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Regular expressions (regex) are widely used to match strings of text. For example, the POSIX grep utility supports regular expressions for finding patterns in the specified text. For introductory information on regular expressions, see the Java Tutorials [Java Tutorials 08]. The java.util.regex package provides the Pattern class that encapsulates a compiled representation of a regular expression and the Matcher class, which is an engine that uses a Pattern to perform matching operations on a CharSequence.

Java's powerful regular expression ( regex ) facilities must be protected from misuse. An attacker may supply a malicious input that modifies the original regular expression in such a way that the regex fails to comply with the program's specification. This attack vector, called a regex injection, might affect control flow, cause information leaks, or result in denial-of-service (DoS) vulnerabilities.

Certain constructs and properties of Java regular expressions are susceptible to exploitation:

• Matching flags: Untrusted inputs may override matching options that may or may not have been passed to the Pattern.compile() method.
• Greediness: An untrusted input may attempt to inject a regex that changes the original regex to match as much of the string as possible, exposing sensitive information.
• Grouping: The programmer can enclose parts of a regular expression in parentheses to perform some common action on the group. An attacker may be able to change the groupings by supplying untrusted input.

Untrusted input should be sanitized before use to prevent regex injection. When the user must specify a regex as input, care must be taken to ensure that the original regex cannot be modified without restriction. Whitelisting characters (such as letters and digits) before delivering the user-supplied string to the regex parser is a good input sanitization strategy. A programmer must provide only a very limited subset of regular expression functionality to the user to minimize any chance of misuse.

Regex Injection Example

Suppose a system log file contains messages output by various system processes. Some processes produce public messages, and some processes produce sensitive messages marked "private." Here is an example log file:

...

However, if an attacker can substitute any string for <SEARCHTEXT>, he can perform a regex injection with the following text:

...

When injected into the regex, the regex becomes:

(.*? +public\[\d+\] +.*.*)|(.*.*)

This regex will match any line in the log file, including the private ones. 

Noncompliant Code Example

This noncompliant code example searches a log file using search terms from an untrusted user. :

import java.io.FileInputStream;
import java.io.IOException;
import java.nio.CharBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class LogSearch {
	public static void FindLogEntry(String search) {
		// Construct regex dynamically from user string
		String regex = "(.*? +public\\[\\d+\\] +.*" + search + ".*)";
		Pattern keywordPatternsearchPattern = Pattern.compile(regex);
		try (FileInputStream fis = new FileInputStream("log.txt")) {
			FileChannel channel = fis.getChannel();
			// Get the file's size and map it into memory
			long size = channel.size();
			final MappedByteBuffer mappedBuffer = channel.map(
					FileChannel.MapMode.READ_ONLY, 0, size);
			Charset charset = Charset.forName("ISO-8859-15");
			final CharsetDecoder decoder = charset.newDecoder();
			// Read file into char buffer
			CharBuffer log = decoder.decode(mappedBuffer);
			Matcher logMatcher = keywordPatternsearchPattern.matcher(log);
			while (logMatcher.find()) {
				String match = logMatcher.group();
				if (match != null!match.isEmpty()) {
				  	System.out.println(match);
				}
			}
		} catch (IOException ex) {
			System.err.println("thrown exception: " + ex.toString());
			Throwable[] suppressed = ex.getSuppressed();
			for (int i = 0; i < suppressed.length; i++) {
				System.err.println("suppressed exception: "
						+ suppressed[i].toString());
			}
		} 
		return;
	}
	public static void main(String[] args) {
		FindLogEntry(args[0]);
	}
}

This code permits an attacker to perform a regex injection.

Noncompliant Code Example

Compliant Solution (Whitelisting)

This compliant solution sanitizes the search terms at the beginning of the FindLogEntry(), filtering out nonalphanumeric characters (except space and single quote):This noncompliant code example periodically loads the log file into memory and allows clients to obtain keyword search suggestions by passing the keyword as an argument to suggestSearches().

	public class Keywords {
  private static ScheduledExecutorService scheduler
      = Executors.newSingleThreadScheduledExecutor();
  private static CharBuffer log;
  private static final Object lock = new Object();

  // Map log file into memory, and periodically reload
  static {
    try {
      FileChannel channel = new FileInputStream(
          "path").getChannel();

      // Get the file's size and map it into memory
      int size = (int) channel.size();
      final MappedByteBuffer mappedBuffer = channel.map(
          FileChannel.MapMode.READ_ONLY, 0, size);

      Charset charset = Charset.forName("ISO-8859-15");
      final CharsetDecoder decoder = charset.newDecoder();

      log = decoder.decode(mappedBuffer); // Read file into char buffer

      Runnable periodicLogRead = new Runnable() {
        @Override public void run() {
          synchronized(lock) { 
            try {
              log = decoder.decode(mappedBuffer);
            } catch (CharacterCodingException e) {
              // Forward to handler 
            } 
          }
        }
      };
      scheduler.scheduleAtFixedRate(periodicLogRead, 
                                    0, 5, TimeUnit.SECONDS);
    } catch (Throwable t) {
      // Forward to handler
    }
  }


  public static Set<String> suggestSearches(String search) {
    synchronized(lock) {
      Set<String> searches = new HashSet<String>();

      void FindLogEntry(String search) {
		// Sanitize search string
		StringBuilder sb = new StringBuilder(search.length());
		for (int i = 0; i < search.length(); ++i) {
			char ch = search.charAt(i);
			if (Character.isLetterOrDigit(ch) || ch == ' ' || ch == '\'') {
				sb.append(ch);
			}
		}
		search = sb.toString();

		// Construct regex dynamically from user string
      		String regex = "(.*? +public\\[\\d+\\] +.*" + search + ".*)";
  
      Pattern keywordPattern = Pattern.compile(regex);
      Matcher logMatcher = keywordPattern.matcher(log);
      while (logMatcher.find()) {
        String found = logMatcher.group();
        searches.add(found);
      }
      return searches;
    }  
  }

}

This code permits a trusted user to search for public log messages such as "error." However, it also allows a malicious attacker to perform the regex injection previously described.

Compliant Solution (Whitelisting)

// ...
    }

This solution prevents regex injection but also restricts search terms. For example, a user may no longer search for "name =" because nonalphanumeric characters are removed from the search term.

Compliant Solution (Pattern.quote())

This compliant solution sanitizes the search terms by using Pattern.quote() to escape any malicious characters in the search string. Unlike the previous compliant solution, a search string using punctuation characters, such as "name =" is permittedThis compliant solution filters out nonalphanumeric characters (except space and single quote) from the search string, which prevents regex injection.

	public class Keywords {
  // ...
  public static Set<String>void suggestSearchesFindLogEntry(String search) {
    synchronized (lock) {
      Set<String> searches = new HashSet<String>();

      StringBuilder sb = new StringBuilder(search.length());
      for (int i = 0; i < search.length(); ++i) {
   		// Sanitize search string
     char ch = search.charAt(i);
        if (Character.isLetterOrDigit(ch) ||
            ch == ' ' ||
            ch == '\'') {
          sb.append(ch);
        }
      }
      search = sb.toString();

       Pattern.quote(search);
		// Construct regex dynamically from user string
      		String regex = "(.*? +public\\[\\d+\\] +.*" + search + ".*)";
        // ...
    }
  }
}

The  Matcher.quoteReplacement() method can be used to escape strings used when doing regex substitutionThis solution also limits the set of valid search terms. For instance, a user may no longer search for "name =" because the = character would be sanitized out of the regex.

Compliant Solution

Another method of mitigating this vulnerability is to filter out the sensitive information prior to matching. Such a solution would require the filtering to be done every time the log file is periodically refreshed, incurring extra complexity and a performance penalty. Sensitive information may still be exposed if the log format changes but the class is not also refactored to accommodate these changes.

Risk Assessment

Failing to sanitize untrusted data included as part of a regular expression can result in the disclosure of sensitive information.

Automated Detection

Related Guidelines

Bibliography

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