CVE-2026-10556

Q: How do I fix CVE-2026-10556?

Upgrade mattermost server to 11.7.8 or later.

Q: Is CVE-2026-10556 being actively exploited?

Not that we know of. CVE-2026-10556 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.42% is the estimated probability that it will be exploited in the next 30 days. That is higher than 36% of all scored CVEs.

Q: How severe is CVE-2026-10556?

CVE-2026-10556 has a CVSS v3 base score of 5.3, rated medium. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

Q: What does CVE-2026-10556 affect?

Published advisories record a fix for mattermost server (NVD). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

MEDIUM

5.3

CVSS v3

NVD

Description

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to validate null entries in Microsoft Graph webhook notification payloads, which allows an unauthenticated attacker to crash the Microsoft Calendar plugin process and deny calendar integration service to all users on the instance via a crafted {{POST}} request to the public webhook endpoint.. Mattermost Advisory ID: MMSA-2026-00693

CWE-754 CWE-476NULL Pointer Dereference

How to fix

Remediation Available

mattermost serverNVD

Affected:>= 11.7.0, < 11.7.8Fixed in:11.7.8CVE-2026-10556derived from NVD

Affected:>= 10.11.0, < 10.11.23Fixed in:10.11.23CVE-2026-10556derived from NVD

Affected:>= 11.9.0, < 11.9.1Fixed in:11.9.1CVE-2026-10556derived from NVD

Affected:>= 11.8.0, < 11.8.5Fixed in:11.8.5CVE-2026-10556derived from NVD

TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityLow

Privileges RequiredNone

User InteractionNone

ScopeUnchanged

Impact

ConfidentialityNone

IntegrityNone

AvailabilityLow

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Exploit Intelligence

0.42%probability of exploitation in 30 days

36thpercentile

Low risk: more likely to be exploited than 36% of all known CVEs.

References

Vendor Advisory1

mattermost.com

Related Vulnerabilities

Other CWE-754 vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2024-4367	High	8.8	71%	-	Fix
CVE-2022-39288	High	7.5	59%	-	Fix
CVE-2023-30591	High	7.5	59%	-	-
CVE-2024-3393	High	7.5	29%	KEV	Fix
CVE-2024-43044	High	8.8	29%	-	Fix
CVE-2023-41993	High	8.8	24%	KEV	Fix

Common questions

How do I fix CVE-2026-10556?

Upgrade mattermost server to 11.7.8 or later.

Is CVE-2026-10556 being actively exploited?

Not that we know of. CVE-2026-10556 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.42% is the estimated probability that it will be exploited in the next 30 days. That is higher than 36% of all scored CVEs.

How severe is CVE-2026-10556?

CVE-2026-10556 has a CVSS v3 base score of 5.3, rated medium. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

What does CVE-2026-10556 affect?

Published advisories record a fix for mattermost server (NVD). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-10-07.