CVE-2026-55899: microsoft 365_apps Improper Input Validation
CVSS: 7.8v3.1HighAnalyzed
Published 14-07-2026Last modified 08-10-2026
Vulnerability summary
CVE-2026-55899 is a high (CVSS 7.8) Improper Input Validation vulnerability in microsoft 365_apps. EPSS estimates a 0.47% chance of exploitation in the next 30 days, higher than 39.3% of scored vulnerabilities.
CVSS score
7.8/10
010
High · v3.1 (CVE.org)EPSS score
0.47%
0%100%
Low · 0.00 pp in 30 daysKEVNot listedIn no KEV catalog
RansomwareNone recordedNo known use recorded
Public exploits0No public exploits recorded
Timeline
- 14-07-2026PublishedVulnerability first disclosed.
- 08-10-2026Last modifiedVulnerability information updated.
Description
Stack-based buffer overflow in Microsoft Office Excel allows an unauthorized attacker to execute code locally.
CVSS metrics
2 scores · 2 sources · 1 versionHigh 7.8Preferred score · CVE.org (CNA)
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:CSourcesAll 2 agree
- NVDNIST7.8HighCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
EPSS trend
Probability
0.47%
Chance of exploitation in the next 30 days
Percentile
39%
Versus all other CVEs
30-day change
0.00 pp
From 0.47% on 21-09-2026
21-09-202621-10-2026
EPSS score0.47%
Known exploited
Compare KEV catalogsNot in any of the 3 tracked catalogs.
Public exploits
0 foundNo public exploits indexed.
Weaknesses
CWE · root cause- CWE-20Improper Input ValidationThe product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.
- CWE-121Stack-based Buffer OverflowA stack-based buffer overflow condition is a condition where the buffer being overwritten is allocated on the stack (i.e., is a local variable or, rarely, a parameter to a function).
Affected systems
Microsoft15 products
- 365_appsna
- excel2016
- microsoft_365na
- microsoft 365 apps for enterprise≥ 16.0.1, < https://aka.ms/OfficeSecurityReleases
- microsoft excel 2016≥ 16.0.0.0, < 16.0.5561.1001
1 vendor · 15 products
