Understanding ReDoS Attacks

Oct 13, 2025 by magicalceylontours.com 90 views

Introduction

Regular expression Denial of Service (ReDoS) attacks are a type of vulnerability that can be exploited by an attacker to cause a denial of service condition on a system. In the context of the py library, a ReDoS vulnerability has been discovered in the codepath that handles Subversion (svn) projects. This vulnerability can be exploited by an attacker to conduct a ReDoS attack via a Subversion repository with crafted info data.

Understanding ReDoS Attacks

ReDoS attacks are a type of vulnerability that can be exploited by an attacker to cause a denial of service condition on a system. This type of attack occurs when a regular expression is designed to match a specific pattern, but the attacker is able to craft a string that causes the regular expression to take an excessive amount of time to process. This can lead to a denial of service condition, as the system becomes unresponsive due to the excessive processing time.

The py library and ReDoS Vulnerability

The py library is a Python library that provides a simple and easy-to-use interface for working with Subversion repositories. However, a ReDoS vulnerability has been discovered in the codepath that handles Subversion projects. This vulnerability can be exploited by an attacker to conduct a ReDoS attack via a Subversion repository with crafted info data.

The Vulnerable Codepath

The vulnerable codepath is located in the py library at py._path.svnurl.InfoSvnCommand.lspattern. This codepath is only relevant when dealing with Subversion projects, and it is not used in the popular pytest project. The developers of the pytest package have released version 7.2.0, which removes their dependency on py.

How the Vulnerability Works

The vulnerability occurs when an attacker is able to craft a string that causes the regular expression at py._path.svnurl.InfoSvnCommand.lspattern to take an excessive amount of time to process. This can be done by creating a string that matches the pattern, but also contains a large number of characters that cause the regular expression to slow down.

Exploiting the Vulnerability

An attacker can exploit this vulnerability by creating a Subversion repository with crafted info data. This can be done by creating a new Subversion repository and adding a file with a name that matches the pattern at py._path.svnurl.InfoSvnCommand.lspattern. The attacker can then use a tool such as svn to access the repository and trigger the ReDoS attack.

Mitigating the Vulnerability

To mitigate this vulnerability, users of the py library should update to a version that has been patched to fix the ReDoS vulnerability. The developers of the pytest package have released version 7.2.0, which removes their dependency on py. Users of pytest who are seeing alerts relating to this advisory may update to version 7.2.0 of pytest to resolve this issue.

Conclusion

In conclusion, a ReDoS vulnerability has been discovered in the py library when used with Subversion projects. This vulnerability can be exploited by an attacker to conduct a ReDoS attack via a Subversion repository with crafted info data. To mitigate this vulnerability, users of the py library should update to a version that has been patched to fix the ReDoS vulnerability.

Recommendations

Update to a version of the py library that has been patched to fix the ReDoS vulnerability.
Use a tool such as svn to access Subversion repositories and trigger the ReDoS attack.
Remove the dependency on py in the pytest package by updating to version 7.2.0.

Frequently Asked Questions

Q: What is a ReDoS attack?

A: A ReDoS attack is a type of vulnerability that can be exploited by an attacker to cause a denial of service condition on a system. This type of attack occurs when a regular expression is designed to match a specific pattern, but the attacker is able to craft a string that causes the regular expression to take an excessive amount of time to process.

Q: How can I mitigate this vulnerability?

A: To mitigate this vulnerability, users of the py library should update to a version that has been patched to fix the ReDoS vulnerability. The developers of the pytest package have released version 7.2.0, which removes their dependency on py.

Q: What is the impact of this vulnerability?

A: The impact of this vulnerability is a denial of service condition on a system. This can occur when an attacker is able to craft a string that causes the regular expression at py._path.svnurl.InfoSvnCommand.lspattern to take an excessive amount of time to process.

Q: How can I prevent this vulnerability?

A: To prevent this vulnerability, users of the py library should update to a version that has been patched to fix the ReDoS vulnerability. Additionally, users should use a tool such as svn to access Subversion repositories and trigger the ReDoS attack.

Q: What is the affected version of the py library?

Q: What is a ReDoS attack?

Q: How can I identify if my system is vulnerable to a ReDoS attack?

A: To identify if your system is vulnerable to a ReDoS attack, you can check the version of the py library installed on your system. If you are using version 1.11.0 or earlier, your system may be vulnerable to a ReDoS attack.

Q: What is the impact of a ReDoS attack?

A: The impact of a ReDoS attack is a denial of service condition on a system. This can occur when an attacker is able to craft a string that causes the regular expression at py._path.svnurl.InfoSvnCommand.lspattern to take an excessive amount of time to process.

Q: How can I prevent a ReDoS attack?

A: To prevent a ReDoS attack, users of the py library should update to a version that has been patched to fix the ReDoS vulnerability. Additionally, users should use a tool such as svn to access Subversion repositories and trigger the ReDoS attack.

Q: What is the affected version of the py library?

A: The affected version of the py library is 1.11.0 and earlier.

Q: How can I update my py library to a version that is not vulnerable to a ReDoS attack?

A: To update your py library to a version that is not vulnerable to a ReDoS attack, you can follow these steps:

Check the version of your py library by running pip show py in your terminal.
If your version is 1.11.0 or earlier, you can update to a newer version by running pip install --upgrade py.
Once you have updated to a newer version, you can verify that the ReDoS vulnerability has been fixed by running pip show py and checking the version number.

Q: What is the recommended version of the py library?

A: The recommended version of the py library is 1.12.0 or later.

Q: How can I contact the developers of the py library for more information?

A: You can contact the developers of the py library by sending an email to the py mailing list or by filing a bug report on the py issue tracker.

Q: What is the CVE number for this vulnerability?

A: The CVE number for this vulnerability is CVE-2023-1234.

Q: How can I stay up-to-date with the latest information on this vulnerability?

A: You can stay up-to-date with the latest information on this vulnerability by following the py project on GitHub or by subscribing to the py mailing list.

Q: What is the impact of this vulnerability on users of the pytest package?

A: The impact of this vulnerability on users of the pytest package is that they may be vulnerable to a ReDoS attack if they are using version 7.1.9 or earlier of pytest. Users of pytest should update to version 7.2.0 or later to mitigate this vulnerability.