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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 an attacker 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 misleadingto mislead an auditor. 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 an attacker might inject a script into a log file such that when the file is viewed using a web browser, 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 sanitizationbrowser could provide the attacker with a copy of the administrator's cookie so that the attacker might gain access as the 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 untrusted data from an unauthenticated user without data sanitization.

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 guest might look like this:

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

If the username that is used in a log message was is not david, guest but rather a multiline string like this:

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

...

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

Compliant Solution (Sanitized User)

This compliant solution just validates sanitizes the username input before  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 (loginSuccessful) {
  logger.severe("User login succeeded for: " + sanitizeUser(username));
} else {
  logger.severe("User login failed for: " + sanitizeUser(username));
}

The sanitization is done by a dedicated method for sanitizing user names:

ifpublic String (!sanitizeUser(String username) {
  return Pattern.matches("[A-Za-z0-9_]+", username)) {
  // Unsanitized username
  logger.severe("User login failed for ? username : "unauthorized user");
}

Compliant Solution (Sanitized Logger)

This compliant solution uses a text logger that automatically sanitizes its input. A sanitized logger saves the developer from having to worry about unsanitized log messages.

Logger sanLogger = new SanitizedTextLogger(logger);

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

The sanitized text logger takes as delegate an actual logger. We assume the logger outputs text log messages to a file, network, or the console, and each log message has no indented lines. The sanitized text logger sanitizes all text to be logged by indenting every line except the first by two spaces. While a malicious user can indent text by more, a malicious user cannot create a fake log entry because all of her output will be indented, except for the real log output.

class SanitizedTextLogger extends Logger {
  Logger delegate;

  public SanitizedTextLogger(Logger delegate) {
    super(delegate.getName(), delegate.getResourceBundleName());
    this.delegate = delegate;
  }

  public String sanitize(String msg) {
    Pattern newline = Pattern.compile("\n");
    Matcher matcher = newline.matcher(msg);
    return matcher.replaceAll("\n  ");
  }

  public void severe(String msg) {
    delegate.severe(sanitize(msg));
  }

  // .. Other Logger methods which must also sanitize their log messages
}

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.

Automated Detection

Related Guidelines

Bibliography

[API 2006]	Java Platform, Standard Edition 6 API Specification
[OWASP 2008]
[

PCI DSS Standard

Seacord 2015]

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