Authentication Bypass in Microsoft Exchange Online (CVE-2026-56191)
Summary
Authentication bypass in Microsoft Exchange Online (CVE-2026-56191). Successful exploitation can lead to full system takeover.
Original description
The description as published by the source.
Improper authentication in Microsoft Exchange Online allows an unauthorized attacker to perform tampering over a network.
Source: NIST National Vulnerability Database NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-56191 has been found in Microsoft Exchange Online.
Successful exploitation can lead to full system takeover. CVSS score: 10.0/10.
What to do: apply the vendor's official patched release.
If unsure, ask your IT team or search "Microsoft Exchange Online CVE-2026-56191" on the vendor's site.
CVE-2026-56191 (Microsoft Exchange Online) — CWE-287 / CVSS v3 10.0
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
Authentication bypass (CWE-287) exists in Microsoft Exchange Online.
Check the vendor advisory for the affected versions of Microsoft Exchange Online.
Severity: Critical (CVSS 10.0/10). Successful exploitation can lead to full system takeover.
Update to the patched release per vendor advisory.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Audit SBOM/dependencies to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
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1Identify exposure identify
Audit SBOM/dependencies for affected components.資産台帳・SBOM・依存マニフェストで Microsoft Exchange Online を使っているサービスとバージョンを洗い出す。
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2Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
References
- patch secure@microsoft.com
- web nvd.nist.gov
- web github.com