CVE-2021-26701
HIGHCVE-2021-26701 is a high-severity (CVSS 8.1) remote code execution vulnerability in System.Text.Encodings.Web. O3 Security confirms whether CVE-2021-26701 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
.NET Core Remote Code Execution Vulnerability
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.
Real-World Exposure
System.Text.Encodings.Web.NETSystem.Text.Encodings.Web.NETSystem.Text.Encodings.WebReal-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
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | System.Text.Encodings.Web | ≥ 4.0.0&&< 4.5.1 | 4.5.1 |
| .NETNuGet | System.Text.Encodings.Web | ≥ 4.6.0&&< 4.7.2 | 4.7.2 |
| .NETNuGet | System.Text.Encodings.Web | ≥ 5.0.0&&< 5.0.1 | 5.0.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for System.Text.Encodings.Web. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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.
How O3 protects you
O3 pinpoints whether CVE-2021-26701 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2021-26701. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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.
Frequently Asked Questions
Is CVE-2021-26701 in your dependencies?
O3 detects CVE-2021-26701 across NuGet dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.