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.NET
.NET NuGet
Not in CISA KEV
HIGH severity

CVE-2020-1045 — Microsoft.AspNetCore.Http

HIGH

CVE-2020-1045 is a high-severity (CVSS 7.5) vulnerability in Microsoft.AspNetCore.Http. A fix is available for Microsoft.AspNetCore.Http — see the affected versions and patch details below.

Cookie parsing failure

Also known asBIT-aspnet-core-2020-1045GHSA-hxrm-9w7p-39cc
Published
Sep 11, 2020
Updated
Jul 8, 2026
Affected
13 pkgs
Patched
13 / 13
Exploits
None indexed
Exploitation data as of Sep 18, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
  • 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-2020-1045.

EPSS Exploitation Probability

via FIRST.org ↗
5.9%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs93th percentile — riskier than 93% of all scored CVEsHighest risk

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

How urgent is this, really

CVE-2020-1045 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.

Where this sits among everything scored

Of 377,166 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.

Real-World Exposure

13 pkgs affected
.NETMicrosoft.AspNetCore.Http.NETMicrosoft.AspNetCore.App.NETMicrosoft.Owin.NETMicrosoft.AspNetCore.App.Runtime.linux-arm.NETMicrosoft.AspNetCore.App.Runtime.linux-arm64.NETMicrosoft.AspNetCore.App.Runtime.linux-musl-x64.NETMicrosoft.AspNetCore.App.Runtime.linux-x64.NETMicrosoft.AspNetCore.App.Runtime.osx-x64+5 more

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

<p>A security feature bypass vulnerability exists in the way Microsoft ASP.NET Core parses encoded cookie names.</p> <p>The ASP.NET Core cookie parser decodes entire cookie strings which could allow a malicious attacker to set a second cookie with the name being percent encoded.</p> <p>The security update addresses the vulnerability by fixing the way the ASP.NET Core cookie parser handles encoded names.</p>

Affected Packages

13 total 13 fixed
EcosystemPackageVulnerable rangeFix
.NETNuGetMicrosoft.AspNetCore.Httpall versions2.1.22dotnet add package Microsoft.AspNetCore.Http --version 2.1.22
.NETNuGetMicrosoft.AspNetCore.Appall versions2.1.22dotnet add package Microsoft.AspNetCore.App --version 2.1.22
.NETNuGetMicrosoft.Owinall versions4.1.1dotnet add package Microsoft.Owin --version 4.1.1
.NETNuGetMicrosoft.AspNetCore.App.Runtime.linux-arm≥ 3.1.0&&< 3.1.83.1.8dotnet add package Microsoft.AspNetCore.App.Runtime.linux-arm --version 3.1.8
.NETNuGetMicrosoft.AspNetCore.App.Runtime.linux-arm64≥ 3.1.0&&< 3.1.83.1.8dotnet add package Microsoft.AspNetCore.App.Runtime.linux-arm64 --version 3.1.8
.NETNuGetMicrosoft.AspNetCore.App.Runtime.linux-musl-x64≥ 3.1.0&&< 3.1.83.1.8dotnet add package Microsoft.AspNetCore.App.Runtime.linux-musl-x64 --version 3.1.8

Detection & mitigation playbook

Open-source dependency

  1. Detect

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

  2. Fix

    Update Microsoft.AspNetCore.Http to 2.1.22 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-1045 is resolved across your whole dependency graph.

  3. 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.

  4. How O3 protects you

    O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2020-1045 can be triaged on real exposure rather than presence alone.

Tailored to CVE-2020-1045. 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.

Red HatImportant

The "Cookie Prefixes" feature is not used by default in ASP.NET. Successful exploitation likely requires a secondary vulnerability, for example a cross-site scripting issue.

ProductFixed inAdvisory
.NET Core on Red Hat Enterprise Linuxrh-dotnet31-dotnet-0:3.1.108-1.el7RHSA-2020:3697
Red Hat Enterprise Linux 8dotnet3.1-0:3.1.108-2.el8_2RHSA-2020:3699

Frequently Asked Questions

<p>A security feature bypass vulnerability exists in the way Microsoft ASP.NET Core parses encoded cookie names.</p> <p>The ASP.NET Core cookie parser decodes entire cookie strings which could allow a malicious attacker to set a second cookie with the name being percent encoded.</p> <p>The security update addresses the vulnerability by fixing the way the ASP.NET Core cookie parser handles encoded names.</p>
O3 Security · Impact-Aware SCA

Is CVE-2020-1045 in your dependencies?

O3 Security finds CVE-2020-1045 across NuGet dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.

Scan my dependencies How O3 SCA works
CVE-2020-1045: Microsoft.AspNetCore.Http | O3 Security