CVE-2024-38030

What it actually does

This vulnerability allows a crafted Windows theme-related file or reference to cause Windows to access attacker-controlled remote resources, potentially disclosing NTLM authentication material. The flaw requires user interaction, such as opening, previewing, or otherwise processing a malicious theme file or theme package. It is primarily a confidentiality issue: successful exploitation can expose credential challenge-response data that may be cracked offline or relayed to other services if NTLM is permitted. The supplied affected-package list appears inconsistent with the Windows Themes description; validate product applicability against authoritative vendor advisories before relying on Linux package mappings.

Why it matters to the business

An attacker who obtains reusable Windows authentication material may be able to impersonate users, access internal file shares or applications, and move laterally through the organization. Even without direct malware execution, credential exposure can lead to account compromise, data theft, and follow-on ransomware or intrusion activity if NTLM relay paths exist. The issue is not listed in CISA KEV and no ransomware association is provided, but the EPSS value is elevated, so organizations should treat exposure as a practical phishing-driven credential theft risk. If the listed Debian or Ubuntu kernel packages are in scope for separate vendor advisories, they should also be updated, but they do not align with the Windows Themes vulnerability description.

How an attack unfolds

An attacker prepares a malicious Windows theme file, theme package, shortcut, archive, or document-delivered attachment that references an external UNC, SMB, or WebDAV resource under attacker control. The attacker then convinces a user to open or preview the content through phishing, a malicious download, a shared drive, or a collaboration platform. When Windows processes the theme metadata or associated resource path, it attempts to retrieve the remote resource and may automatically authenticate using NTLM. The attacker captures the NetNTLM challenge-response or relays the authentication attempt to another service that accepts NTLM. If relay succeeds, the attacker may gain access as the victim user; if capture succeeds, the attacker may attempt offline cracking and reuse recovered credentials elsewhere.

How to fix it

Apply the relevant Microsoft security updates for the Windows Themes spoofing vulnerability across supported Windows systems. Disable or restrict NTLM where feasible, especially outbound NTLM to untrusted networks, and enforce SMB signing/EPA on services that still require NTLM to reduce relay impact. Block outbound SMB traffic to the Internet, particularly TCP 445 and 139, and restrict WebDAV where not required. Harden email and web gateways to block or detonate theme-related attachments such as .theme, .themepack, .deskthemepack, and archives containing them. Educate users not to open unexpected theme files or visual customization packages. If the provided Linux package data is independently confirmed relevant to a separate advisory, update to at least: Debian 11 linux 5.10.70-1; Debian 12/13 linux 5.14.6-1; Ubuntu 20.04 linux-aws-5.15 5.15.0-1014.18~20.04.1, linux-azure-5.15 5.15.0-1007.8~20.04.1, linux-azure-fde-5.15 5.15.0-1019.24~20.04.1.1, linux-gcp-5.15 5.15.0-1006.9~20.04.1, linux-gkeop-5.15 5.15.0-1003.5~20.04.2, linux-hwe-5.15 5.15.0-33.34~20.04.1, linux-ibm-5.15 5.15.0-1033.36~20.04.1, linux-intel-iotg-5.15 5.15.0-1003.5~20.04.1, linux-iot 5.4.0-1001.3, linux-lowlatency-hwe-5.15 5.15.0-33.34~20.04.1, linux-oracle-5.15 5.15.0-1007.9~20.04.1, linux-riscv-5.15 5.15.0-1015.17~20.04.1, linux-xilinx-zynqmp 5.4.0-1020.24; Ubuntu 22.04 linux 5.13.0-19.19, linux-aws 5.13.0-1005.6, linux-aws-6.5 6.5.0-1008.8~22.04.1, linux-azure 5.13.0-1006.7, linux-azure-6.5 6.5.0-1007.7~22.04.1, and linux-azure-fde 5.15.0-1019.24.1.

How to confirm the fix worked

Confirm Windows endpoints have the applicable Microsoft security update installed by checking OS build and cumulative update level against Microsoft’s advisory. Validate that outbound SMB to the Internet is blocked by attempting controlled egress tests to TCP 445 and 139 from representative network segments. Verify that NTLM restrictions, SMB signing, and Extended Protection for Authentication are enabled where applicable. Search endpoint inventories for theme-related file types received from external sources and inspect suspicious files for remote UNC or WebDAV paths. For the provided Linux package list, if applicable, confirm installed package versions meet or exceed the listed fixed versions using native package managers such as dpkg, apt, or vulnerability scanners.

Typical attack path

1. Create a crafted Windows theme, theme package, or related file that references an attacker-controlled remote SMB, UNC, or WebDAV path.
2. Deliver the file to a target user through phishing, a download link, a compressed archive, a file share, or a collaboration tool.
3. Wait for the user to open, preview, extract, or otherwise cause Windows to process the theme content.
4. Capture the outbound NTLM authentication attempt on attacker-controlled infrastructure or relay it to an internal or externally reachable service.
5. Use relayed access, cracked credentials, or captured authentication material to access resources, escalate privileges, or move laterally.

How to detect it

Review email security logs, web proxy logs, EDR telemetry, and file-system events for delivery or execution of .theme, .themepack, .deskthemepack, or archive files containing theme files. Inspect suspicious theme files for remote paths beginning with \\, file://, smb://, or WebDAV-style URLs. On endpoints, look for explorer.exe or related shell processes initiating outbound SMB, WebDAV, or HTTP requests shortly after a user opened a theme-related file. In network logs, hunt for outbound TCP 445, 139, or WebDAV traffic to public IP addresses or newly observed domains, especially where NTLMSSP authentication is present. In Windows Security logs, investigate NTLM network logons, unusual Event ID 4624 Type 3 activity, authentication failures, and access to resources from unexpected source systems following the suspected exposure. In domain controller logs, review NTLM authentication events and anomalous account usage after outbound SMB/WebDAV attempts. If packet capture, proxy, or IDS telemetry is available, search for NTLMSSP negotiation to unfamiliar destinations. Also look for signs that captured credentials were used later: impossible travel, first-time access to file shares, unusual administrative actions, remote service creation, scheduled tasks, or lateral movement from accounts that received suspicious theme files.

• *.theme
• *.themepack
• *.deskthemepack
• %LOCALAPPDATA%\Microsoft\Windows\Themes\*.theme
• %APPDATA%\Microsoft\Windows\Themes\*.theme
• Wallpaper=\\\\
• IconFile=\\\\
• VisualStyle=\\\\
• BrandImage=\\\\
• ScreenSaveActive
• NTLMSSP
• Authorization: NTLM
• WWW-Authenticate: NTLM
• TCP/445 outbound
• TCP/139 outbound
• WebClient
• explorer.exe outbound SMB
• explorer.exe outbound WebDAV
• Event ID 4624 Logon Type 3
• Event ID 4776
• Microsoft-Windows-SmbClient
• Microsoft-Windows-WebClnt

What your security team should do

• Patch affected Windows systems with the relevant Microsoft security updates and verify installation across workstation fleets.
• Block outbound SMB traffic to untrusted networks and monitor for failed or blocked attempts to external TCP 445 and 139 destinations.
• Reduce NTLM exposure by disabling NTLM where possible, restricting outbound NTLM, requiring SMB signing, and enabling Extended Protection for Authentication on services that support it.
• Add mail, proxy, EDR, and file-content detections for .theme, .themepack, and .deskthemepack files containing UNC, SMB, or WebDAV references.
• Hunt for recent outbound NTLM authentication attempts to unfamiliar hosts, especially immediately after users received or opened theme-related files.
• Review authentication logs for suspicious NTLM logons, relay-like access patterns, and lateral movement following outbound SMB or WebDAV activity.
• Validate the supplied affected-product list against vendor advisories because it appears to describe Linux kernel packages rather than Windows Themes.