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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. For example, an attacker might inject a script into the a log file such that , if when the file is viewed using a web browser, the browser could provide the attacker with a copy of the operator/ administrator's cookie so that he the attacker might gain access as that user. See IDS00-J. Prevent SQL Injection for more details on input sanitizationthe administrator.

Noncompliant Code Example

This noncompliant code example logs 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);
}

...

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

...

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

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:

public 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]

 

[Seacord 2015]	Image Added IDS03-J. Do not log unsanitized user input LiveLesson

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