CVE-2026-55009: Deserialization of untrusted data in Microsoft Exchange Server allows an authorized attacker to elevate privil

Q: How do I fix CVE-2026-55009?

exchange server: downgrade or apply the listed mitigation; exchange server subscription edition: upgrade to 15.02.2562.045. Exact commands for each OS are on this page.

Q: Is CVE-2026-55009 being exploited?

CVE-2026-55009 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-55009 affect?

CVE-2026-55009 affects exchange server, exchange server subscription edition. Severity is high (CVSS 7.8). Deserialization of untrusted data in Microsoft Exchange Server allows an authorized attacker to elevate privileges locally.

Q: How urgent is CVE-2026-55009?

Priority 53/100. High severity; On your watchlist

This month

HIGH 7.8WATCHLISTDOWNGRADEUPGRADE

CVE-2026-55009 is a high-severity flaw (CVSS 7.8) affecting exchange server, exchange server subscription edition. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

High severity
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	7.8 (High)
Attack vector	Local
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	2.5% (top 15%)
CISA exploited list	No
Weakness	CWE-502
Published	2026-07-14
Last modified	2026-07-24

Publisher (CNA): Microsoft Corporation · Vendor: microsoft

Last updated 2026-07-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-14 17:17 UTCPublished CVE record published
2026-07-24 19:19 UTCUpdated Record last modified

microsoft exchange server DOWNGRADE

Affected: 2016 cumulative_update_23 · 2019 cumulative_update_14 · 2019 cumulative_update_15

Fixed in: no fixed release known

Safe to downgrade to: a release before 2016

1. Check installed version

apt-cache policy exchange-server || rpm -q exchange-server || exchange-server --version

2. No fixed release: DOWNGRADE to a release before 2016 (Debian / Ubuntu)

apt-cache madison exchange-server sudo apt install exchange-server=<VERSION_BEFORE_2016> sudo apt-mark hold exchange-server

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list exchange-server sudo dnf downgrade exchange-server-<VERSION_BEFORE_2016> sudo dnf versionlock add exchange-server

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

microsoft exchange server subscription edition UPGRADE

Affected: < 15.02.2562.045

Fixed in: 15.02.2562.045

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy exchange-server-subscription-edition || rpm -q exchange-server-subscription-edition || exchange-server-subscription-edition --version

2. UPGRADE to 15.02.2562.045 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade exchange-server-subscription-edition

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade exchange-server-subscription-edition

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-55009?

exchange server: downgrade or apply the listed mitigation; exchange server subscription edition: upgrade to 15.02.2562.045. Exact commands for each OS are on this page.

Is CVE-2026-55009 being exploited?

CVE-2026-55009 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-55009 affect?

CVE-2026-55009 affects exchange server, exchange server subscription edition. Severity is high (CVSS 7.8). Deserialization of untrusted data in Microsoft Exchange Server allows an authorized attacker to elevate privileges locally.

How urgent is CVE-2026-55009?

Priority 53/100. High severity; On your watchlist

Software pages

Microsoft Exchange vulnerabilities

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CVE-2026-62911 HIGH 8.0 DOWNGRADE UPGRADE Authentication bypass by capture-replay in Microsoft Exchange Server allows an authorized attacker to elevate
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References

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