OnlyPatch
Windows Universal Disk Format File System Driver (UDFS)

CVE-2026-62699 — Windows Universal Disk Format File System Driver (UDFS) Remote Code Execution Vulnerability

Important CVSS 6.8 EPSS 0.00409 🔬 Patch diffed 2026-08 archive

Executive Summary

Heap-based buffer overflow in Windows Universal Disk Format File System Driver (UDFS) allows an unauthorized attacker to execute code with a physical attack.

Overview

6.8
CVSS MEDIUM
Important
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 0.00409 — 0.33931 percentile
NVD CVSS 6.8 MEDIUM — matches MSRC

CVSS Vector

CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C
ATTACK VECTOR
Physical
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: 5.9

EPSS Score

0.00409
probability of exploitation in the next 30 days
0.33931 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) Important Remote Code Execution Yes
Windows 10 Version 1607 for x64-based Systems 5120418 (Security Update) Important Remote Code Execution Yes
Windows 10 Version 1809 for 32-bit Systems 5120238 (Security Update) Important Remote Code Execution Yes
Windows 10 Version 1809 for x64-based Systems 5120238 (Security Update) Important Remote Code Execution Yes
Windows 10 Version 21H2 for 32-bit Systems 5120249 (Security Update) Important Remote Code Execution Yes
Windows 10 Version 21H2 for ARM64-based Systems 5120249 (Security Update) Important Remote Code Execution Yes
Windows 10 Version 21H2 for x64-based Systems 5120249 (Security Update) Important Remote Code Execution Yes
Windows 10 Version 22H2 for 32-bit Systems 5120249 (Security Update) Important Remote Code Execution Yes
Windows 10 Version 22H2 for ARM64-based Systems 5120249 (Security Update) Important Remote Code Execution Yes
Windows 10 Version 22H2 for x64-based Systems 5120249 (Security Update) Important Remote Code Execution Yes
Windows 11 Version 23H2 for ARM64-based Systems 5120240 (Security Update) Important Remote Code Execution Yes
Windows 11 Version 23H2 for x64-based Systems 5120240 (Security Update) Important Remote Code Execution Yes
Windows 11 Version 24H2 for ARM64-based Systems 5120994 (Security Hotpatch Update) 5121003 (Security Update) Important Remote Code Execution 5101650 Base: 6.8 Temporal: 5.9 Vector: CVSS:3.1/AV:P/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) Important Remote Code Execution 5101650 Base: 6.8 Temporal: 5.9 Vector: CVSS:3.1/AV:P/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) Important Remote Code Execution 5101650 Base: 6.8 Temporal: 5.9 Vector: CVSS:3.1/AV:P/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) Important Remote Code Execution 5101650 Base: 6.8 Temporal: 5.9 Vector: CVSS:3.1/AV:P/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) Important Remote Code Execution Yes
Windows 11 version 26H1 for x64-based Systems 5121000 (Security Update) Important Remote Code Execution Yes
Windows Server 2012 5120386 (Monthly Rollup) Important Remote Code Execution Yes
Windows Server 2012 (Server Core installation) 5120386 (Monthly Rollup) Important Remote Code Execution Yes
Windows Server 2012 R2 5120385 (Monthly Rollup) Important Remote Code Execution Yes
Windows Server 2012 R2 (Server Core installation) 5120385 (Monthly Rollup) Important Remote Code Execution Yes
Windows Server 2016 5120418 (Security Update) Important Remote Code Execution Yes
Windows Server 2016 (Server Core installation) 5120418 (Security Update) Important Remote Code Execution Yes
Windows Server 2019 5120238 (Security Update) Important Remote Code Execution Yes
Windows Server 2019 (Server Core installation) 5120238 (Security Update) Important 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 · udfs.sys (KB5121003)

Integer overflow (CWE-190) leading to a heap-based buffer overflow (CWE-122) in the Windows UDF file system driver udfs.sys volume-structure parsing, code execution via a malicious UDF disc (physical attack, AV:P). When mounting a UDF volume, udfs.sys reads on-disk volume-descriptor structures and, in UdfQueryOnDiskVolInfo / UdfUpdateVolumeStructures, computes a pool allocation size from an attacker-controlled count field taken from the disc (size = count << 3, i.e. count * 8, where count is *(uint*)(desc+0x48)). PRE: that size computation was not range-checked, so a large on-disk count makes the 32-bit size calculation wrap (integer overflow), yielding an undersized heap allocation that is then filled from the on-disk data - a heap buffer overflow. Because UDF volume mount runs in the kernel and is triggered by inserting/mounting a crafted UDF disc, this is a code-execution primitive (Microsoft classifies it RCE via physical attack). Diff of udfs.sys 10.0.26100.8972 -> .9168 (Aug 11 2026, KB5121003) confirms the fix: gated behind CFR flag Feature_2661530937, the size derived from the on-disk count is now bounded before use - the 64-bit product (count << 3) is checked to be below 0x100000000 (and related descriptor sizes are validated) so the allocation size can no longer wrap, closing the integer-overflow-to-heap-overflow. UdfUpdateVcbPhase0 / UdfVerifyVolume / UdfQueryFreeBlocksAndNWA were updated under the same gate.

Pre-patch version 10.0.26100.8972 Download
Post-patch version 10.0.26100.9168 Download
Function Address Change Note
UdfQueryOnDiskVolInfo code change code (allocation size from on-disk count now overflow-checked, CFR-gated) Post (Feature_2661530937): pcVar11 = (count /*(uint*)(desc+0x48)*/ << 3); if (pcVar11 < 0x100000000) { ...validate descriptor sizes... } - bounds the 64-bit size so count*8 cannot wrap into an undersized allocation.
UdfUpdateVolumeStructures code change code (descriptor size handling hardened, CFR-gated) Reworked reads of the on-disk partition/volume descriptor (stride 0x38 entries) with the gated size validation (e.g. local_58 = count<<3) before allocating/reading.
UdfUpdateVcbPhase0 / UdfVerifyVolume / UdfQueryFreeBlocksAndNWA code change code (mount-path validation updated) Volume-mount / verify path updated in concert under Feature_2661530937.
Feature_2661530937 gate added (CFR gate) CFR flag gating the on-disk size-overflow validation; the original unchecked size computation still ships when disabled.
View full diff report View RCA report

Attack Path

A malicious UDF disc supplies a large descriptor count so the mount allocation size wraps and the heap buffer overflows

Attack path for CVE-2026-62699 A malicious UDF disc supplies a large descriptor count so the mount allocation size wraps and the heap buffer overflows 01 — ENTRY A crafted UDF disc is inserted/mounted udfs.sys parses on-disk volume descriptors during mount (UdfMountVolume -> UdfUpdateVolumeStructures -> UdfQueryOnDiskVolInfo). AV:P/PR:N. 02 — CONTROLLED INPUT The disc declares a large count field in a volume/partition descriptor count = *(uint*)(desc+0x48); the allocation size is derived as count << 3 (count*8). 03 — MISSING CHECK Size computation overflows without a range check (CWE-190) The 32-bit size wraps for large count, producing an undersized pool allocation. 04 — PATH On-disk data is copied into the undersized heap buffer (CWE-122) The mount code fills the buffer from the disc, overflowing the heap allocation in kernel. 05 — PRIMITIVE Kernel heap overflow -> code execution The Aug 2026 fix (Feature_2661530937) bounds the size (count<<3 < 0x100000000) before allocation.

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

Detection Rules

Acknowledgments

Thanatos Tian (HKPolyU) & npc0vo & @2st__ with Diffract

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