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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]. 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 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

Java's regular expression facilities are wide ranging and powerful which can lead to unwanted modification of the original regular expression string to form a pattern that matches too widely, possibly resulting in far too much information being matched.

The primary means of preventing this vulnerability is to sanitize a regular expression string coming from untrusted input. Additionally, the programmer should look into ways of avoiding using regular expressions from untrusted input, or perhaps provide only a very limited subset of regular expression functionality to the user

Constructs and properties of Java regular expressions to watch out for includeare susceptible to exploitation:

• match flags used in non-capturing groups (These Matching flags: Untrusted inputs may override matching options that may or may not have been passed into to the Pattern.compile() method.
• Greediness

Since Java regular expressions are similar to Perl, it is a good idea to apply lessons learned from Perl regex.

Noncompliant Code Example

This class does not sanitize the incoming regular expression, and as a result, exposes too much information to the user.

This program searches a database of users for usernames that match a regular expression.

A non-malicious example would be to search for 'John.*'. A malicious example would be to search for '(?s)John.*'

import java.util.regex.Pattern;
import java.util.regex.Matcher;
import java.util.Set;
import java.util.HashSet;

public class ForumUserMan
{
    private final String userCSV = "JohnPaul,HearsGodsVoice\nJohnJackson,OlympicBobsleder\nJohnMayer,MakesBadMusic\n";

&nbsp;&nbsp;&nbsp; public Set<String> searchUser(String name)
&nbsp;&nbsp;&nbsp; {
        Set<String> matchedUsers = new HashSet<String>();

        String regex = name + ","; //supposedly this forces the regex to only match names
        Pattern p = Pattern.compile(regex, 0);
        Matcher m = p.matcher(userCSV);

        while (m.find())
            matchedUsers.add(m.group());

        return matchedUsers;
    }
}

When searching using the regex '(?s)John.*', the program returns all the users' passwords. The (?s) turns on single-line matching support, which means new lines are ignored.

Compliant Solution

It is very difficult to filter out overly permissive regular expressions. It might be easier and more secure to rewrite the application to limit the usage of regular expressions.

For the above code sample, the easy solution is to parse the CSV into a class and limit the regular expression over the name field of the User class.

• : 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:

10:47:03 private[423] Successful logout  name: usr1 ssn: 111223333
10:47:04 public[48964] Failed to resolve network service
10:47:04 public[1] (public.message[49367]) Exited with exit code: 255
10:47:43 private[423] Successful login  name: usr2 ssn: 444556666
10:48:08 public[48964] Backup failed with error: 19

A user wishes to search the log file for interesting messages but must be prevented from seeing the private messages. A program might accomplish this by permitting the user to provide search text that becomes part of the following regex:

(.*? +public\[\d+\] +.*<SEARCHTEXT>.*)

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.Pattern;
import java.util.regex.Matcher;
import java.util.regex.HashMapPattern;

/\*public Usageclass Test2LogSearch <regex>
&nbsp;\* Regex is used directly without santization causing sensitive data to be exposed
&nbsp;\*
&nbsp;\* Imagine this program searches a database of users for usernames that match a regex
&nbsp;\* Non malicious usage: Test1 John.\*
&nbsp;\* Malicious usage: (?s)John.\*
&nbsp;*/
public class Test2
{
&nbsp;&nbsp; &nbsp;public static class User
&nbsp;&nbsp; &nbsp;{
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;String name, password;
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;public User(String name, String password)
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;{
            setName(name);
            setPassword(password);
        }
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;private void setName(String n) { name = n; }
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;private void setPassword(String pw) { password = pw; }
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;public String getName() { return name; }
&nbsp;&nbsp; &nbsp;}

&nbsp;&nbsp; &nbsp;public static void main(String\[\] args)
&nbsp;&nbsp; &nbsp;{
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;if (args.length < 1) {
            System.err.println("Failed to specify a regex");
            return;
        }

&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;String sensitiveData; //represents sensitive data from a file or something
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;int flags;
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;String regex;
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;Pattern p;
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;Matcher m;
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;HashMap<String, User> userMap = new HashMap<String, User>();

&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;//imagine a CSV style database: user,password
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;sensitiveData = "JohnPaul,HearsGodsVoice\nJohnJackson,OlympicBobsleder\nJohnMayer,MakesBadMusic\n";
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;String\[\] csvUsers = sensitiveData.split("\n");
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;for (String csvUser : csvUsers) {
            String[] csvUserSplit = csvUser.split(",");
            String name = csvUserSplit[0];
            String pw = csvUserSplit[1];
            User u = new User(name, pw);
            userMap.put(name, u);
        }


&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;regex = args[0];
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;flags = 0;

&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;System.out.println("Pattern: \'" + regex + "\'");
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;p = Pattern.compile(regex, flags);


&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;for (String u : userMap.keySet()) {
            m = p.matcher(u);
            while (m.find())
                System.out.println("Found \'" + m.group() + "\'");
        }
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;System.err.println("DONE");
&nbsp;&nbsp; &nbsp;}
}

Risk Assessment

References

CWE ID 625 Permissive Regular Expressions

{
	public static void FindLogEntry(String search) {
		// Construct regex dynamically from user string
		String regex = "(.*? +public\\[\\d+\\] +.*" + search + ".*)";
		Pattern searchPattern = 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 = searchPattern.matcher(log);
			while (logMatcher.find()) {
				String match = logMatcher.group();
				if (!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;
	}

This code permits an attacker to perform a regex injection.  

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):

	public static 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 + ".*)";
        // ...
    }

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 permitted.

	public static void FindLogEntry(String search) {
		// Sanitize search string
        search = 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 substitution.

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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