In a recent investigation involving the Remote Desktop Protocol (RDP), cybersecurity analysts gained extraordinary insights into an attacker’s lateral movements through bitmap cache analysis.
This case underscores how seemingly mundane technical mechanisms can yield crucial forensic evidence for defending compromised systems.
The attackers initially leveraged RDP a widely used protocol facilitating remote access to computer desktops for lateral movement across the environment.
This activity was aligned with MITRE ATT&CK techniques T1570 and T1021.
By exploiting RDP’s bitmap cache, a performance optimization feature that caches graphical screen tiles, analysts were able to reconstruct portions of the attackers’ activity.
This allowed the team to view desktop screens as the attackers saw them and retrieve sensitive details, such as commands typed and applications used.
The bitmap cache, stored locally on the initiating host, holds fragmented screen data to minimize bandwidth requirements.
By analyzing this artifact, investigators extracted visual traces of the attacker’s activity, providing a first-person perspective that complemented traditional logs and network evidence.
Unpacking the Bitmap Cache Artifacts
Bitmap caches save graphical elements from RDP sessions in either bcache.bmc or Cache.bin file formats, depending on the Windows version.
On modern systems, cache files, such as Cache0000.bin, store 64×64-pixel tiles in 32-bit-per-pixel (BPP) format. These files use cryptographic keys to manage persistent caching across RDP sessions.
With tools like BMC-Tools, developed by the French National Agency for Information Systems Security (ANSSI), these tiles can be extracted as BMP images.
However, interpreting the cache data presents challenges, including the absence of metadata like screen location and time stamps.
By stitching together extracted tiles using software like RdpCacheStitcher, developed by Germany’s BSI (Federal Office for Information Security), analysts reconstructed partial screen views.
In this case, investigators identified significant details, including active commands (who, whoami), file downloads using certutil.exe, private browser sessions, and suspicious file transfers such as a svchost.exe file to a public directory.
While incomplete, these fragments provided crucial breadcrumbs on attacker behavior.
Real-World Applications and Limitations
According to the Insinuator report, the use of RDP bitmap caches in investigations has some inherent constraints.
Cache data is stored only on the initiating host, not the target system, restricting access if the attacker’s machine isn’t retrieved.
Moreover, cache files capture only fragments of on-screen content based on optimization policies, leaving portions of an attacker’s activity undocumented.
Despite these challenges, reconstructed screen segments can reveal invaluable contextual data, such as accessed directories, ongoing RDP connections, and system configurations.
In real-world incidents, analysts have successfully uncovered Indicators of Compromise (IoCs) and reconstructed pivotal actions undertaken by adversaries.
For example, details like URLs used for malicious downloads and specific applications accessed helped paint a clearer picture of the adversary’s objectives.
The forensic utility of RDP bitmap caches, while not exhaustive, adds a powerful dimension to incident response.
When correlated with network logs, registry data, and event logs, these artifacts offer a deeper understanding of attacker workflows.
As highlighted in this investigation, careful examination of such overlooked data sources can bolster detection and response strategies, particularly in lateral movement scenarios.
This case demonstrates the importance of leveraging innovative methodologies to counter increasingly sophisticated cyberattacks, emphasizing that sometimes the most vital clues lie in the least-expected places.
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