CVE-2023-38146 Detection: Windows “ThemeBleed” RCE Bug Poses Growing Risks with the PoC Exploit Release

The new Microsoft Windows Themes security bug tracked as CVE-2023-38146, which enables attackers to perform RCE, emerges in the cyber threat arena. The proof-of-concept (PoC) exploit for this vulnerability, also known as “ThemeBleed,” has recently been released on GitHub, posing a threat to potentially infected Windows instances and arresting the attention of defenders.

CVE-2023-38146 Detection

Having a PoC exploit publicly released on the web, adversaries turn their sight to weaponize Windows ThemeBleed vulnerability enabling remote code execution on the affected instances. To detect suspicious activity associated with CVE-2023-38146 exploitation attempts on time, SOC Prime Platform aggregates a set of relevant Sigma rules. All detections are compatible with leading SIEM, EDR, XDR, and Data Lake technology formats and aligned with MITRE ATT&CK® framework to streamline threat hunting procedures. 

To explore the full list of curated rules and smooth the deep dive into the CVE-2023-28146 threat, hit the Explore Detections button below. Security professionals can reach extensive cyber threat context accompanied by ATT&CK references and CTI links and obtain more insightful metadata matching current security needs and boosting threat investigation.

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CVE-2023-38146 Analysis

The newly discovered Windows Themes vulnerability identified as CVE-2023-38146, aka ThemeBleed, with a high CVSS score reaching 8.8, can lead to arbitrary code execution. With the ThemeBleed PoC exploit publicly available on GitHub, the flaw requires immediate attention to timely identify the threat. 

On September 12, 2023, Microsoft covered the details of the potential CVE-2023-38146 exploitation attempts. The infection chain is triggered by loading a weaponized THEME file on a compromised system with access to an attacker-controlled SMB share. 

The researcher Gabe Kirkpatrick who was the first to report ThemeBleed and the developer of the PoC code has covered the in-depth attack details. Leveraging the version number “999” creates a significant time gap in the process of verifying the DLL signature and library load within the routine for handling the MSSTYLES file, which can cause the emergence of a race condition. Further on, by applying the specifically generated MSSTYLES file, adversaries can exploit a race window to apply malicious DLL instead of a verified one, which enables them to run arbitrary code on the impacted system. In addition, the researcher adds that downloading a malicious Windows Theme file from the web triggers the ‘mark-of-the-web’ warning, which can notify a user of the potential threat. However, adversaries can bypass this alert by wrapping the theme into a THEMEPACK archive file.

Microsoft addressed security updates for CVE-2023-38146 in the recent September 2023 Patch Tuesday by removing the “version 999” functionality. However, Kirkpatrick points out that the fundamental race condition still exists while posing potential risks to targeted users. In addition, an absence of mark-of-the-web warnings for THEMEPACK files still needs addressing to prevent adversaries from evading security protection measures.

To mitigate the threat, defenders recommend applying the latest Microsoft security updates pack addressing CVE-2023-38146 along with 50+ other bugs, removing the “version 999” functionality, as well as preventing the loading of resources from remote shares within Windows theme files.

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