When building an application that uses a client-server model, storing sensitive information at client-side may result in its disclosure if an application is vulnerable to attacks that can compromise the information. For example, consider the use of a cookie for storing sensitive information such as user credentials. Cookies are set by a web server and are stored for a certain period of time on the client-side. All subsequent requests to the domain identified by the cookie are made to contain information that was saved in the cookie. If the web application is vulnerable to a cross-site scripting (XSS) vulnerability, an attacker may be able to read any unencrypted information contained in the cookie.
A partial list of sensitive information includes user names, passwords, credit card numbers, social security numbers, and any other personally identifiable information about the user. For more details about managing passwords, see 13. Store passwords using a hash function. For more information about securing the memory that holds sensitive information, see 02. Limit the lifetime of sensitive data.
Noncompliant Code Example
In this noncompliant code example, the login servlet stores the user name and password in the cookie to identify the user for subsequent requests:
protected void doPost(HttpServletRequest request,
HttpServletResponse response) {
String username = request.getParameter("username");
char[] password = request.getParameter("password").toCharArray();
boolean rememberMe = Boolean.valueOf(request.getParameter("rememberme"));
LoginService loginService = new LoginServiceImpl();
if (rememberMe) {
if (request.getCookies()[0] != null && request.getCookies()[0].getValue() != null) {
String[] value = request.getCookies()[0].getValue().split(";");
if (!loginService.isUserValid(value[0], value[1].toCharArray())) {
// Set error and return
} else {
// Forward to welcome page
}
} else {
boolean validated = loginService.isUserValid(username, password);
if (validated) {
Cookie loginCookie = new Cookie("rememberme", username
+ ";" + new String(password));
response.addCookie(loginCookie);
// ... forward to welcome page
} else {
// Set error and return
}
}
} else { // No remember-me functionality selected
// Proceed with regular authentication, if it fails set error and return
}
Arrays.fill(password, ' ');
}
However, the attempt to implement the remember-me functionality is insecure because sensitive information should not be stored at client-side without strong encryption. This code also violates 13. Store passwords using a hash function.
Compliant Solution (Session)
This compliant solution implements the remember-me functionality by storing the user name and a secure random string in the cookie. It also maintains state in the session using HttpSession:
protected void doPost(HttpServletRequest request,
HttpServletResponse response) {
// Validate input (omitted)
String username = request.getParameter("username");
char[] password = request.getParameter("password").toCharArray();
boolean rememberMe = Boolean.valueOf(request.getParameter("rememberme"));
LoginService loginService = new LoginServiceImpl();
boolean validated = false;
if (rememberMe) {
if (request.getCookies()[0] != null
&& request.getCookies()[0].getValue() != null) {
String[] value = request.getCookies()[0].getValue().split(";");
if(value.length != 2) {
// Set error and return
}
if (!loginService.mappingExists(value[0], value[1])) { // (username, random)
// Set error and return
}
} else {
validated = loginService.isUserValid(username, password);
if (!validated) {
// Set error and return
}
}
String newRandom = loginService.getRandomString();
// Reset the random every time
loginService.mapUserForRememberMe(username, newRandom);
HttpSession session = request.getSession();
session.invalidate();
session = request.getSession(true);
// Set session timeout to one hour
session.setMaxInactiveInterval(60 * 60);
// Store user attribute and a random attribute in session scope
session.setAttribute("user", loginService.getUsername());
Cookie loginCookie = new Cookie("rememberme", username + ";"
+ newRandom);
response.addCookie(loginCookie);
// ... forward to welcome page
} else {
// ... authenticate using isUserValid() and if failed, set error
}
Arrays.fill(password, ' ');
}
The server maintains a mapping table that contains user name and secure random string pairs. When a user selects "Remember me," the doPost() method checks whether the supplied cookie contains a valid user name and random string pair. If the mapping contains a matching pair, the server authenticates the user and forwards him or her to the welcome page. If not, the server returns an error to the client. If the user selects "Remember me" but the client fails to supply a valid cookie, the server requires the user to authenticate using his or her credentials. If the authentication is successful, the server issues a new cookie with remember-me characteristics.
This solution avoids session-fixation attacks by invalidating the current session and creating a new session. It also reduces the window during which an attacker could perform a session-hijacking attack by setting the session timeout to 1.
Applicability
Violation of this rule places sensitive information within cookies, making the information vulnerable to packet sniffing or XSS attacks.
Bibliography