# CVE-2026-56191: Microsoft Exchange Online Tampering Vulnerability

> An unauthorized attacker can bypass authentication to tamper with data in Microsoft Exchange Online via network access.

- **Severity:** Critical 10.0 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H/E:U/RL:O/RC:C`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 24 Jul 2026
- **Updated:** 28 Oct 2026
- **Also known as:** GHSA-w7qh-jvr9-qmx9

## What is CVE-2026-56191?

CVE-2026-56191 is a critical authentication bypass vulnerability in Microsoft Exchange Online, published on 24 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Improper authentication in Microsoft Exchange Online allows an unauthorized attacker to perform tampering over a network.

Weakness: CWE-287 (Improper Authentication)

## Which versions are affected by CVE-2026-56191?

It affects 1 product.


Products: Microsoft Exchange Online.

## How do I fix CVE-2026-56191?

No fix has been published for CVE-2026-56191 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-56191 being exploited?

There is no known exploitation of CVE-2026-56191 in the wild and no public exploit code.

## Other vulnerabilities in Microsoft Exchange Online

- [CVE-2026-65801](https://insights.sbomify.com/CVE-2026-65801/): Microsoft Exchange Online Elevation of Privilege Vulnerability (critical 10.0)
- [CVE-2026-48582](https://insights.sbomify.com/CVE-2026-48582/): Microsoft Exchange Online Elevation of Privilege Vulnerability (critical 9.6)
- [CVE-2026-48579](https://insights.sbomify.com/CVE-2026-48579/): Microsoft Exchange Online Information Disclosure Vulnerability (critical 9.1)
- [CVE-2026-54998](https://insights.sbomify.com/CVE-2026-54998/): Microsoft Exchange Online Elevation of Privilege Vulnerability (high 8.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-56191 and fix it if it is affected.

What it is: CVE-2026-56191 is a critical authentication bypass vulnerability in Microsoft Exchange Online, published on 24 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-56191 in the wild and no public exploit code.

Affected product: Microsoft Exchange Online. 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-2026-56191/ (Markdown: https://insights.sbomify.com/CVE-2026-56191/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-56191
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-56191
- OSV: https://osv.dev/vulnerability/CVE-2026-56191

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-56191/
Summaries are generated and may contain mistakes; the official description above is the record.