular/core/testing';
 
 
 describe('comment node text escaping', () => {
-  it('should not be possible to do XSS through comment reflect data', () => {
-    @Component({template: `<div><span *ngIf="xssValue"></span><div>`})
-    class XSSComp {
-      xssValue: string = '--> --><script>"evil"</script>';
-    }
+  // see: https://html.spec.whatwg.org/multipage/syntax.html#comments
+  ['>',         // self closing
+   '-->',       // standard closing
+   '--!>',      // alternate closing
+   '<!-- -->',  // embedded comment.
+  ].forEach((xssValue) => {
+    it('should not be possible to do XSS through comment reflect data when writing: ' + xssValue,
+       () => {
+         @Component({template: `<div><span *ngIf="xssValue"></span><div>`})
+         class XSSComp {
+           // ngIf serializes the `xssValue` into a comment for debugging purposes.
+           xssValue: string = xssValue + '<script>"evil"</script>';
+         }
 
-    TestBed.configureTestingModule({declarations: [XSSComp]});
-    const fixture = TestBed.createComponent(XSSComp);
-    fixture.detectChanges();
-    const div = fixture.nativeElement.querySelector('div') as HTMLElement;
-    // Serialize into a string to mimic SSR serialization.
-    const html = div.innerHTML;
-    // This must be escaped or we have XSS.
-    expect(html).not.toContain('--><script');
-    // Now parse it back into DOM (from string)
-    div.innerHTML = html;
-    // Verify that we did not accidentally deserialize the `<script>`
-    const script = div.querySelector('script');
-    expect(script).toBeFalsy();
+         TestBed.configureTestingModule({declarations: [XSSComp]});
+         const fixture = TestBed.createComponent(XSSComp);
+         fixture.detectChanges();
+         const div = fixture.nativeElement.querySelector('div') as HTMLElement;
+         // Serialize into a string to mimic SSR serialization.
+         const html = div.innerHTML;
+         // This must be escaped or we have XSS.
+         expect(html).not.toContain('--><script');
+         // Now parse it back into DOM (from string)
+         div.innerHTML = html;
+         // Verify that we did not accidentally deserialize the `<script>`
+         const script = div.querySelector('script');
+         expect(script).toBeFalsy();
+       });
   });
 });
\ No newline at end of file

programming_language: TypeScript
filename: dom_spec.ts
change_type: ModificationType.MODIFY
diff: @@ -14,13 +14,35 @@ describe('comment node text escaping', () => {
       expect(escapeCommentText('text')).toEqual('text');
     });
 
+    it('should escape "<" or ">"', () => {
+      expect(escapeCommentText('<!--')).toEqual('\u200b<\u200b!--');
+      expect(escapeCommentText('<!--<!--')).toEqual('\u200b<\u200b!--\u200b<\u200b!--');
+      expect(escapeCommentText('>')).toEqual('\u200b>\u200b');
+      expect(escapeCommentText('>-->')).toEqual('\u200b>\u200b--\u200b>\u200b');
+    });
+
     it('should escape end marker', () => {
-      expect(escapeCommentText('before-->after')).toEqual('before-\u200b-\u200b>after');
+      expect(escapeCommentText('before-->after')).toEqual('before--\u200b>\u200bafter');
     });
 
     it('should escape multiple markers', () => {
       expect(escapeCommentText('before-->inline-->after'))
-          .toEqual('before-\u200b-\u200b>inline-\u200b-\u200b>after');
+          .toEqual('before--\u200b>\u200binline--\u200b>\u200bafter');
+    });
+
+    it('should caver the spec', () => {
+      // https://html.spec.whatwg.org/multipage/syntax.html#comments
+      expect(escapeCommentText('>')).toEqual('\u200b>\u200b');
+      expect(escapeCommentText('->')).toEqual('-\u200b>\u200b');
+      expect(escapeCommentText('<!--')).toEqual('\u200b<\u200b!--');
+      expect(escapeCommentText('-->')).toEqual('--\u200b>\u200b');
+      expect(escapeCommentText('--!>')).toEqual('--!\u200b>\u200b');
+      expect(escapeCommentText('<!-')).toEqual('\u200b<\u200b!-');
+
+      // Things which are OK
+      expect(escapeCommentText('.>')).toEqual('.>');
+      expect(escapeCommentText('.->')).toEqual('.->');
+      expect(escapeCommentText('<!-.')).toEqual('<!-.');
     });
   });
 });

programming_language: TypeScript