IDS18-J Prevent Regex Injection

Regular expressions (regexes) are commonly used to match strings of text. As such, regular expressions can be found in applications that must search through text. A notable example includes the POSIX grep utility. For example, a programmer may want this kind of functionality for searching through log files.

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, or matches occuring when not expected.

One method of preventing this vulnerability is to filter out the sensitive information prior to matching and then running the user-supplied regex against the remaining non-sensitive information. However, if the log format changes without a corresponding change in the class, sensitive information may be exposed. Furthermore, depending on how encapsulated the search keywords are, a malicious user may be able to grab a list of all the keywords. (If there are a lot of keywords, this may cause a denial of service.)

The primary means of preventing this vulnerability is to sanitize a regular expression string coming from untrusted input. Whitelisting certain characters (such as letters and digits) before passing the user supplied string to the regex parser is a common strategy. Blacklisting certain operators might be difficult due to the variability of the regex language, and consequently whitelisting is preferred over blacklisting.

Additionally, the programmer could 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 include:

• Matching flags used in non-capturing groups (These override matching options that may or may not have been passed into the compile() method.)
• greediness (The regular expression tries to match as much of the string as possible. This may expose too much information.)
• grouping (The programmer can define certain smaller parts of the regular expression to capture and return, but a malicious user may be able to make their own groupings)

For introductory information on regular expressions, see [[Tutorials 08]].

Noncompliant Code Example

This noncompliant code example searches a log file of previous searches for keywords that match a regular expression to present search suggestions to the user. The suggestSearches() method is repeatedly called to provide suggestions for the user for completion of the search text. The full log of previous searches is stored in the logBuffer object. The strings in logBuffer are periodically copied to the log object for use in searchSuggestions().

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

public final class ExploitableLog {
  private static final StringBuilder logBuffer = new StringBuilder();
  private static String log = logBuffer.toString();

  static {
    // this is supposed to come from a file, but its here as a string for
    // illustrative purposes. Each line's format is: name,id,timestamp
    append("Alice,1267773881,2147651408\n");
    append("Bono,1267774881,2147351708\n");
    append("Charles,1267775881,1175523058\n");
    append("Cecilia,1267773222,291232332\n");
  }
      
  private static void append(CharSequence str) {
    logBuffer.append(str);
    log = logBuffer.toString(); // update log string on append
  }

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

    // Construct regex from user string
    String regex = "^(" + search + ".*),[0-9]+?,[0-9]+?$";
    int flags = Pattern.MULTILINE;
    Pattern keywordPattern = Pattern.compile(regex, flags);

    // Match regex
    Matcher logMatcher = keywordPattern.matcher(log);
    while (logMatcher.find()) {
      String found = logMatcher.group(1);
      searches.add(found);
    }

    return searches;
  }
}

The regex used to search the log is:

^^(" + search + ".*),[0-9]+?,[0-9]+?$"

This regex matches against an entire line of the log and searches for old searches beginning with the entered keyword. The anchoring operators and use of the reluctance operators mitigate some greediness concerns. The grouping characters allow the program to grab only the keyword while still matching the IP and timestamp. Because the log String contains multiple lines, the MULTILINE flag must be active to force the anchoring operators to match against newlines. By all appearances, this is a strong regex.

However, this class does not sanitize the incoming regular expression, and as a result, exposes too much information from the log file to the user.

A non-malicious use of the searchSuggestions() method would be to enter "C" to match "Charles" and "Cecilia". However, a malicious user could enter

 ?:)(^.*,[0-9]+?,[0-9]+?$)|(?:

which grabs the entire log line rather than just the old keywords. The first close parentheses of the malicious search string defeats the grouping protection. Using the OR operator allows injection of any arbitrary regex. Now this regex will reveal all IPs and timestamps of past searches.

Compliant Solution

This compliant solution filters out non-alphanumeric characters from the search string using Java's Character.isLetterOrDigit(). This removes the grouping parentheses and the OR operator which triggers the injection.

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

public final class FilteredLog {
  // ...

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

    // Filter bad chars from user input
    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 from user string
    String regex = "^(" + search + ".*),[0-9]+?,[0-9]+?$";
    int flags = Pattern.MULTILINE;
    Pattern keywordPattern = Pattern.compile(regex, flags);

    // Match regex
    Matcher logMatcher = keywordPattern.matcher(log);
    while (logMatcher.find()) {
      String found = logMatcher.group(1);
      searches.add(found);
    }

    return searches;
  }
}

Risk Assessment

Violating this guideline may result in sensitive information disclosure.

References

[[Tutorials 08]] Regular Expressions
[[MITRE 09]] CWE ID 625 "Permissive Regular Expressions"
[[CVE 05]] CVE-2005-1949