# CVE-2021-34473: Microsoft Exchange Server Remote Code Execution

> Attackers can exploit a Server-Side Request Forg flaw in Microsoft Exchange Server to achieve remote code execution.

- **Severity:** Critical 9.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:U/RL:O/RC:C`)
- **Exploited in the wild:** Yes, since 3 Nov 2021 (CISA KEV), used in ransomware
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 14 Jul 2021
- **Updated:** 10 Aug 2026
- **Also known as:** GHSA-fgq9-p33g-xcfc

## What is CVE-2021-34473?

CVE-2021-34473 is a critical server-side request forgery vulnerability in Microsoft Exchange Server, published on 14 Jul 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Microsoft Exchange Server Remote Code Execution Vulnerability

Weakness: CWE-918 (Server-Side Request Forgery)

## Which versions are affected by CVE-2021-34473?

It affects 1 product.


Products: Microsoft Exchange Server.

## How do I fix CVE-2021-34473?

No fix has been published for CVE-2021-34473 yet. Limit exposure of the affected software until one is available. US federal agencies must fix it by 17 Nov 2021.

## Is CVE-2021-34473 being exploited?

Yes. Attackers have exploited CVE-2021-34473 in the wild since at least 3 Nov 2021, including in ransomware campaigns.

Public exploit code:

- Proof of Concept for CVE-2021-34473, CVE-2021-34523, and CVE-2021-31207 (github_poc): https://github.com/horizon3ai/proxyshell
- nuclei scanner for proxyshell ( CVE-2021-34473 ) (github_poc): https://github.com/cyberheartmi9/Proxyshell-Scanner
- CVE-2021-34473 Microsoft Exchange Server Remote Code Execution Vulnerability (github_poc): https://github.com/kh4sh3i/ProxyShell
- CVE-2021-34473 Microsoft Exchange Server Remote Code Execution Vulnerability (github_poc): https://github.com/p2-98/CVE-2021-34473
- 对Exchange Proxyshell 做了二次修改，精确的拆分、实现辅助性安全测试。 (github_poc): https://github.com/je6k/CVE-2021-34473-Exchange-ProxyShell
- Scanner for CVE-2021-34473, ProxyShell, A Microsoft Exchange On-premise Vulnerability (github_poc): https://github.com/RaouzRouik/CVE-2021-34473-scanner

## 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-2021-26855](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)
- [CVE-2022-41080](https://insights.sbomify.com/CVE-2022-41080/): Microsoft Exchange Server Privilege Escalation (high 8.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-34473 and fix it if it is affected.

What it is: CVE-2021-34473 is a critical server-side request forgery vulnerability in Microsoft Exchange Server, published on 14 Jul 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: Yes. Attackers have exploited CVE-2021-34473 in the wild since at least 3 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-2021-34473/ (Markdown: https://insights.sbomify.com/CVE-2021-34473/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-34473
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-34473
- OSV: https://osv.dev/vulnerability/CVE-2021-34473

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-34473/
Summaries are generated and may contain mistakes; the official description above is the record.