Microsoft Internet Messaging API: Heap buffer overflow enables remote code execution
Summary
A heap-based buffer overflow in the Microsoft Internet Messaging API, a Windows component, allows an unauthorized attacker to execute code over a network. The CVSS vector requires user interaction (UI:R), which lowers practical risk. CISA rates exploitation as none, and no public proof of concept was found. Exposure depends on whether the component is reachable from outside the network.
Affected
- Windows 10 Version 1607 (builds before 10.0.14393.9612)
- Windows 10 Version 1809 (builds before 10.0.17763.9376)
- Windows 10 Version 21H2 (builds before 10.0.19044.7807)
- Windows 10 Version 22H2 (builds before 10.0.19045.7807)
- Windows 11 Version 23H2 (builds before 10.0.22631.7652)
- Windows 11 Version 24H2 (builds before 10.0.26100.9711)
- Windows 11 Version 25H2 (builds before 10.0.26200.9711)
- Windows 11 Version 26H1 (builds before 10.0.28000.3231)
- Windows 11 Version 26H2 (builds before 10.0.26300.9711)
- Windows Server 2012, including Server Core (builds before 6.2.9200.26434)
- Windows Server 2012 R2, including Server Core (builds before 6.3.9600.23470)
- Windows Server 2016 (builds before 10.0.14393.9612)
Fixed in
- Fixed builds as listed in the official CNA record, to be verified against the MSRC advisory before rollout.
- Possible October 2026 cumulative updates: KB5126646 (Windows Server 2025), KB5126651 (Windows 10 1809 / Server 2019), KB5126868 (Windows Server 2016, per a third-party page). These are not confirmed as the fixes.
Workaround
Unknown. No vendor workaround or interim mitigation was identified; patching is the only documented remedy.
Exploitation
- Not known to be exploited in the wild.
- CISA SSVC: exploitation none, automatable no, technical impact total.
- No public proof of concept found; this is not confirmation that none exists.
Our recommendation
- Verify the fixed KB per product in the MSRC Security Update Guide before deployment.
- Patch internet-facing and server systems first, then workstations.
- Also patch the related local variant CVE-2026-83919 together with this CVE.
- Check whether Windows systems with this component are reachable from outside the network and restrict access where possible.
- Monitor for unusual process activity and network traffic on affected hosts.
Information from Microsoft (Patch Tuesday October 2026)
- Severity (Microsoft)
- Important
- Impact
- Remote code execution – Microsoft Internet Messaging API
- Status on Patch Tuesday
- Neither exploited nor publicly disclosed
- Exploitability assessment
- Exploitation less likely
- Product family
- Windows, Windows ESU (Extended Security Updates)
- Updates (KB)
- KB5126651, KB5126655, KB5126657, KB5126646, KB5127648, KB5126653
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Microsoft Security Update Guide: CVE-2026-83778 msrc.microsoft.com
- OpenCVE: CVE-2026-83778 app.opencve.io
- Vulners NVD record: CVE-2026-83778 vulners.com
- Strix AI: CVE-2026-83778 (third-party aggregator, contains errors) www.strix.ai
- Senserva: KB5126868 (Server 2016) senserva.com
- Vulners PT-2026-112680 (related CVE-2026-83919) vulners.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 31 Oct 2026, 18:32.
Original description (NVD)
Heap-based buffer overflow in Microsoft Internet Messaging API allows an unauthorized attacker to execute code 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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