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CWE-537: Information Exposure Through Java Runtime Error Message
 DescriptionDescription Summary In many cases, an attacker can leverage the conditions that cause unhandled exception errors in order to gain unauthorized access to the system.
Time of Introduction
Applicable PlatformsLanguages Java Common Consequences
Demonstrative ExamplesExample 1 In the following Java example the class InputFileRead enables an input file to be read using a FileReader object. In the constructor of this class a default input file path is set to some directory on the local file system and the method setInputFile must be called to set the name of the input file to be read in the default directory. The method readInputFile will create the FileReader object and will read the contents of the file. If the method setInputFile is not called prior to calling the method readInputFile then the File object will remain null when initializing the FileReader object. A Java RuntimeException will be raised, and an error message will be output to the user. (Bad Code) Example
Language: Java public class InputFileRead { private File readFile = null;
private FileReader reader = null;
private String inputFilePath = null;
private final String DEFAULT_FILE_PATH =
"c:\\somedirectory\\";
public InputFileRead() {
inputFilePath = DEFAULT_FILE_PATH;
}
public void setInputFile(String inputFile) {
/* Assume appropriate validation / encoding is used and
privileges / permissions are preserved */
}
public void readInputFile() {
try {
reader = new FileReader(readFile);
...
} catch (RuntimeException rex) {
System.err.println("Error: Cannot open input file in
the directory " + inputFilePath);
System.err.println("Input file has not been set, call
setInputFile method before calling
readInputFile");
} catch (FileNotFoundException ex) {...}
}
} However, the error message output to the user contains information regarding the default directory on the local file system. This information can be exploited and may lead to unauthorized access or use of the system. Any Java RuntimeExceptions that are handled should not expose sensitive information to the user. Example 2 In the example below, the BankManagerLoginServlet servlet class will process a login request to determine if a user is authorized to use the BankManager Web service. The doPost method will retrieve the username and password from the servlet request and will determine if the user is authorized. If the user is authorized the servlet will go to the successful login page. Otherwise, the servlet will raise a FailedLoginException and output the failed login message to the error page of the service. (Bad Code) Example
Language: Java public class BankManagerLoginServlet extends HttpServlet { protected void doPost(HttpServletRequest request,
HttpServletResponse response) throws ServletException,
IOException {
try {
// Get username and password from login page
request
String username = request.getParameter("username");
String password = request.getParameter("password");
// Authenticate user
BankManager bankMgr = new BankManager();
boolean isAuthentic =
bankMgr.authenticateUser(username, password);
// If user is authenticated then go to successful
login page
if (isAuthentic) {
request.setAttribute("login", new String("Login
Successful."));
getServletContext().getRequestDispatcher("/BankManagerServiceLoggedIn.jsp").
forward(request, response);
}
else {
// Otherwise, raise failed login exception and
output unsuccessful login message to error page
throw new FailedLoginException("Failed Login for
user " + username + " with password " +
password);
}
} catch (FailedLoginException ex) {
// output failed login message to error page
request.setAttribute("error", new String("Login
Error"));
request.setAttribute("message",
ex.getMessage());
getServletContext().getRequestDispatcher("/ErrorPage.jsp").forward(request,
response);
}
}
However, the output message generated by the FailedLoginException includes the user-supplied password. Even if the password is erroneous, it is probably close to the correct password. Since it is printed to the user's page, anybody who can see the screen display will be able to see the password. Also, if the page is cached, the password might be written to disk. Potential Mitigations
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Page Last Updated:
September 12, 2011
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