A log injection vulnerability arises when a log entry contains unsanitized user input. A malicious user can insert fake log data and consequently deceive system administrators as to the system's behavior [OWASP 2008]. 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. Log injection attacks can be prevented by sanitizing and validating any untrusted input sent to a log.

Logging unsanitized user input can also result in leaking sensitive data across a trust boundary, or storing sensitive data in a manner that violates local law or regulation. For example, if a user can inject an unencrypted credit card number into a log file, the system could violate PCI DSS regulations [PCI 2010]. See rule IDS00-J. Sanitize untrusted data passed across a trust boundary for more details on input sanitization.

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);
}

Without sanitization, a log injection attack is possible. A standard log message when username is david 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 a multiline string like this:

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

Compliant Solution

This compliant solution just validates the username input before logging it, preventing injection attacks. 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 username
  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 violates a local law or regulation.

Related Guidelines

Bibliography

IDS02-J. Canonicalize path names before validating them            IDS04-J. Limit the size of files passed to ZipInputStream