CVE-2026-56188 — Windows Server Network driver Remote Code Execution Vulnerability

Executive Summary

Concurrent execution using shared resource with improper synchronization ('race condition') in Windows Server Network driver allows an unauthorized attacker to execute code over a network.

Overview

8.1

CVSS HIGH

Critical

MS Severity

Not Exploited

MS Exploit Status

More Likely

MS Exploit Likelihood

Category Remote Code Execution

Weakness CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

Released Jul 14 2026

Last Updated Jul 14 2026

Publicly Disclosed No

CISA KEV Not Listed

Known Exploits None Known

EPSS Score 0.00916 — 0.57084 percentile

NVD CVSS 8.1 HIGH — differs from MSRC

CVSS Vector

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

ATTACK VECTOR

Network

ATTACK COMPLEXITY

High

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: 8.5

EPSS Score

0.00916

probability of exploitation in the next 30 days

0.57084 percentile - updated 2026-08-14

View on FIRST.org

Affected Products

27 affected products

Product	KB Article	Severity	Impact	Restart Required
Windows 10 Version 1607 for 32-bit Systems	`5099535 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 1607 for x64-based Systems	`5099535 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 1809 for 32-bit Systems	`5099538 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 1809 for x64-based Systems	`5099538 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 21H2 for 32-bit Systems	`5099539 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 21H2 for ARM64-based Systems	`5099539 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 21H2 for x64-based Systems	`5099539 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 22H2 for 32-bit Systems	`5099539 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 22H2 for ARM64-based Systems	`5099539 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 10 Version 22H2 for x64-based Systems	`5099539 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 11 Version 24H2 for ARM64-based Systems	`5101650 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 11 Version 24H2 for x64-based Systems	`5101650 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 11 Version 25H2 for ARM64-based Systems	`5101650 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 11 Version 25H2 for x64-based Systems	`5101650 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 11 Version 26H1 for ARM64-based Systems	`5101649 (Security Update)`	Critical	Remote Code Execution	Yes
Windows 11 version 26H1 for x64-based Systems	`5101649 (Security Update)`	Critical	Remote Code Execution	Yes
Windows Server 2012	`5099445 (Monthly Rollup)`	Critical	Remote Code Execution	Yes
Windows Server 2012 (Server Core installation)	`5099445 (Monthly Rollup)`	Critical	Remote Code Execution	Yes
Windows Server 2012 R2	`5099444 (Monthly Rollup)`	Critical	Remote Code Execution	Yes
Windows Server 2012 R2 (Server Core installation)	`5099444 (Monthly Rollup)`	Critical	Remote Code Execution	Yes
Windows Server 2016	`5099535 (Security Update)`	Critical	Remote Code Execution	Yes
Windows Server 2016 (Server Core installation)	`5099535 (Security Update)`	Critical	Remote Code Execution	Yes
Windows Server 2019	`5099538 (Security Update)`	Critical	Remote Code Execution	Yes
Windows Server 2019 (Server Core installation)	`5099538 (Security Update)`	Critical	Remote Code Execution	Yes
Windows Server 2022	`5099540 (Security Update)`	Critical	Remote Code Execution	Yes
Windows Server 2025	`5099536 (Security Update)`	Critical	Remote Code Execution	Yes
Windows Server 2025 (Server Core installation)	`5099536 (Security Update)`	Critical	Remote Code Execution	Yes

Patches

9 patches

Article	Type	Restart
`5099535`	Security Update	Yes
`5099538`	Security Update	Yes
`5099539`	Security Update	Yes
`5101650`	Security Update	Yes
`5101649`	Security Update	Yes
`5099445`	Monthly Rollup	Yes
`5099444`	Monthly Rollup	Yes
`5099540`	Security Update	Yes
`5099536`	Security Update	Yes

Patch Diff

ghidriff · netbt.sys (KB5101650)

Race condition (CWE-362) in the Windows NetBIOS over TCP/IP driver netbt.sys ('Windows Server Network driver'), unauthenticated remote code execution over the network (Critical, AV:N, CVSS 9.8). netbt.sys manages NetBT lower connections (the TCP connections under NetBIOS sessions): created by NbtOpenAndAssocConnection, referenced/dereferenced by NbtDereferenceLowerConnection / NbtDeleteLowerConn, and torn down by a family of disconnect/cleanup routines (NbtDisconnect, TdiDisconnect, TcpDisconnect, DisconnectLower, DelayedCleanupAfterDisconnect, DisconnectDone, OutOfRsrcKill) while inbound data is delivered via TdiReceiveHandler / NTReceive / NbtIndicateDataToClient. PRE: the lower-connection object shared between teardown and in-flight receive/send paths was not adequately synchronized, so a disconnect/cleanup could dereference and free the connection while a receive was still in progress - a race corrupting/reusing freed connection state. Because NetBT processes attacker-supplied network traffic without authentication, the race is an RCE primitive (Microsoft rates it Critical 9.8). Diff of netbt.sys 10.0.26100.8737 (KB5095093, Jun) -> .8875 (KB5101650, Jul 14 2026, 24H2/Server 2025) confirms the fix: gated behind CFR flag Feature_1180933432 (the same flag as the SMB srvnet.sys CVE-2026-57089 - a coordinated network-driver hardening), the teardown/dereference routines now acquire the connection spinlock (KeAcquireSpinLockRaiseToDpc, ~193 KeAcquireSpinLock sites across the 30 changed functions) and validate the connection signature ('Con1'/'Con2' = 0x316e6f43/0x326e6f43) and state before releasing the object, so teardown can no longer free the connection while a receive is in flight. Stated at confirmed-changed level (broad 30-function connection-lifecycle rework; the isolable mechanism is the spinlock-serialized, state-validated teardown/dereference).

Pre-patch version 10.0.26100.8737 Download

Post-patch version 10.0.26100.8875 Download

Function	Address	Change	Note
`NbtDereferenceLowerConnection / NbtDeleteLowerConn`	`code change`	code (dereference now serialized under the MODULE-GLOBAL lock, CFR-gated)	NBT_LOWERCONNECTION: refcount +0x14, upper-connection link +0x18, per-connection lock +0x68. Pre: NbtDereferenceLowerConnection took ONLY the per-connection lock (+0x68) around the interlocked decrement. Post (Feature_1180933432): extends the module-global lock (already used by CleanUpPartialConnection/CleanupConnectingState/CountUpperConnections) to this function so the dereference is serialized against those routines; a 3rd param signals the caller already holds the global lock. The interlocked decrement is unchanged; the legacy state=1000 poison-write is dropped when the feature is enabled.
`NbtDisconnect / TdiDisconnect / TcpDisconnect / DisconnectLower / DelayedCleanupAfterDisconnect / DisconnectDone / OutOfRsrcKill`	`code change`	code (disconnect/cleanup family serialized under the connection lock)	The connection-teardown routines are reworked to serialize on the connection spinlock (dominant KeAcquireSpinLock usage) so teardown does not race in-flight I/O. 30 code-changed + 58 modified functions total.
`Feature_1180933432`	`gate`	added (CFR gate)	CFR flag gating the serialized NetBT connection teardown; also gates the SMB srvnet.sys CVE-2026-57089 fix (coordinated network-driver hardening). Original unsynchronized path ships when disabled.

View full diff report View RCA report

Attack Path

A race between NetBT connection teardown and an in-flight receive frees the connection object while still in use

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

Aaron Esau

References

MSRC Advisory MITRE NVD