# CVE-2021-33766: Microsoft Exchange Server Information Disclosure

> An attacker can exploit this vulnerability in Microsoft Exchange Server to disclose sensitive information.

- **Severity:** High 7.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L/E:U/RL:O/RC:C`)
- **Exploited in the wild:** Yes, since 18 Jan 2022 (CISA KEV)
- **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-f9f4-96j3-5mvr

## What is CVE-2021-33766?

CVE-2021-33766 is a high-severity 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 Information Disclosure Vulnerability

## Which versions are affected by CVE-2021-33766?

It affects 1 product.


Products: Microsoft Exchange Server.

## How do I fix CVE-2021-33766?

No fix has been published for CVE-2021-33766 yet. Limit exposure of the affected software until one is available. US federal agencies must fix it by 1 Feb 2022.

## Is CVE-2021-33766 being exploited?

Yes. Attackers have exploited CVE-2021-33766 in the wild since at least 18 Jan 2022.

Public exploit code:

- ProxyToken (CVE-2021-33766) : An Authentication Bypass in Microsoft Exchange Server POC exploit (github_poc): https://github.com/bhdresh/CVE-2021-33766
- CVE-2021-33766-poc (github_poc): https://github.com/demossl/CVE-2021-33766-ProxyToken
- Microsoft Exchange - Authentication Bypass (nuclei): http/cves/2021/CVE-2021-33766.yaml

## 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-2021-33766 and fix it if it is affected.

What it is: CVE-2021-33766 is a high-severity 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-2021-33766 in the wild since at least 18 Jan 2022.

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-2021-33766/ (Markdown: https://insights.sbomify.com/CVE-2021-33766/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-33766
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-33766
- OSV: https://osv.dev/vulnerability/CVE-2021-33766

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-33766/
Summaries are generated and may contain mistakes; the official description above is the record.