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  4. CVE 2026 10556 unauthenticated webhook request with null
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Back to active CVEs
Medium · CVSS 5.3CVE-2026-10556

Unauthenticated webhook request with null notification entry could crash the Microsoft Calendar plugin.CVE-2026-10556

Answer in brief

CVE-2026-10556 records a Medium severity (CVSS 5.3) vulnerability in Unauthenticated webhook request with null notification entry could crash the Microsoft Calendar plugin.. The current sources do not mark it as known exploited. The current feed maps Mattermost/Mattermost (generic), Mattermost/Mattermost (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Sep 14, 2026Updated Oct 7, 2026Source checked Oct 29, 2026First seen by HOL Sep 14, 2026Material review Oct 7, 2026
Upstream Advisory

Key facts

Risk
Medium · CVSS 5.3
Exploitation
Not marked as known exploited
Affected software
3 mapped packages or products
Fix availability
Not reported

Why this deserves its current priority

CVSS is 5.3. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps Mattermost/Mattermost (generic), Mattermost/Mattermost (generic). Check affected ranges and fixed versions before updating.

Affected products and reported versions
ProductAffected versionsFixed versions
cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*Not reportedNot reported
Mapped affected packages and fixed versions
PackageAffected rangeFixed version
Mattermost/Mattermostgeneric>=11.9.0 <=11.9.0 || >=11.8.0 <=11.8.4 || >=11.7.0 <=11.7.7 || >=10.11.0 <=10.11.22Not reported
Mattermost/Mattermostgeneric11.9.0 || 11.8.0 || 11.7.0 || 10.11.0Not reported

Recommended response

  1. 1Check inventory. Check lockfiles and deployed manifests for Mattermost/Mattermost, Mattermost/Mattermost.
  2. 2Review the reported fix. Source-reported solution: Update Mattermost to versions 11.10.0, 11.9.1, 11.8.5, 11.7.8, 10.11.23 or higher.

Evidence timeline and material changes

  1. Published upstream

    Sep 14, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  2. Source modified

    Oct 7, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  3. First seen by HOL

    Sep 14, 2026

Sources and claim methodology

  • Source referencemattermost.com
Upstream source 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

Quoted source text, attributed separately from HOL analysis.

Related CVEs

  • NULL pointer dereference on implicit read of AR specific record indexAlso CWE-476
  • NULL pointer dereference on implicit read of user specific record indexAlso CWE-476

Record context

Vulnerability class
Missing Auth
EPSS
Not reported
CWE IDs
CWE-754, CWE-476
Source
CVE List V5
Source checked
Oct 29, 2026
References
1 linked source
Open source record