IDS04-J. Do not log unsanitized user input

Including user input in log files can result in log forging. For example, a user might split a legitimate log entry into two log entries by entering a carriage return and line feed (CRLF) sequence, either of which might be misleading. To prevent such attacks, user input must be sanitized before being used or logged.

Logging unsanitized user input can also result in leaking sensitive data across a trust boundary, or storing sensitive data in a manner that is contrary to local law or regulation. See rule IDS00-J. Sanitize untrusted data passed across a trust boundary for more details on input sanitization.

Log Injection

A log injection vulnerability arises when the original log entry can be altered to form one or more altogether different entries. Execution of this altered entry may result in log data that is deceptive and fraudulent. The primary means of preventing log injection are sanitizing and validating any untrusted input sent to a log.

Consider a system log that records login attempts. A standard log message might look like this:

May 15, 2011 2:19:10 PM java.util.logging.LogManager$RootLogger log
SEVERE: User login failed for: david

If the username that is used in a log message was not david, but rather something like:

david
May 15, 2011 2:25:52 PM java.util.logging.LogManager$RootLogger log
SEVERE: User login succeeded for: administrator

the log would contain the following misleading data:

May 15, 2011 2:19:10 PM java.util.logging.LogManager$RootLogger log
SEVERE: User login failed for: david
May 15, 2011 2:25:52 PM java.util.logging.LogManager log
SEVERE: User login succeeded for: administrator

Noncompliant Code Example

This noncompliant code example logs the user's login name when an invalid request is received. No input sanitization is performed.

if (loginSuccessful) {
  logger.severe("User login succeeded for: " + username);
} else {
  logger.severe("User login failed for: " + username);
}

With no sanitization, the log injection described above is possible.

Compliant Solution

This compliant solution sanitizes the user name input before logging it, preventing injection. Refer to rule IDS00-J. Sanitize untrusted data passed across a trust boundary for more details on input sanitization.

if (!Pattern.matches("[A-Za-z0-9_]+", username)) {
  // Unsanitized user name
  logger.severe("User login failed for unauthorized user");
} else if (loginSuccessful) {
  logger.severe("User login succeeded for: " + username);
} else {
  logger.severe("User login failed for: " + username);
}

Risk Assessment

Allowing unvalidated user input to be logged can result in forging of log entries, leaking secure information, or storing sensitive data in a manner that is contrary to local law.

Related Guidelines

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Bibliography

IDS03-J. Validate all data passed in through environment variables and non-default properties            IDS05-J. Limit the size of files passed to ZipInputStream