Critical CVSS 10
CVE-2026-56191
NVD analysis in progress
Improper authentication in Microsoft Exchange Online allows an unauthorized attacker to perform tampering over a network.
What this means
- Exposure
- Exploitable remotely over the network, without authentication and with no action from the victim.
- Impact
- An attacker can read sensitive data, modify or destroy data and take the service offline. The impact spreads beyond the vulnerable component into other parts of the system.
- Weakness
- The authentication mechanism can be bypassed, letting an attacker pose as a legitimate user.
- Likelihood
- Its EPSS score stays low: nothing points to imminent exploitation, which is no reason to leave it unpatched.
What to doPatch without waiting for the next scheduled cycle. Start with the instances exposed to the internet.
Read automatically from the CVSS vector, the weakness type (CWE) and the EPSS score. The technical description above remains the one published by NIST.