# CVE-2021-42321: Microsoft Exchange Server Remote Code Execution

> Attackers can execute arbitrary code on Microsoft Exchange Server, requiring immediate patching by administrators.

- **Severity:** High 8.8 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C`)
- **Exploited in the wild:** Yes, since 17 Nov 2021 (CISA KEV), used in ransomware
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 10 Nov 2021
- **Updated:** 19 Aug 2026
- **Also known as:** GHSA-3f87-66x4-f3p7

## What is CVE-2021-42321?

CVE-2021-42321 is a high-severity code injection vulnerability in Microsoft Exchange Server, published on 10 Nov 2021. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Microsoft Exchange Server Remote Code Execution Vulnerability

## Which versions are affected by CVE-2021-42321?

It affects 1 product.


Products: Microsoft Exchange Server.

## How do I fix CVE-2021-42321?

No fix has been published for CVE-2021-42321 yet. Limit exposure of the affected software until one is available. US federal agencies must fix it by 1 Dec 2021.

## Is CVE-2021-42321 being exploited?

Yes. Attackers have exploited CVE-2021-42321 in the wild since at least 17 Nov 2021, including in ransomware campaigns.

Public exploit code:

- Microsoft Exchange Server Poc (github_poc): https://github.com/DarkSprings/CVE-2021-42321
- https://github.com/7BitsTeam/exch_CVE-2021-42321 (github_poc): https://github.com/7BitsTeam/exch_CVE-2021-42321
- Microsoft Exchange Server ChainedSerializationBinder RCE (metasploit)
- Clone from gist (github_poc): https://github.com/timb-machine-mirrors/testanull-CVE-2021-42321_poc.py

## Other vulnerabilities in Microsoft Exchange Server

- [CVE-2024-21410](https://insights.sbomify.com/CVE-2024-21410/): Microsoft Exchange Server Privilege Escalation (critical 9.8, exploited in the wild)
- [CVE-2021-34473](https://insights.sbomify.com/CVE-2021-34473/): Microsoft Exchange Server Remote Code Execution (critical 9.1, exploited in the wild)
- [CVE-2021-26855](https://insights.sbomify.com/CVE-2021-26855/): Microsoft Exchange Server Remote Code Execution (critical 9.1, exploited in the wild)
- [CVE-2021-34523](https://insights.sbomify.com/CVE-2021-34523/): Microsoft Exchange Server Privilege Escalation (critical 9.0, exploited in the wild)
- [CVE-2023-21529](https://insights.sbomify.com/CVE-2023-21529/): Microsoft Exchange Server Deserialization of Untrusted Data (high 8.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-42321 and fix it if it is affected.

What it is: CVE-2021-42321 is a high-severity code injection vulnerability in Microsoft Exchange Server, published on 10 Nov 2021. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: Yes. Attackers have exploited CVE-2021-42321 in the wild since at least 17 Nov 2021, including in ransomware campaigns.

Affected product: Microsoft Exchange Server. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-42321/ (Markdown: https://insights.sbomify.com/CVE-2021-42321/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-42321
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-42321
- OSV: https://osv.dev/vulnerability/CVE-2021-42321

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-42321/
Summaries are generated and may contain mistakes; the official description above is the record.