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ReDoS: What It Is, How It Works, and How to Prevent It



How to Download ReDoS Tools and Protect Your Applications from Regular Expression Denial of Service Attacks




Regular expressions (regexes) are powerful tools for validating, parsing, and manipulating strings. However, they can also be a source of security risks if not used carefully. In this article, we will explain what ReDoS is, how it can affect your applications, and how to prevent it. We will also show you how to download ReDoS tools that can help you detect, test, and fix ReDoS vulnerabilities in your code.


What is ReDoS and Why is it Dangerous?




ReDoS Explained




ReDoS stands for Regular Expression Denial of Service. It is a type of algorithmic complexity attack that exploits the fact that some regex implementations may take a very long time to evaluate certain inputs. An attacker can craft a malicious input that causes the regex engine to enter a state of excessive backtracking, consuming a lot of CPU time and memory. This can result in a denial of service for the application or the server that uses the regex.




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Examples of ReDoS Attacks and Vulnerabilities




ReDoS attacks are not uncommon in the real world. For example, Cloudflare's firewall had a ReDoS issue in 2019, and Stack Overflow's back-end had a ReDoS issue in 2016. Here are some examples of regexes that are vulnerable to ReDoS attacks:



Regex


Description


Malicious Input


^(a+)+$


This regex matches one or more repetitions of one or more 'a' characters.


aaaaaaaaaaaaaaaaaaaaaaaaa!


([a-zA-Z]+)*


This regex matches zero or more repetitions of one or more alphabetic characters.


aaaaaaaaaaaaaaaaaaaaaaaaa1


(aaa)+


This regex matches one or more repetitions of either 'a' or 'aa'.


aaaaaaaaaaaaaaaaaaaaaaaaa!


(.*a)x


This regex matches x repetitions of any number of characters followed by 'a'.


a! (for x > 10)


In all these cases, the malicious input causes the regex engine to try many possible paths before failing to match, resulting in exponential time complexity.


How to Prevent ReDoS Attacks




Avoid Using Regex or Use Safe Regex Engines




The most foolproof way of avoiding ReDoS attacks is to avoid using regex altogether. However, this may not be feasible or desirable, as regexes are very useful for many tasks. In that case, you should use safe regex engines that do not exhibit pathological behavior. For example, you can use NFA-based engines that simulate all possible paths in parallel, or DFA-based engines that convert the regex to a deterministic automaton. Some examples of safe regex engines are RE2, Hyperscan, and Rust's regex crate.


Detect and Sanitize Evil Regexes




You can also prevent ReDoS by detecting evil regexes in your code or in user input, then sanitizing them. An evil regex is one that contains grouping with repetition, inside which there is repetition or alternation with overlapping. For example, (a+)+ is an evil regex because it contains a+ inside (a+)+. You can use tools like RXXR2 or SafeRegex to detect evil regexes and suggest safer alternatives. You can also use libraries like OWASP Java Encoder or OWASP ESAPI to sanitize user input before applying regexes to it.


Use Timeouts and Limits for Regex Matching




Another way of preventing ReDoS is to use timeouts and limits for regex matching. This means that you set a maximum amount of time or resources that the regex engine can use to evaluate an input. If the limit is exceeded, the matching is aborted and an error is returned. This way, you can avoid blocking your application or server due to a malicious input. You can use libraries like RegexTimeout or RegExLib to implement timeouts and limits for regex matching in various languages.


How to Download ReDoS Tools




Tools for Detecting ReDoS Vulnerabilities




If you want to download ReDoS tools that can help you detect ReDoS vulnerabilities in your code, you can use the following tools:



  • RXXR2: A tool that can statically analyze regexes and find out whether they are vulnerable to ReDoS attacks. It can also suggest safer alternatives for evil regexes. You can download it from .



  • SafeRegex: A tool that can check regexes for ReDoS vulnerabilities using a dynamic approach. It can also generate test cases that trigger the worst-case performance of the regex. You can download it from .



  • ReScue: A tool that can automatically patch ReDoS vulnerabilities in Java applications using a combination of static and dynamic analysis. It can also generate test cases that verify the correctness and effectiveness of the patches. You can download it from .



Tools for Testing and Benchmarking ReDoS Performance




If you want to download ReDoS tools that can help you test and benchmark the performance of your regexes, you can use the following tools:


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  • RegexBuddy: A tool that can help you create, test, debug, and optimize your regexes. It can also measure the speed and efficiency of your regexes and compare them with different engines and options. You can download it from .



  • RegexPerf: A tool that can help you measure the performance of your regexes on various inputs and engines. It can also generate graphs and reports that show the results of your tests. You can download it from .



  • RegexPal: A tool that can help you test your regexes online using a web browser. It can also highlight the matches and capture groups of your regexes on different inputs. You can access it from .



Tools for Generating Safe and Efficient Regexes




If you want to download ReDoS tools that can help you generate safe and efficient regexes, you can use the following tools:



  • Rex: A tool that can help you generate regexes from natural language specifications. It can also check the correctness and coverage of your regexes using examples and counterexamples. You can download it from .



  • Rexygen: A tool that can help you generate optimal regexes from examples. It can also rank the generated regexes based on their simplicity, readability, and performance. You can download it from .



  • Regulex: A tool that can help you generate visual diagrams of your regexes. It can also help you understand and debug your regexes by showing their structure and behavior. You can access it from .



Conclusion




In this article, we have explained what ReDoS is, how it can affect your applications, and how to prevent it. We have also shown you how to download ReDoS tools that can help you detect, test, and fix ReDoS vulnerabilities in your code. We hope that this article has been useful for you and that you have learned something new about ReDoS.


FAQs




Here are some frequently asked questions about ReDoS and ReDoS tools:


What is the difference between ReDoS and DoS?




ReDoS is a specific type of DoS (Denial of Service) attack that targets the regex engine of an application or a server. DoS is a general term that refers to any attack that aims to disrupt the normal functioning of a system or a network by overwhelming it with requests or traffic.


How can I tell if my regex is vulnerable to ReDoS?




You can use tools like RXXR2 or SafeRegex to analyze your regex and check if it is vulnerable to ReDoS. You can also use tools like RegexBuddy or RegexPerf to test your regex on different inputs and engines and measure its performance.


How can I fix a ReDoS vulnerability in my code?




You can fix a ReDoS vulnerability in your code by using one or more of the following methods:



  • Avoid using regex or use safe regex engines that do not exhibit pathological behavior.



  • Detect and sanitize evil regexes in your code or in user input.



  • Use timeouts and limits for regex matching.



  • Use tools like ReScue to automatically patch ReDoS vulnerabilities in your code.



What are some best practices for writing safe and efficient regexes?




Some best practices for writing safe and efficient regexes are:



  • Use simple and specific regexes that match only what you need.



  • Avoid using unnecessary grouping, repetition, alternation, or backreferences.



  • Use anchors, literals, character classes, and quantifiers to reduce ambiguity and backtracking.



  • Use tools like Rex or Rexygen to generate optimal regexes from natural language or examples.



  • Use tools like Regulex to visualize and understand your regexes.



Where can I learn more about ReDoS and ReDoS tools?




You can learn more about ReDoS and ReDoS tools from the following resources:



  • : A website that provides an overview of ReDoS, its causes, effects, and solutions.



  • : A wiki page that explains ReDoS, its impact, prevention, and testing methods.



  • : A GitHub repository that lists various tools for dealing with ReDoS issues.



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