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Java servlets often must store information associated with each client that connects to them. Using member fields in the javax.servlet.http.HttpServlet to store information specific to individual clients is a common, simple practice. However, doing so is a mistake for the following reasons:

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In any Java servlet container, such as Apache Tomcat,

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HttpServlet

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is a singleton class (see MSC07-J. Prevent multiple instantiations of singleton objects for information related to singleton classes). Therefore, there can be only one instance of member variables, even if they are not declared static.
A servlet container is permitted to invoke the servlet from multiple threads. Consequently, accessing fields in the servlet can lead to data races.
If two clients initiate sessions with the servlet, the servlet can leak information from one client to the other client.

Java servlets provide an HttpSession class for storing session-specific data, which is encoded in each web request. Use of this class prevents both cross-session information leakage and data races Consequently, any fields in a subclass are only instantiated once, just like static fields. A common mistake is to use fields in this class to store information specific to individual clients. Because this information can leak to other users, classes that inherit from HttpServlet must not contain non-static fields.

Noncompliant Code Example

This noncompliant code example creates a servlet that echos a parameter passed to it, as well as the previous parameter passed to itprompts the user for an email address, then repeats the address back to the user. The previous parameter address is stored in the last lastAddr variable, which is an instance field.

Code Block

bgColor	#ffcccc
lang	java

public class SampleServlet extends HttpServlet {

  private String lastlastAddr = "lastnobody@nowhere.com";

  public void doGet(HttpServletRequest request, HttpServletResponse response)
    throws IOException, ServletException {
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    out.println("<html>");

    String currentemailAddr = request.getParameter("currentemailAddr");

    if (currentemailAddr != null) {
      out.println("CurrentEmail ParameterAddress:");
      out.println(sanitize(currentemailAddr));
      out.println("<br>Last<br>Previous ParameterAddress:");
      out.println(sanitize(lastlastAddr));
    };

    out.println("<p>");
    out.print("<form action=\"");
    out.print("SampleServlet\" ");
    out.println("method=POST>");
    out.println("Parameter:");
    out.println("<input type=text size=20 name=current>emailAddr>");
    out.println("<br>");
    out.println("<input type=submit>");
    out.println("</form>");

    lastlastAddr = currentemailAddr;
  }

  public void doPost(HttpServletRequest request, HttpServletResponse response)
    throws IOException, ServletException {
    doGet(request, response);
  }

  // Filter the specified message string for characters
  // that are sensitive in HTML.
  public static String sanitize(String message) {
    // ...
  }
}

Because the HttpServlet class is a singleton, there is only one last lastAddr field that is shared by every client who accesses the servlet. Therefore Consequently, the contents of the last lastAddr field can be the previous setting of the field by a different client. Also, since there is no because this code example lacks thread-safety, it is possible for the last lastAddr field to take on a stale value should two clients request the parameter simultaneously, which violates VNA01-J. Ensure visibility of shared references to immutable objects.

Noncompliant Code Example

In this noncompliant code example, the last lastAddr field is static. This It more accurately reflects the fact that there is never more than a single instance of the field. This However, this code has the same behavior as the previous noncompliant code example and also violates VNA01-J. Ensure visibility of shared references to immutable objects.

Code Block

bgColor	#ffcccc
lang	java

public class SampleServlet extends HttpServlet {

  private static String lastlastAddr = "lastnobody@nowhere.com";

  // ... otherOther methods unchanged
}

...

Noncompliant Code Example

In this compliant solutionnoncompliant code example, the lastlastAddr field is static , and is protected from concurrent access by a separate lock object, as is recommended by LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code. This approach guarantees thread-safety in in the servlet. The However, the servlet can still return the last parameter entered email address provided by a different session, however.

Code Block

bgColor	#ccccff#ffcccc
lang	java

public class SampleServlet extends HttpServlet {
 
  private static String lastlastAddr = "lastnobody@nowhere.com";
  private static final Object lastLocklastAddrLock = new Object();

  public void doGet(HttpServletRequest request, HttpServletResponse response)
    throws IOException, ServletException {
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    out.println("<html>");
 
    String currentemailAddr = request.getParameter("currentemailAddr");
 
    if (currentemailAddr != null) {
      out.println("CurrentEmail ParameterAddress::");
      out.println(sanitize(currentemailAddr));
      synchronized (lock) {
        out.println("<br>Last Parameter<br>Previous Email Address::");
        out.println(sanitize(lastlastAddr));
      }
    };
 
    out.println("<p>");
    out.print("<form action=\"");
    out.print("SampleServlet\" ");
    out.println("method=POST>");
    out.println("Parameter:");
    out.println("<input type=text size=20 name=current>emailAddr>");
    out.println("<br>");
    out.println("<input type=submit>");
    out.println("</form>");
 
    synchronized (lock) {
      lastlastAddr = currentemailAddr;
    }
  }
 
  public void doPost(HttpServletRequest request, HttpServletResponse response)
    throws IOException, ServletException {
    doGet(request, response);
  }

  // Filter the specified message string for characters
  // that are sensitive in HTML.
  public static String sanitize(String message) {
    // ...
  }
}

Compliant Solution

This compliant solution stores the last lastAddr parameter in the HttpSession object, which is provided as part of the HttpServletRequest. The servlet mechanism keeps track of the session, providing the client with the session's ID, which is stored as a cookie by the client's browser. The other information in the session, including the last lastAddr attribute, are is stored by the server. Consequently, the servlet provides the last value email address that was presented to the servlet in the same session (avoiding race conditions with requests from other sessions). The local variables, which temporarily hold data in this example, are not vulnerable to race conditions in the singleton.

Code Block

bgColor	#ccccff
lang	java

public class SampleServlet extends HttpServlet {

  public void doGet(HttpServletRequest request, HttpServletResponse response)
    throws IOException, ServletException {
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    out.println("<html>");

    String currentemailAddr = request.getParameter("currentemailAddr");
    HttpSession session = request.getSession();
    Object attr = session.getAttribute("lastlastAddr");
    String lastlastAddr = (attr == null) ? "null" : attr.toString();

    if (currentemailAddr != null) {
      out.println("CurrentEmail ParameterAddress::");
      out.println(sanitize(currentemailAddr));
      out.println("<br>Last Parameter<br>Previous Email Address::");
      out.println(sanitize(lastlastAddr));
    };

    out.println("<p>");
    out.print("<form action=\"");
    out.print("SampleServlet\" ");
    out.println("method=POST>");
    out.println("Parameter:");
    out.println("<input type=text size=20 name=current>emailAddr>");
    out.println("<br>");
    out.println("<input type=submit>");
    out.println("</form>");

    session.setAttribute("lastlastAddr", currentemailAddr);
  }

  public void doPost(HttpServletRequest request, HttpServletResponse response)
    throws IOException, ServletException {
    doGet(request, response);
  }

  // Filter the specified message string for characters
  // that are sensitive in HTML.
  public static String sanitize(String message) {
    // ...
  }
}

Risk Assessment

Use of non-static nonstatic member fields in a servlet can result in information leakage.

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
MSC11-J	Medium	Likely	High	P6	L2

Automated Detection

Tool

Version

Checker

Description

Findbugs

2.0.3

MSF_MUTABLE_SERVLET_FIELD
MTIA_SUSPECT_STRUTS_INSTANCE_FIELD
MTIA_SUSPECT_SERVLET_INSTANCE_FIELD

Implemented

Fortify

6.10.0120

Singleton_Member_Field

Implemented

SonarQube

Include Page

	SonarQube_V
	SonarQube_V

S2226

Related Guidelines

MITRE CWE

CWE-543: , Use of Singleton Pattern Without Synchronization in a Multithreaded Context

Bibliography

[Apache 2015]	Apache Tomcat
[Fortify 2014]	Fortify Diagnostic	Apache Tomcat
[J2EE API 2013]	HttpServletRequest

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Noncompliant Code Example

Noncompliant Code Example

Noncompliant Code Example

Compliant Solution

Risk Assessment

Automated Detection

Related Guidelines

Bibliography