CVE-2021-26701: .NET Core Text Encoding Remote Code Execution Vulnerability

CVE-2021-26701 identifies a critical remote code execution vulnerability in .NET Core's text encoding implementation that enables attackers to execute arbitrary code through improper handling of text encoding operations in the System.Text.Encodings.Web package. This vulnerability affects .NET 5.0, .NET Core 3.1, and .NET Core 2.1 applications, achieving a maximum CVSS score of 9.8 (Critical severity) with an EPSS score of 79.5 percentile and 1.4% exploitation probability, indicating extremely high risk for .NET applications processing text encoding operations. The vulnerability details reveal that flawed text encoding logic in System.Text.Encodings.Web versions 4.0.0-4.5.0, 4.6.0-4.7.1, and 5.0.0 allows attackers to bypass security controls and achieve code execution without authentication or user interaction. This creates substantial exploit risk for .NET web applications, API services, and enterprise systems that perform text encoding, HTML encoding, or URL encoding operations on user-supplied data, particularly affecting applications that process untrusted input through encoding mechanisms without proper validation and sanitization controls. The technical root cause lies in .NET Core's System.Text.Encodings.Web package implementation, where the TextEncoder.Encode method and related encoding primitives contain insufficient input validation that allows malicious payloads to bypass sanitization mechanisms, creating a vector for known exploited vulnerabilities targeting .NET text processing systems. The vulnerability specifically affects how text encoding is performed across all derived encoders, where improper escaping mechanisms enable attackers to inject malicious content that executes during encoding operations, circumventing intended security boundaries. With over 31 affected packages including dotnet-host-fxr-2.1 and dotnet-hostfxr-3.1, this vulnerability demonstrates widespread exposure across .NET deployment scenarios and enterprise environments. Mitigation strategies require upgrading to patched System.Text.Encodings.Web versions 4.5.1, 4.7.2, or 5.0.1 and later, which implement proper input validation and encoding security controls to prevent code injection attacks through text processing operations. Organizations should prioritize identifying all .NET applications using vulnerable System.Text.Encodings.Web versions, audit text encoding workflows for user input processing, implement strict input validation and output encoding for all text operations, and maintain updated CVE database records to track similar encoding vulnerabilities that could compromise .NET application security through unsafe text processing and encoding manipulation attacks in web applications and data processing systems.