CVE-2016-9910 (published 2017-02-22T16:59Z)

The serializer in html5lib before 0.99999999 might allow remote attackers to conduct cross-site scripting (XSS) attacks by leveraging mishandling of special characters in attribute values, a different vulnerability than CVE-2016-9909.

Severity: MEDIUM
CVSS v2 base score: 4.3 (AV:N/AC:M/Au:N/C:N/I:P/A:N)
CVSS v3 base score: 6.1, MEDIUM (CVSS:3.0/AV:N/AC:L/PR:N/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/html5lib/html5lib-python
commit_details: repo_url: https://github.com/html5lib/html5lib-python
author: Geoffrey Sneddon
author_date: 2013-07-19 23:15:47+01:00
msg: Fix #11, #12: quote attributes that need escaping in legacy browsers

These are mostly out of the market now, so this isn't massively
needed any more; nevertheless, avoiding XSS as much as possible is
inevitably desirable.

This alters the API so that quote_attr_values is now a ternary
setting, choosing between legacy-safe behaviour, spec behaviour, and
always quoting.
repository: repo_url: https://github.com/html5lib/html5lib-python
repo_name: html5lib/html5lib-python
description: Standards-compliant library for parsing and serializing HTML documents and fragments in Python
repo_language: Python
owner: html5lib
file_changes: filename: CHANGES.rst
change_type: ModificationType.MODIFY
diff: @@ -33,6 +33,13 @@ Released on XXX
 * **Use scripting disabled by default (as we don't implement
   scripting).**
 
+* **Fix #11, avoiding the XSS bug potentially caused by serializer
+  allowing attribute values to be escaped out of in old browser versions,
+  changing the quote_attr_values option on serializer to take one of
+  three values, "always" (the old True value), "legacy" (the new option,
+  and the new default), and "spec" (the old False value, and the old
+  default).**
+
 
 0.9999999/1.0b8
 ~~~~~~~~~~~~~~~

programming_language: Markdown
filename: htmlserializer.py
change_type: ModificationType.MODIFY
diff: @@ -10,7 +10,16 @@ from xml.sax.saxutils import escape
 
 spaceCharacters = "".join(spaceCharacters)
 
-quoteAttributeSpec = re.compile("[" + spaceCharacters + "\"'=<>`]")
+quoteAttributeSpecChars = spaceCharacters + "\"'=<>`"
+quoteAttributeSpec = re.compile("[" + quoteAttributeSpecChars + "]")
+quoteAttributeLegacy = re.compile("[" + quoteAttributeSpecChars +
+                                  "\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n"
+                                  "\x0b\x0c\r\x0e\x0f\x10\x11\x12\x13\x14\x15"
+                                  "\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f"
+                                  "\x20\x2f\x60\xa0\u1680\u180e\u180f\u2000"
+                                  "\u2001\u2002\u2003\u2004\u2005\u2006\u2007"
+                                  "\u2008\u2009\u200a\u2028\u2029\u202f\u205f"
+                                  "\u3000]")
 
 try:
     from codecs import register_error, xmlcharrefreplace_errors
@@ -72,7 +81,7 @@ else:
 class HTMLSerializer(object):
 
     # attribute quoting options
-    quote_attr_values = False
+    quote_attr_values = "legacy"  # be secure by default
     quote_char = '"'
     use_best_quote_char = True
 
@@ -108,9 +117,9 @@ class HTMLSerializer(object):
         inject_meta_charset=True|False
           Whether it insert a meta element to define the character set of the
           document.
-        quote_attr_values=True|False
+        quote_attr_values="legacy"|"spec"|"always"
           Whether to quote attribute values that don't require quoting
-          per HTML5 parsing rules.
+          per legacy browser behaviour, when required by the standard, or always.
         quote_char=u'"'|u"'"
           Use given quote character for attribute quoting. Default is to
           use double quote unless attribute value contains a double quote,
@@ -239,10 +248,15 @@ class HTMLSerializer(object):
                         (k not in booleanAttributes.get(name, tuple()) and
                          k not in booleanAttributes.get("", tuple())):
                         yield self.encodeStrict("=")
-                        if self.quote_attr_values:
+                        if self.quote_attr_values == "always" or len(v) == 0:
                             quote_attr = True
+                        elif self.quote_attr_values == "spec":
+                            quote_attr = quoteAttributeSpec.search(v) is not None
+                        elif self.quote_attr_values == "legacy":
+                            quote_attr = quoteAttributeLegacy.search(v) is not None
                         else:
-                            quote_attr = len(v) == 0 or quoteAttributeSpec.search(v)
+                            raise ValueError("quote_attr_values must be one of: "
+                                             "'always', 'spec', or 'legacy'")
                         v = v.replace("&", "&amp;")
                         if self.escape_lt_in_attrs:
                             v = v.replace("<", "&lt;")

programming_language: Python
filename: core.test
change_type: ModificationType.MODIFY
diff: @@ -242,7 +242,7 @@
         },
         {
             "expected": [
-                "<span title=foo\u000bbar>"
+                "<span title=\"foo\u000bbar\">"
             ],
             "input": [
                 [

programming_language: CoffeeScript
filename: options.test
change_type: ModificationType.MODIFY
diff: @@ -41,9 +41,9 @@
                     ]
                 ]
             ],
-            "description": "quote_attr_values=true",
+            "description": "quote_attr_values='always'",
             "options": {
-                "quote_attr_values": true
+                "quote_attr_values": "always"
             }
         },
         {
@@ -64,9 +64,78 @@
                     ]
                 ]
             ],
-            "description": "quote_attr_values=true with irrelevant",
+            "description": "quote_attr_values='always' with irrelevant",
             "options": {
-                "quote_attr_values": true
+                "quote_attr_values": "always"
+            }
+        },
+        {
+            "expected": [
+                "<div class=\"foo\">"
+            ],
+            "input": [
+                [
+                    "StartTag",
+                    "http://www.w3.org/1999/xhtml",
+                    "div",
+                    [
+                        {
+                            "namespace": null,
+                            "name": "class",
+                            "value": "foo"
+                        }
+                    ]
+                ]
+            ],
+            "description": "non-minimized quote_attr_values='always'",
+            "options": {
+                "quote_attr_values": "always"
+            }
+        },
+        {
+            "expected": [
+                "<div class=foo>"
+            ],
+            "input": [
+                [
+                    "StartTag",
+                    "http://www.w3.org/1999/xhtml",
+                    "div",
+                    [
+                        {
+                            "namespace": null,
+                            "name": "class",
+                            "value": "foo"
+                        }
+                    ]
+                ]
+            ],
+            "description": "non-minimized quote_attr_values='legacy'",
+            "options": {
+                "quote_attr_values": "legacy"
+            }
+        },
+        {
+            "expected": [
+                "<div class=foo>"
+            ],
+            "input": [
+                [
+                    "StartTag",
+                    "http://www.w3.org/1999/xhtml",
+                    "div",
+                    [
+                        {
+                            "namespace": null,
+                            "name": "class",
+                            "value": "foo"
+                        }
+                    ]
+                ]
+            ],
+            "description": "non-minimized quote_attr_values='spec'",
+            "options": {
+                "quote_attr_values": "spec"
             }
         },
         {

programming_language: Swift
filename: test_serializer.py
change_type: ModificationType.MODIFY
diff: @@ -146,6 +146,37 @@ def testComment():
     throwsWithLatin1([["Comment", "\u0101"]])
 
 
+@pytest.mark.parametrize("c", list("\t\n\u000C\x20\r\"'=<>`"))
+def testSpecQuoteAttribute(c):
+    input_ = [["StartTag", "http://www.w3.org/1999/xhtml", "span",
+               [{"namespace": None, "name": "foo", "value": c}]]]
+    if c == '"':
+        output_ = ["<span foo='%s'>" % c]
+    else:
+        output_ = ['<span foo="%s">' % c]
+    options_ = {"quote_attr_values": "spec"}
+    runSerializerTest(input_, output_, options_)
+
+
+@pytest.mark.parametrize("c", list("\t\n\u000C\x20\r\"'=<>`"
+                                   "\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n"
+                                   "\x0b\x0c\r\x0e\x0f\x10\x11\x12\x13\x14\x15"
+                                   "\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f"
+                                   "\x20\x2f\x60\xa0\u1680\u180e\u180f\u2000"
+                                   "\u2001\u2002\u2003\u2004\u2005\u2006\u2007"
+                                   "\u2008\u2009\u200a\u2028\u2029\u202f\u205f"
+                                   "\u3000"))
+def testLegacyQuoteAttribute(c):
+    input_ = [["StartTag", "http://www.w3.org/1999/xhtml", "span",
+               [{"namespace": None, "name": "foo", "value": c}]]]
+    if c == '"':
+        output_ = ["<span foo='%s'>" % c]
+    else:
+        output_ = ['<span foo="%s">' % c]
+    options_ = {"quote_attr_values": "legacy"}
+    runSerializerTest(input_, output_, options_)
+
+
 @pytest.fixture
 def lxml_parser():
     return etree.XMLParser(resolve_entities=False)

programming_language: Python