CVE-2026-62816 — Windows Reliable Multicast Transport Driver (RMCAST) Remote Code Execution Vulnerability

Executive Summary

Heap-based buffer overflow in Reliable Multicast Transport Driver (RMCAST) allows an unauthorized attacker to execute code over an adjacent network.

Overview

8.8

CVSS HIGH

Critical

MS Severity

Not Exploited

MS Exploit Status

Less Likely

MS Exploit Likelihood

Category Remote Code Execution

Weakness CWE-122: Heap-based Buffer Overflow; CWE-190: Integer Overflow or Wraparound

Released Aug 11 2026

Last Updated Aug 11 2026

Publicly Disclosed No

CISA KEV Not Listed

Known Exploits None Known

EPSS Score 0.00401 — 0.33242 percentile

NVD CVSS 8.8 HIGH — matches MSRC

CVSS Vector

CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C

ATTACK VECTOR

Adjacent_network

ATTACK COMPLEXITY

Low

PRIVILEGES REQUIRED

None

USER INTERACTION

None

SCOPE

Unchanged

CONFIDENTIALITY

High

INTEGRITY

High

AVAILABILITY

High

EXPLOIT CODE MATURITY

Unproven

REMEDIATION LEVEL

Official Fix

REPORT CONFIDENCE

Confirmed

Temporal Score: 7.7

EPSS Score

0.00401

probability of exploitation in the next 30 days

0.33242 percentile - updated 2026-08-14

View on FIRST.org

Affected Products

22 affected products

Product	KB Article	Severity	Impact	Restart Required
Windows 10 Version 1607 for 32-bit Systems	`5120418 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 1607 for x64-based Systems	`5120418 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 1809 for 32-bit Systems	`5120238 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 1809 for x64-based Systems	`5120238 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 21H2 for 32-bit Systems	`5120249 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 21H2 for ARM64-based Systems	`5120249 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 21H2 for x64-based Systems	`5120249 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 22H2 for 32-bit Systems	`5120249 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 22H2 for ARM64-based Systems	`5120249 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 22H2 for x64-based Systems	`5120249 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 11 Version 23H2 for ARM64-based Systems	`5120240 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 11 Version 23H2 for x64-based Systems	`5120240 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 11 Version 24H2 for ARM64-based Systems 5120994 (Security Hotpatch Update) 5121003 (Security Update) Critical Remote Code Execution 5101650 Base: 8.8 Temporal: 7.7 Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C 10.0.26100.9106 10.0.26100.9168 Yes None Windows 11 Version 24H2 for x64-based Systems 5121003 (Security Update) 5120994 (Security Hotpatch Update) Critical Remote Code Execution 5101650 Base: 8.8 Temporal: 7.7 Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C 10.0.26100.9168 10.0.26000.9106 Yes None Windows 11 Version 25H2 for ARM64-based Systems 5121003 (Security Update) 5120994 (Security Hotpatch Update) Critical Remote Code Execution 5101650 Base: 8.8 Temporal: 7.7 Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C 10.0.26200.9168 10.0.26100.9106 Yes None Windows 11 Version 25H2 for x64-based Systems 5121003 (Security Update) 5120994 (Security Hotpatch Update) Critical Remote Code Execution 5101650 Base: 8.8 Temporal: 7.7 Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C 10.0.26200.9168 10.0.26100.9106 Yes None Windows 11 Version 26H1 for ARM64-based Systems	`5121000 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 11 version 26H1 for x64-based Systems	`5121000 (Security Update)`	Critical	Remote Code Execution	Yes
Windows Server 2012	`5120386 (Monthly Rollup)`	Critical	Remote Code Execution	Yes
Windows Server 2012 (Server Core installation)	`5120386 (Monthly Rollup)`	Critical	Remote Code Execution	Yes
Windows Server 2012 R2	`5120385 (Monthly Rollup)`	Critical	Remote Code Execution	Yes
Windows Server 2012 R2 (Server Core installation)	`5120385 (Monthly Rollup)`	Critical	Remote Code Execution	Yes
Windows Server 2016	`5120418 (Security Update)`	Critical	Remote Code Execution	Yes
Windows Server 2016 (Server Core installation)	`5120418 (Security Update)`	Critical	Remote Code Execution	Yes
Windows Server 2019	`5120238 (Security Update)`	Critical	Remote Code Execution	Yes
Windows Server 2019 (Server Core installation)	`5120238 (Security Update)`	Critical	Remote Code Execution	Yes

Patches

7 patches

Article	Type	Restart
`5120418`	Security Update	Yes
`5120238`	Security Update	Yes
`5120249`	Security Update	Yes
`5120240`	Security Update	Yes
`5121000`	Security Update	Yes
`5120386`	Monthly Rollup	Yes
`5120385`	Monthly Rollup	Yes

Patch Diff

ghidriff · rmcast.sys (KB5121003)

Integer overflow (CWE-190) leading to a heap-based buffer overflow (CWE-122) in the Windows Reliable Multicast Transport Driver rmcast.sys (PGM - Pragmatic General Multicast), unauthenticated remote code execution over an adjacent network (Critical, AV:A, CVSS 8.8). PGM provides reliability via NAK-driven retransmission of repair data (RData). When PgmSendRData services a received NAK, it derives an index from the NAK (GetNextNakIndex -> local_a7) and combines it into local_a4, then computes the repair-data buffer offset as roughly (local_a4 - window_trail) * per_packet_stride. PRE: the NAK-derived index was attacker-influenced and the subtract/multiply were performed without adequate range validation, so a crafted NAK could wrap the arithmetic (CWE-190) and produce a miscalculated offset/length that was then used for the repair-data copy - an out-of-bounds heap write (CWE-122). Because RMCAST processes multicast packets without authentication over an adjacent network, the overwrite is an RCE primitive (Microsoft rates it Critical 8.8). Diff of rmcast.sys 10.0.26100.8972 -> .9168 (Aug 11 2026, KB5121003) shows PgmSendRData (with AdvanceWindow, SendNextPacket, DestroyEntry) reworked under CFR flag Feature_4002387257: the repair-data copy is converted from memmove to a length-bounded memcpy and the NAK-derived index local_a4 is range-validated before it drives the (local_a4 - trail) * stride offset multiply, closing the overflow. Stated at confirmed-changed level (the update is a substantial rework of the PGM send path; 3 code changes + 2 new helpers).

Pre-patch version 10.0.26100.8972 Download

Post-patch version 10.0.26100.9168 Download

Function	Address	Change	Note
`PgmSendRData`	`code change`	code (NAK-index range validation + bounded copy, CFR-gated)	Pre: (local_a4 - trail) * stride offset computed from an attacker-influenced NAK-derived index without range validation, then used to copy repair data (memmove) - integer overflow -> OOB heap write. Post (Feature_4002387257): local_a4 range-validated before the offset multiply and the copy converted to a length-bounded memcpy.
`AdvanceWindow / SendNextPacket / DestroyEntry`	`code change`	code (PGM send-window path reworked in same update)	Transmit-window advancement, next-packet send, and entry teardown reworked alongside PgmSendRData; AdvanceWindow return type and SendNextPacket signature changed. Two new helper functions added.
`Feature_4002387257`	`gate`	added (CFR gate)	Dominant CFR flag gating the validated PGM send path; the original unvalidated arithmetic still ships when the flag is disabled.

View full diff report View RCA report

Attack Path

A crafted PGM NAK overflows the repair-data offset arithmetic, overflowing the heap during RData retransmission

Derived from the patch delta: the checks added by the vendor identify which fields crossed a trust boundary unvalidated. Reachability and privilege are taken from the call chain in the RCA report.

Exploits & PoC

No public exploit or PoC has been linked for this CVE yet. When available, references are sourced from public repositories and may be unverified or non-functional — review carefully before use.

Detection Rules

No public Sigma or Nuclei detection rule has been mapped to this CVE yet. Coverage is concentrated on exploited / high-profile vulnerabilities; check SigmaHQ for updates.

Acknowledgments

Seung Chan Kim

References

MSRC Advisory MITRE NVD