Summary: I’m disclosing a full-chain CVSS 10.0 RCE affecting Microsoft Semantic Kernel (.NET v1.74) and the new Agent Framework 1.0.

The Timeline & Conflict: > * March 24: Initial disclosure sent to MSRC with PoC.

• April 8: MSRC closed the case as "Developer Error / Configuration Issue."
• The Reality: Despite the rejection, Microsoft silently merged mitigations in PRs #13683 and #13702 without assigning a CVE. This results in a "False Green" for enterprise SCA tools (Snyk/Checkmarx/Dependabot) while the bypasses remain functional.

Technical Scope:

• Architectural Trust Gap (CWE-1039): Auto-invocation logic treats non-deterministic LLM output as a high-privilege system coordinator without a sandbox boundary.
• 6 Day-Zero Bypasses: Discovery of Type Confusion and Unicode homoglyphs that defeat the "hardened" baseline in the April 2026 releases.
• Versioning: Persistence confirmed from .NET v1.7x through the Agent Framework 1.0 re-baseline.

Full paper, .cast exploit recordings, and a production-ready C# remediation filter are available at the link.

The core issue: Windows RPC runtime doesn't verify whether the server a high-privileged client connects to is legitimate. If a target RPC server is unavailable, an attacker with SeImpersonatePrivilege can spin up a fake RPC server mimicking the same endpoint, wait for a SYSTEM-level client to connect, then call RpcImpersonateClient to escalate privileges.

Five confirmed escalation paths:

- gpupdate /force → SYSTEM (coerces Group Policy service)

- Microsoft Edge launch → Administrator (no coercion needed)

- WDI background service → SYSTEM (fires every 5–15 min automatically)

- ipconfig + disabled DHCP → Administrator

- w32tm.exe → Administrator via non-existent named pipe

Microsoft assessed this as moderate severity, issued no CVE, and has no patch planned — justification being that SeImpersonatePrivilege is a prerequisite.

Questions for the community:

1. Are you monitoring for RPC_S_SERVER_UNAVAILABLE (Event ID 1 via ETW) in your environment?

2. Any Sigma/Defender rules already written for this?

3. Do you agree with Microsoft's severity assessment given how common SeImpersonatePrivilege is on IIS/SQL servers?

Kaspersky's full write-up + PoC: https://securelist.com/phantomrpc-rpc-vulnerability/119428/

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