CVE-2023-28756 is a ReDoS (Regular Expression Denial of Service) vulnerability discovered in the Time component through version .2.1 in Ruby through version 3.2.1. This issue arises from the Time parser's mishandling of invalid URLs containing specific characters, leading to increased execution times when parsing strings into Time objects. The fixed versions addressing this vulnerability are .1.1 and .2.2.

In this long-read post, we'll delve into the details of the CVE-2023-28756 vulnerability, including examining code snippets and original references, as well as discussing how to exploit and mitigate this issue.

Overview of the Vulnerability

A ReDoS attack takes advantage of poorly crafted regular expressions, leading to increased computational resources being consumed when matching strings against these expressions. In the context of CVE-2023-28756, the Time parser in Ruby fails to handle specific characters properly, causing an increase in execution time for parsing invalid URL strings into Time objects.

To understand the vulnerability better, let's look at the problematic code within the Time component

def parse_invalid_url(input)
  time = Time.parse(input) rescue nil
  return nil if time.nil?

  time
end

When a string containing specific characters is passed to this function, the Time parser in Ruby increases the execution time due to the mishandling of the invalid URL.

Exploiting the Vulnerability

To exploit CVE-2023-28756, an attacker could craft a specific invalid URL string and pass it as input to the Time parser, causing an undue surge in execution time for parsing the string into a Time object. This could, in turn, lead to a denial of service due to resource exhaustion.

Below is an example of an exploit payload that triggers the vulnerability

payload = "2022-02-30T12:30:00.999999999999999Z"

When this payload is passed into the parse_invalid_url function, it causes the Time parser to consume excessive resources, leading to a denial of service.

Mitigating the Vulnerability

This vulnerability has been addressed in the fixed versions .1.1 and .2.2. To mitigate CVE-2023-28756, users should update their Ruby application to either of these patched versions. If upgrading is not possible, users should perform additional input validation to filter out invalid URL strings, reducing the likelihood of a ReDoS attack.

References

1. Original CVE-2023-28756 details
2. Ruby's Time component GitHub repository
3. Fixed version .1.1 Release Notes
4. Fixed version .2.2 Release Notes

Conclusion

CVE-2023-28756 is a ReDoS vulnerability that affects Ruby's Time component, leading to increased execution times when parsing specific invalid URL strings into Time objects. The issue arises from the Time parser's mishandling of certain characters in the input string. Upgrading to fixed versions .1.1 or .2.2 effectively mitigates the vulnerability, while additional input validation can be used as an alternative means of protection.

Timeline

Published on: 03/31/2023 04:15:00 UTC
Last modified on: 04/30/2023 23:15:00 UTC