How severe is CVE-2021-31819?

CVE-2021-31819 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-31819?

CVE-2021-31819 affects the following packages: Halibut (NuGet). Ecosystems affected: NuGet.

How do I fix CVE-2021-31819?

Update Halibut to 4.4.7 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-31819 is resolved across your whole dependency graph.

How do I detect CVE-2021-31819 in my NuGet dependencies?

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

How do I mitigate CVE-2021-31819 if there is no patch (or I can't update yet)?

Do not deserialise data from untrusted sources: where the format allows it, restrict deserialisation to an explicit allowlist of expected types, and prefer a data-only format (JSON, Protobuf) over one that can reconstruct arbitrary objects until you can upgrade.

Is CVE-2021-31819 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-31819 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-2021-31819?

CVE-2021-31819 has an EPSS (Exploit Prediction Scoring System) score of 2.4%, placing it in the 83th 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 indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-31819?

CVE-2021-31819 is classified as Deserialization of Untrusted Data (CWE-502). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2021-31819 published, and has it been updated?

CVE-2021-31819 was published on September 22, 2021 and was last updated on July 9, 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

CRITICAL severity

CVE-2021-31819 — Halibut

CRITICAL

CVE-2021-31819 is a critical-severity (CVSS 9.8) Deserialization of Untrusted Data vulnerability in Halibut. A fix is available for Halibut — see the affected versions and patch details below.

Remote Code Execution in Halibut

Also known asGHSA-hpf7-4c2g-9chf

Published

Sep 22, 2021

Updated

Jul 9, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 11, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

2.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs83th percentile — riskier than 83% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2021-31819 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

.NETHalibut

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

In Halibut versions prior to 4.4.7 there is a deserialisation vulnerability that could allow remote code execution on systems that already trust each other based on certificate verification.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
.NETNuGet	`Halibut`	all versions	4.4.7`dotnet add package Halibut --version 4.4.7`

Affected Products

1 product · 1 configurations

Application

halibutoctopus

< 4.4.7

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for Halibut, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update Halibut to 4.4.7 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-31819 is resolved across your whole dependency graph.
Workarounds
Do not deserialise data from untrusted sources: where the format allows it, restrict deserialisation to an explicit allowlist of expected types, and prefer a data-only format (JSON, Protobuf) over one that can reconstruct arbitrary objects until you can upgrade.

Frequently Asked Questions

In Halibut versions prior to 4.4.7 there is a deserialisation vulnerability that could allow remote code execution on systems that already trust each other based on certificate verification.

O3 Security · Impact-Aware SCA

Is CVE-2021-31819 in your dependencies?

Find it across NuGet, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

9.8/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

High

Raw vector

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

CWE Classification

CWE-502Deserialization of Untrusted Data

Weaponization

No known public exploit tooling found.

Timeline

Disclosed

Sep 22, 2021

Last updated

Jul 9, 2026

Identifiers

CVE-2021-31819

GHSA-hpf7-4c2g-9chf

References

Advisoryadvisories.octopus.com Advisorynvd.nist.gov Webadvisories.octopus.com Packagegithub.com

Related vulnerabilities

This CVE is critical severity; 4 of 8 other CVEs with CWE-502 are rated as high or higher (top: critical).

CVE-2025-66455CVE-2025-59953CVE-2026-49400CVE-2026-50646CVE-2026-78683CVE-2026-79657GHSA-gqvg-gmmx-x4hmCVE-2026-55175Browse all CWE-502

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedJul 9, 2026

CVE-2021-31819 — Halibut

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Frequently Asked Questions

Is CVE-2021-31819 in your dependencies?