CVE-2026-55009

HIGH 7.8 ALL-YEARS

Published 2026-07-14 · Last modified 2026-10-08 · microsoft · NVD: Analyzed

Deserialization of untrusted data in Microsoft Exchange Server allows an authorized attacker to elevate privileges locally.

ELEVATED IMPACT

Severe if exploited (CVSS 7.8), but no confirmed exploitation. Patch on a normal cadence.

Exploitation likelihood

2.5%chance of exploitation in 30 days · 85th percentile

○ In CISA KEV○ Catalogued PoC◆ SSVC: exploitation none, automatable no

Impact if exploited

7.8CVSS 3.1 · HIGH · CNA

ConfidentialityHigh
IntegrityHigh
AvailabilityHigh

What an attacker needs

⚠Access: Requires local access to the host
⚠Privileges: Requires a low-privilege account
✓User interaction: No user interaction needed
✓Complexity: No special conditions — reliably repeatable

✓ lowers the bar for an attacker · ⚠ raises it

Proof of concept

No public PoC is currently catalogued.

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Tracked sources are checked weekly for the full archive, including PoCs published after the original disclosure. A later release appears after a tracked source lists it and a refresh succeeds.

Affected

Vendors Microsoft

Products Microsoft Exchange Server 2016 Cumulative Update 23 Microsoft Exchange Server 2019 Cumulative Update 14 Microsoft Exchange Server 2019 Cumulative Update 15 Microsoft Exchange Server Subscription Edition Rtm Exchange Server Exchange Server Subscription Edition

Weakness (CWE)

CWE-502: : Deserialization of Untrusted Data

CVSS vector

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C

References and research data (JSON)

Known Affected Software Configurations

Vendor	Product	Version range
Microsoft	Exchange Server	2016
Microsoft	Exchange Server	2019
Microsoft	Exchange Server	2019
Microsoft	Exchange Server Subscription Edition	< 15.02.2562.045

All CVSS metrics

HIGH 7.8 v3.1 · CNA Primary
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C
HIGH 7.8 v3.1 · NVD Secondary
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

References

Patches & mitigations

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55009 vendor-advisorypatchVendor AdvisoryCVE record · microsoft · Updated 2026-10-08T18:13:15.616Z
NVD · CVE-2026-55009 · Updated 2026-07-24T19:19:42.720

Advisories

https://euvd.enisa.europa.eu/vulnerability/EUVD-2026-44027 ADVISORYEUVD · EUVD-2026-44027

Sources: NVD · CVE.org · All sources and attribution · EPSS