CVE-2021-27293 in RestSharp
Summary
by MITRE • 07/12/2021
RestSharp < 106.11.8-alpha.0.13 uses a regular expression which is vulnerable to Regular Expression Denial of Service (ReDoS) when converting strings into DateTimes. If a server responds with a malicious string, the client using RestSharp will be stuck processing it for an exceedingly long time. Thus the remote server can trigger Denial of Service.
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Analysis
by VulDB Data Team • 07/15/2021
The vulnerability identified as CVE-2021-27293 affects RestSharp versions prior to 106.11.8-alpha.0.13 and represents a critical security flaw categorized under CWE-400, which specifically addresses Uncontrolled Resource Consumption. This issue manifests as a Regular Expression Denial of Service (ReDoS) vulnerability that occurs during the conversion of string representations into DateTime objects within the HTTP client library. The flaw exploits the use of a vulnerable regular expression pattern that exhibits exponential backtracking behavior when processing maliciously crafted input strings, causing the application to consume excessive computational resources and potentially leading to complete system unresponsiveness.
The technical implementation of this vulnerability resides in RestSharp's internal string parsing mechanisms that attempt to convert HTTP response data into structured DateTime objects. When a remote server responds with a specially crafted string that matches the vulnerable regular expression pattern, the parsing algorithm enters a state of exponential time complexity where each additional character in the malicious input dramatically increases the processing time required to validate the string. This behavior creates a denial of service condition where legitimate requests cannot be processed because the client application becomes trapped in the resource-intensive regular expression evaluation loop. The vulnerability specifically impacts applications that rely on RestSharp for HTTP communication and automatic date parsing, making it particularly dangerous in environments where automated processing of external data is common.
From an operational perspective, this vulnerability presents significant risks to systems that depend on RestSharp for web service communication, particularly in scenarios involving external API interactions, data processing pipelines, or automated service integrations. Attackers can exploit this weakness by crafting HTTP responses containing maliciously formatted date strings that trigger the vulnerable parsing logic, causing the affected client applications to become unresponsive for extended periods. The impact extends beyond simple service disruption to potentially affect entire application availability, especially in high-throughput environments where multiple concurrent requests could be simultaneously affected. This vulnerability also aligns with ATT&CK technique T1499.004, which covers Network Denial of Service through resource exhaustion, making it a particularly concerning threat in distributed systems where client applications may be exposed to untrusted network inputs.
Organizations should prioritize immediate patching of RestSharp libraries to versions 106.11.8-alpha.0.13 or later, which contain the fixed regular expression patterns that eliminate the exponential backtracking behavior. Additional mitigations include implementing input validation layers that sanitize date string inputs before they reach the RestSharp parsing logic, establishing timeout mechanisms for HTTP request processing to prevent indefinite hanging, and monitoring network traffic for suspicious patterns that might indicate exploitation attempts. Security teams should also consider implementing network segmentation and access controls to limit exposure of systems that utilize RestSharp, particularly in environments where external services cannot be fully trusted. The vulnerability demonstrates the importance of regular security audits of third-party libraries and the necessity of maintaining up-to-date dependencies to prevent exploitation of known weaknesses in commonly used software components.