Microsoft Exchange Server: Denial of Service through inefficient algorithm
Summary
Microsoft Exchange Server Subscription Edition RTM (x64) builds before 15.02.2562.057 contain an inefficient algorithmic complexity flaw (CWE-407). An unauthenticated attacker with network access can exhaust server resources and cause a denial of service. The attacker cannot read or change mail, but mail and calendar services can become unavailable, and exposure is highest if OWA, ActiveSync or EWS is reachable from the internet.
Affected
- Microsoft Exchange Server Subscription Edition RTM (x64): builds from 15.02.0.0 before 15.02.2562.057
- Other Exchange versions (for example 2016 or 2019) and hybrid-only configurations: Unknown. Check the MSRC advisory product list.
Fixed in
- Upgrade Exchange Server Subscription Edition RTM to build 15.02.2562.057 or later.
- The fix is part of the Microsoft security update of November 10, 2026 (KB5133453). The cumulative update name differs between sources (SU11 or SU12); verify it against Microsoft's advisory.
Workaround
Unknown. No vendor-documented workaround was identified; patching is the only remediation identified.
Exploitation
- Microsoft states that no publicly available exploit code exists or that an exploit is only theoretical.
- Rapid7 rates it 'Exploitation Less Likely' and not publicly disclosed.
- CISA SSVC exploitation status: none. Strix lists it as not in KEV; this was not verified against the CISA KEV catalogue.
- No public proof-of-concept is known.
Our recommendation
- Check whether any server runs Exchange Server Subscription Edition RTM below build 15.02.2562.057.
- Install the November 2026 security update (KB5133453) or the latest cumulative update in the normal monthly cycle; patch sooner if the server is internet-facing.
- Check the MSRC advisory for other Exchange versions and any mitigation text.
- Monitor availability and resource usage of mail services and keep a fallback for mail and calendar.
Information from Microsoft (Patch Tuesday November 2026)
- Severity (Microsoft)
- Important
- Impact
- Denial of service – Microsoft Exchange Server
- Status on Patch Tuesday
- Neither exploited nor publicly disclosed
- Exploitability assessment
- Exploitation less likely
- Product family
- Server software (Exchange, SharePoint …)
- Updates (KB)
- KB5133453
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Microsoft MSRC Security Update Guide: CVE-2026-118153 msrc.microsoft.com
- CVE Record CVE-2026-118153 www.cve.org
- Icewolf: Exchange Server SE security update November 2026 blog.icewolf.ch
- Rapid7: Patch Tuesday November 2026 www.rapid7.com
- Strix: CVE-2026-118153 www.strix.ai
- OffSeq Radar: Exchange Server vulnerabilities fixed radar.offseq.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 11 Nov 2026, 05:36.
Original description (NVD)
Inefficient algorithmic complexity in Microsoft Exchange Server allows an unauthorized attacker to deny service over a network.
NVD references (1)
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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