Failure to filter sensitive information when propagating exceptions often results in information leaks that can assist an attacker's efforts to develop further exploits. An attacker may craft input arguments to expose internal structures and mechanisms of the application. Both the exception message text and the type of an exception can leak information. For example, the
FileNotFoundException message reveals information about the file system layout, and the exception type reveals the absence of the requested file.
This rule applies to server-side applications as well as to clients. Attackers can glean sensitive information not only from vulnerable web servers but also from victims who use vulnerable web browsers. In 2004, Schönefeld discovered an exploit for the Opera v7.54 web browser in which an attacker could use the
sun.security.krb5.Credentials class in an applet as an oracle to "retrieve the name of the currently logged in user and parse his home directory from the information which is provided by the thrown
java.security.AccessControlException" [Schönefeld 2004].
All exceptions reveal information that can assist an attacker's efforts to carry out a denial of service (DoS) against the system. Consequently, programs must filter both exception messages and exception types that can propagate across trust boundaries. The following table lists several problematic exceptions.
Description of Information Leak or Threat
Underlying file system structure, user name enumeration
Database structure, user name enumeration
Enumeration of open ports when untrusted client can choose server port
May provide information about thread-unsafe code
Insufficient server resources (may aid DoS)
Underlying file system structure
Printing the stack trace can also result in unintentionally leaking information about the structure and state of the process to an attacker. When a Java program that is run within a console terminates because of an uncaught exception, the exception's message and stack trace are displayed on the console; the stack trace may itself contain sensitive information about the program's internal structure. Consequently, any program that may be run on a console accessible to an untrusted user must never abort due to an uncaught exception.
Noncompliant Code Example (Leaks from Exception Message and Type)
In this noncompliant code example, the program must read a file supplied by the user, but the contents and layout of the file system are sensitive. The program accepts a file name as an input argument but fails to prevent any resulting exceptions from being presented to the user.
When a requested file is absent, the
FileInputStream constructor throws a
FileNotFoundException, allowing an attacker to reconstruct the underlying file system by repeatedly passing fictitious path names to the program.
Noncompliant Code Example (Wrapping and Rethrowing Sensitive Exception)
This noncompliant code example logs the exception and then wraps it in a more general exception before rethrowing it:
Even when the logged exception is not accessible to the user, the original exception is still informative and can be used by an attacker to discover sensitive information about the file system layout.
Note that this example also violates FIO04-J. Release resources when they are no longer needed, as it fails to close the input stream in a
finally block. Subsequent code examples also omit this
finally block for brevity.
Noncompliant Code Example (Sanitized Exception)
This noncompliant code example logs the exception and throws a custom exception that does not wrap the
Although this exception is less likely than the previous noncompliant code examples to leak useful information, it still reveals that the specified file cannot be read. More specifically, the program reacts differently to nonexistent file paths than it does to valid ones, and an attacker can still infer sensitive information about the file system from this program's behavior. Failure to restrict user input leaves the system vulnerable to a brute-force attack in which the attacker discovers valid file names by issuing queries that collectively cover the space of possible file names. File names that cause the program to return the sanitized exception indicate nonexistent files, whereas file names that do not return exceptions reveal existing files.
Compliant Solution (Security Policy)
This compliant solution implements the policy that only files that live in
c:\homepath may be opened by the user and that the user is not allowed to discover anything about files outside this directory. The solution issues a terse error message when the file cannot be opened or the file does not live in the proper directory. Any information about files outside
c:\homepath is concealed.
The compliant solution also uses the
File.getCanonicalFile() method to canonicalize the file to simplify subsequent path name comparisons (see FIO16-J. Canonicalize path names before validating them for more information).
Compliant Solution (Restricted Input)
This compliant solution operates under the policy that only
c:\homepath\file2 are permitted to be opened by the user. It also catches
Throwable, as permitted by exception ERR08-J-EX2 (see ERR08-J. Do not catch NullPointerException or any of its ancestors). It uses the
MyExceptionReporter class described in ERR00-J. Do not suppress or ignore checked exceptions, which filters sensitive information from any resulting exceptions.
Compliant solutions must ensure that security exceptions such as
java.lang.SecurityException continue to be logged and sanitized appropriately (see ERR02-J. Prevent exceptions while logging data for additional information). The
MyExceptionReporter class from ERR00-J. Do not suppress or ignore checked exceptions demonstrates an acceptable approach for this logging and sanitization.
For scalability, the
switch statement should be replaced with some sort of mapping from integers to valid file names or at least an enum type representing valid files.
Exceptions may inadvertently reveal sensitive information unless care is taken to limit the information disclosure.
|Do not call the 'printStackTrace()' method of "Throwable" objects|
Catch all exceptions which may be thrown within Servlet methods
Avoid writing to Consoles
|S1989||Exceptions should not be thrown from servlet methods|
CVE-2009-2897 describes several cross-site scripting (XSS) vulnerabilities in several versions of SpringSource Hyperic HQ. These vulnerabilities allow remote attackers to inject arbitrary web script or HTML via invalid values for numerical parameters. They are demonstrated by an uncaught
java.lang.NumberFormatException exception resulting from entering several invalid numeric parameters to the web interface.
CVE-2015-2080 describes a vulnerability in the Jetty web server, versions 9.2.3 to 9.2.8, where an illegal character passed in an HTML request causes the server to respond with an error message containing the text with the illegal character. But this error message can also contain sensitive information, such as cookies from previous web requests.
9.1, Security Exceptions
|\[Gotham 2015\]||JetLeak Vulnerability: Remote Leakage Of Shared Buffers In Jetty Web Server|