CVE-2021-27293: RestSharp ReDoS Denial of Service Vulnerability

CVE-2021-27293 Overview

CVE-2021-27293 is a Regular Expression Denial of Service (ReDoS) vulnerability affecting RestSharp, a popular .NET HTTP client library. The vulnerability exists in versions prior to 106.11.8-alpha.0.13 where a poorly constructed regular expression is used when converting strings into DateTimes. When a malicious server responds with a specially crafted string, the RestSharp client becomes stuck processing the input for an extended period, effectively causing a Denial of Service condition.

Critical Impact

Remote attackers can exploit this vulnerability to cause client-side Denial of Service by sending malicious responses that trigger catastrophic backtracking in the regex engine, rendering applications unresponsive.

Affected Products

• RestSharp versions prior to 106.11.8-alpha.0.13
• RestSharp 106.11.8-alpha0.2 through 106.11.8-alpha0.12
• All RestSharp installations using vulnerable DateTime parsing functionality

Discovery Timeline

• 2021-07-12 - CVE-2021-27293 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-27293

Vulnerability Analysis

This vulnerability is classified under CWE-697 (Incorrect Comparison), which in this context manifests as an algorithmic complexity attack through Regular Expression Denial of Service. The flaw resides in the DateTime string parsing functionality of RestSharp, where a vulnerable regular expression pattern exhibits catastrophic backtracking behavior when processing maliciously crafted input strings.

ReDoS vulnerabilities occur when regular expressions with specific patterns (often involving nested quantifiers or overlapping alternatives) are fed input that causes the regex engine to explore an exponentially growing number of possible matches. This transforms what should be a simple string parsing operation into a computationally expensive process that can consume CPU resources for extended periods.

The attack is particularly concerning because it is remotely triggerable - a malicious server can craft responses that exploit this vulnerability, causing any client application using the affected RestSharp versions to hang or become unresponsive.

Root Cause

The root cause lies in the implementation of the regular expression used for DateTime string conversion within RestSharp. The regex pattern contains constructs that lead to exponential time complexity when processing certain input patterns. When the regex engine encounters a string that partially matches multiple potential paths, it must backtrack and try alternative matches, leading to what is known as "catastrophic backtracking."

This architectural flaw in the regex pattern design allows external input to directly influence application responsiveness, violating the principle of bounded resource consumption.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker controlling a server that a RestSharp client connects to can craft malicious HTTP responses containing strings designed to trigger the ReDoS condition. When the RestSharp library attempts to parse these responses and convert date strings, the vulnerable regex causes the application to hang.

The attack scenario involves:

1. A target application using a vulnerable version of RestSharp makes HTTP requests to an attacker-controlled or compromised server
2. The malicious server responds with carefully crafted strings in fields that RestSharp attempts to parse as DateTime values
3. The vulnerable regex processes the malicious input, triggering catastrophic backtracking
4. The client application becomes unresponsive until the regex engine eventually completes or times out

This vulnerability demonstrates the risks of using unbounded regex patterns when processing untrusted external data. Technical details and the original vulnerability report can be found in the GitHub Issue #1556.

Detection Methods for CVE-2021-27293

Indicators of Compromise

• Unusually high CPU utilization in processes using RestSharp library
• Application threads stuck in regex processing operations
• Timeouts or hangs when processing HTTP responses from external servers
• Memory profile showing extended regex backtracking operations

Detection Strategies

• Implement dependency scanning to identify RestSharp versions below 106.11.8-alpha.0.13
• Monitor application performance metrics for anomalous CPU spikes during HTTP response processing
• Utilize Software Composition Analysis (SCA) tools to track vulnerable library versions
• Review application logs for timeout exceptions related to DateTime parsing

Monitoring Recommendations

• Enable detailed logging for RestSharp HTTP operations to identify slow response processing
• Set up alerting for process CPU utilization thresholds that may indicate ReDoS exploitation
• Monitor response times for API calls made using RestSharp clients
• Implement application performance monitoring (APM) to detect regex-related performance degradation

How to Mitigate CVE-2021-27293

Immediate Actions Required

• Upgrade RestSharp to version 106.11.8-alpha.0.13 or later immediately
• Audit all applications and services that depend on RestSharp for vulnerable versions
• Implement request timeouts in applications to limit the impact of potential ReDoS attacks
• Review and update dependency management policies to prevent deployment of vulnerable versions

Patch Information

The vulnerability has been addressed in RestSharp version 106.11.8-alpha.0.13 and later releases. The fix involves replacing the vulnerable regex pattern with a safer implementation that does not exhibit catastrophic backtracking behavior. Organizations should update to the latest stable release of RestSharp to ensure they receive this fix along with any subsequent security improvements.

For detailed patch information and release notes, refer to the RestSharp Official Website and the GitHub Issue #1556 which tracks this vulnerability.

Workarounds

• Implement connection timeouts at the HTTP client level to limit execution time for any single request
• Add application-level timeout wrappers around RestSharp API calls to prevent indefinite hangs
• Consider input validation or sanitization for responses from untrusted external servers
• Deploy network-level protections to limit exposure to potentially malicious external endpoints

# Example: Update RestSharp via NuGet Package Manager
dotnet add package RestSharp --version 110.0.0
# Or update via Package Manager Console
Install-Package RestSharp -Version 110.0.0