How severe is CVE-2021-26701?

CVE-2021-26701 has a CVSS score of 8.1/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-26701?

CVE-2021-26701 affects the following packages: System.Text.Encodings.Web (NuGet), System.Text.Encodings.Web (NuGet), System.Text.Encodings.Web (NuGet). Ecosystems affected: NuGet.

How do I fix CVE-2021-26701?

Update System.Text.Encodings.Web to 4.5.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-26701 is resolved across your whole dependency graph.

How do I detect CVE-2021-26701 in my NuGet dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for System.Text.Encodings.Web, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-26701 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2021-26701 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-26701 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-26701?

CVE-2021-26701 has an EPSS (Exploit Prediction Scoring System) score of 30.1%, placing it in the 98th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score warrants monitoring and prompt remediation.

When was CVE-2021-26701 published, and has it been updated?

CVE-2021-26701 was published on February 25, 2021 and was last updated on September 22, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

.NET

.NET NuGet

Not in CISA KEV

HIGH severity

CVE-2021-26701 — System.Text.Encodings.Web

HIGH

CVE-2021-26701 is a high-severity (CVSS 8.1) remote code execution vulnerability in System.Text.Encodings.Web. EPSS puts its 30-day exploitation probability at 30.1% (98th percentile). A fix is available for System.Text.Encodings.Web — see the affected versions and patch details below.

.NET Core Remote Code Execution Vulnerability

Also known asBIT-dotnet-2021-26701BIT-dotnet-sdk-2021-26701GHSA-ghhp-997w-qr28

Published

Feb 25, 2021

Updated

Sep 22, 2026

Affected

3 pkgs

Patched

3 / 3

Exploits

None indexed

Exploitation data as of Oct 12, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

A successful exploit gives an attacker total control of the affected component, not partial access.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2021-26701.

EPSS Exploitation Probability

via FIRST.org ↗

30.1%probability of exploitation in next 30 days

Moderate Risk0.00%

Lower risk than most CVEs98th percentile — riskier than 98% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-26701 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 386,684 CVEs with a current EPSS score, this one falls in the 10–50% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

3 pkgs affected

.NETSystem.Text.Encodings.Web.NETSystem.Text.Encodings.Web.NETSystem.Text.Encodings.Web

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.

Description

.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.

Affected Packages

3 total 3 fixed

Ecosystem	Package	Vulnerable range	Fix
.NETNuGet	`System.Text.Encodings.Web`	≥ 4.0.0&&< 4.5.1	4.5.1`dotnet add package System.Text.Encodings.Web --version 4.5.1`
.NETNuGet	`System.Text.Encodings.Web`	≥ 4.6.0&&< 4.7.2	4.7.2`dotnet add package System.Text.Encodings.Web --version 4.7.2`
.NETNuGet	`System.Text.Encodings.Web`	≥ 5.0.0&&< 5.0.1	5.0.1`dotnet add package System.Text.Encodings.Web --version 5.0.1`

Affected Products

5 products · 10 configurations

fedorafedoraproject

3 versions

323334

Application

.netmicrosoft

≥ 5.0 && < 5.0.4

range

Application

.net coremicrosoft

≥ 3.1 && < 3.1.15

range

Application

powershell coremicrosoft

2 versions

7.07.1

Application

visual studio 2019microsoft

≥ 16.0 && ≤ 16.9

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for System.Text.Encodings.Web, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update System.Text.Encodings.Web to 4.5.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-26701 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

Product	Fixed in	Advisory
.NET Core on Red Hat Enterprise Linux	`rh-dotnet21-0:2.1-25.el7_9`	RHSA-2021:0787
.NET Core on Red Hat Enterprise Linux	`rh-dotnet31-dotnet-0:3.1.113-1.el7_9`	RHSA-2021:0789
.NET Core on Red Hat Enterprise Linux	`rh-dotnet50-dotnet-0:5.0.104-1.el7_9`	RHSA-2021:0794
Red Hat Enterprise Linux 8	`dotnet-0:2.1.522-1.el8_3`	RHSA-2021:0788
Red Hat Enterprise Linux 8	`dotnet3.1-0:3.1.113-1.el8_3`	RHSA-2021:0790
Red Hat Enterprise Linux 8	`dotnet5.0-0:5.0.104-1.el8_3`	RHSA-2021:0793

Frequently Asked Questions

.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

O3 Security · Impact-Aware SCA

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CVSS Score

8.1/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

High

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

High

Raw vector

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Weaponization

No known public exploit tooling found.

Timeline

Disclosed

Feb 25, 2021

Last updated

Sep 22, 2026

Identifiers

CVE-2021-26701

GHSA-ghhp-997w-qr28

BIT-dotnet-2021-26701

BIT-dotnet-sdk-2021-26701

Related vulnerabilities

This CVE is high severity; 2 of 4 other CVEs affecting System.Text.Encodings.Web are rated as high or higher (top: high).

CVE-2017-0247CVE-2017-0248CVE-2017-0256CVE-2017-0249

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedSep 22, 2026

CVE-2021-26701 — System.Text.Encodings.Web

Exploitation Status

No confirmed exploitation observed yet

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Executive summary

Discussion

Mitigation factors

Affected software

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Fixing This On Your OS

Frequently Asked Questions

Is CVE-2021-26701 in your dependencies?