Summary
.NET Core Remote Code Execution Vulnerability
.NET Core Remote Code Execution Vulnerability This CVE ID is unique from CVE-2021-24112.
Executive summary
Microsoft is releasing this security advisory to provide information about a vulnerability in .NET 5.0, .NET Core 3.1, and .NET Core 2.1. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability.
A remote code execution vulnerability exists in .NET 5 and .NET Core due to how text encoding is performed.
Discussion
Discussion for this issue can be found at dotnet/runtime#49377
Mitigation factors
Microsoft has not identified any mitigating factors for this vulnerability.
Affected software
The vulnerable package is System.Text.Encodings.Web . Upgrading your package and redeploying your app should be sufficient to address this vulnerability.
Vulnerable package versions:
Any .NET 5, .NET Core, or .NET Framework based application that uses the System.Text.Encodings.Web package with a vulnerable version listed below.
| Package Name | Vulnerable Versions | Secure Versions |
|---|---|---|
| System.Text.Encodings.Web | 4.0.0 - 4.5.0 | 4.5.1 |
| System.Text.Encodings.Web | 4.6.0-4.7.1 | 4.7.2 |
| System.Text.Encodings.Web | 5.0.0 | 5.0.1 |
Please validate that each of the .NET versions you are using is in support. Security updates are only provided for supported .NET versions.
Impact
CVE-2021-26701 has a CVSS score of 9.8 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.5.1, 4.7.2, 5.0.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
System.Text.Encodings.Web to 4.5.1 or later; System.Text.Encodings.Web to 4.7.2 or later; System.Text.Encodings.Web to 5.0.1 or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2021-26701? CVE-2021-26701 is a critical-severity security vulnerability in System.Text.Encodings.Web (nuget), affecting versions >= 4.0.0, < 4.5.1. It is fixed in 4.5.1, 4.7.2, 5.0.1.
- How severe is CVE-2021-26701? CVE-2021-26701 has a CVSS score of 9.8 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of System.Text.Encodings.Web are affected by CVE-2021-26701? System.Text.Encodings.Web (nuget) versions >= 4.0.0, < 4.5.1 is affected.
- Is there a fix for CVE-2021-26701? Yes. CVE-2021-26701 is fixed in 4.5.1, 4.7.2, 5.0.1. Upgrade to this version or later.
- Is CVE-2021-26701 exploitable, and should I be worried? Whether CVE-2021-26701 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2021-26701 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2021-26701?
- Upgrade
System.Text.Encodings.Webto 4.5.1 or later - Upgrade
System.Text.Encodings.Webto 4.7.2 or later - Upgrade
System.Text.Encodings.Webto 5.0.1 or later
- Upgrade