# CVE-2021-26855: 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:F/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:** 2 Mar 2021
- **Updated:** 19 Aug 2026
- **Also known as:** GHSA-6784-2mh5-cq56

## What is CVE-2021-26855?

CVE-2021-26855 is a critical server-side request forgery vulnerability in Microsoft Exchange Server, published on 2 Mar 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-2021-26855?

It affects 1 product.


Products: Microsoft Exchange Server.

## How do I fix CVE-2021-26855?

No fix has been published for CVE-2021-26855 yet. Limit exposure of the affected software until one is available. US federal agencies must fix it by 3 May 2022.

## Is CVE-2021-26855 being exploited?

Yes. Attackers have exploited CVE-2021-26855 in the wild since at least 3 Nov 2021, including in ransomware campaigns.

Public exploit code:

- C# POC for CVE-2021-26855 aka ProxyLogon, supports the classically semi-interactive web shell as well as shellcode injection  (github_poc): https://github.com/Flangvik/SharpProxyLogon
- [ProxyLogon] CVE-2021-26855 & CVE-2021-27065 Fixed RawIdentity Bug Exploit. [ProxyOracle] CVE-2021-31195 & CVE-2021-31196 Exploit Chains. [ProxyShell] CVE-2021-34473 & CVE-2021-34523 & CVE-2021-31207 Exploit Chains. (github_poc): https://github.com/hosch3n/ProxyVulns
- A fast tool to mass scan for a vulnerability on Microsoft Exchange Server that allows an attacker bypassing the authentication and impersonating as the admin (CVE-2021-26855). (github_poc): https://github.com/dwisiswant0/proxylogscan
- ProxyLogon(CVE-2021-26855+CVE-2021-27065) Exchange Server RCE(SSRF->GetWebShell) (github_poc): https://github.com/p0wershe11/ProxyLogon
- Detect webshells dropped on Microsoft Exchange servers exploited through "proxylogon" group of vulnerabilites (CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, CVE-2021-27065) (github_poc): https://github.com/cert-lv/exchange_webshell_detection
- https://github.com/h4x0r-dz/CVE-2021-26855 (github_poc): https://github.com/h4x0r-dz/CVE-2021-26855

## 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-2021-34523](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-2021-26855 and fix it if it is affected.

What it is: CVE-2021-26855 is a critical server-side request forgery vulnerability in Microsoft Exchange Server, published on 2 Mar 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: Yes. Attackers have exploited CVE-2021-26855 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-2021-26855/ (Markdown: https://insights.sbomify.com/CVE-2021-26855/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-26855
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-26855
- OSV: https://osv.dev/vulnerability/CVE-2021-26855

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-26855/
Summaries are generated and may contain mistakes; the official description above is the record.