VDB-340774 · CVE-2026-20839 · GCVE-0-2026-20839

Microsoft Windows up to Server 2025 Client-Side Caching access control

CVSS Meta Temp Score CVSS is a standardized scoring system to determine possibilities of attacks. The Temp Score considers temporal factors like disclosure, exploit and countermeasures. The unique Meta Score calculates the average score of different sources to provide a normalized scoring system.	Current Exploit Price (≈) Our analysts are monitoring exploit markets and are in contact with vulnerability brokers. The range indicates the observed or calculated exploit price to be seen on exploit markets. A good indicator to understand the monetary effort required for and the popularity of an attack.	CTI Interest Score Our Cyber Threat Intelligence team is monitoring different web sites, mailing lists, exploit markets and social media networks. The CTI Interest Score identifies the interest of attackers and the security community for this specific vulnerability in real-time. A high score indicates an elevated risk to be targeted for this vulnerability.
4.4	$5k-$25k	0.00

Summaryinfo

A vulnerability classified as problematic was found in Microsoft Windows. Impacted is an unknown function of the component Client-Side Caching. Executing a manipulation can lead to access control. The identification of this vulnerability is CVE-2026-20839. The attack can only be executed locally. There is no exploit available. It is best practice to apply a patch to resolve this issue.

Detailsinfo

A vulnerability was found in Microsoft Windows. It has been declared as problematic. This vulnerability affects an unknown function of the component Client-Side Caching. The manipulation with an unknown input leads to a access control vulnerability. The CWE definition for the vulnerability is CWE-284. The product does not restrict or incorrectly restricts access to a resource from an unauthorized actor. As an impact it is known to affect confidentiality. CVE summarizes:

Improper access control in Windows Client-Side Caching (CSC) Service allows an authorized attacker to disclose information locally.

The weakness was shared 01/13/2026 as confirmed security update guide (Website). The advisory is available at msrc.microsoft.com. This vulnerability was named CVE-2026-20839. The exploitation appears to be easy. Local access is required to approach this attack. The technical details are unknown and an exploit is not available. The structure of the vulnerability defines a possible price range of USD $5k-$25k at the moment (estimation calculated on 01/16/2026). This vulnerability is assigned to T1068 by the MITRE ATT&CK project.

Applying a patch is able to eliminate this problem. A possible mitigation has been published immediately after the disclosure of the vulnerability.

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Productinfo

Type

Operating System

Vendor

Microsoft

Name

Windows

Version

License

commercial

Website

Vendor: https://www.microsoft.com/
Product: https://www.microsoft.com/en-us/windows

CPE 2.3info

CPE 2.2info

CVSSv4info

VulDB Vector: 🔒
VulDB Reliability: 🔍

CVSSv3info

VulDB Meta Base Score: 4.8
VulDB Meta Temp Score: 4.4

VulDB Base Score: 3.3
VulDB Temp Score: 2.9
VulDB Vector: 🔒
VulDB Reliability: 🔍

Vendor Base Score (Microsoft): 5.5
Vendor Vector (Microsoft): 🔒

CNA Base Score: 5.5
CNA Vector (Microsoft): 🔒

CVSSv2info

AV	AC	Au	C	I	A
💳	💳	💳	💳	💳	💳
💳	💳	💳	💳	💳	💳
💳	💳	💳	💳	💳	💳

Vector	Complexity	Authentication	Confidentiality	Integrity	Availability
Unlock	Unlock	Unlock	Unlock	Unlock	Unlock
Unlock	Unlock	Unlock	Unlock	Unlock	Unlock
Unlock	Unlock	Unlock	Unlock	Unlock	Unlock

VulDB Base Score: 🔒
VulDB Temp Score: 🔒
VulDB Reliability: 🔍

Exploitinginfo

Class: Access control
CWE: CWE-284 / CWE-266
CAPEC: 🔒
ATT&CK: 🔒

Physical: Partially
Local: Yes
Remote: No

Availability: 🔒
Status: Unproven

EPSS Score: 🔒
EPSS Percentile: 🔒

Price Prediction: 🔍
Current Price Estimation: 🔒

0-Day	Unlock	Unlock	Unlock	Unlock
Today	Unlock	Unlock	Unlock	Unlock

Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍

Countermeasuresinfo

Recommended: Patch
Status: 🔍

Reaction Time: 🔒
0-Day Time: 🔒
Exposure Time: 🔒

Timelineinfo

01/13/2026 VulDB entry created
01/13/2026 +0 days Advisory disclosed
01/13/2026 +0 days Countermeasure disclosed
01/16/2026 +3 days VulDB entry last update

Sourcesinfo

Vendor: microsoft.com
Product: microsoft.com

Advisory: msrc.microsoft.com
Status: Confirmed

CVE: CVE-2026-20839 (🔒)
GCVE (CVE): GCVE-0-2026-20839
GCVE (VulDB): GCVE-100-340774
scip Labs: https://www.scip.ch/en/?labs.20161215

Entryinfo

Created: 01/13/2026 19:26
Updated: 01/16/2026 11:33
Changes: 01/13/2026 19:26 (34), 01/13/2026 19:27 (1), 01/13/2026 19:30 (34), 01/16/2026 11:33 (12)
Complete: 🔍
Cache ID: 216::103