A new wave of malware, dubbed UULoader, has been identified targeting users via weaponized PDF documents, with a particular focus on Korean and Chinese speakers. This sophisticated malware disguises itself as legitimate software through malicious .msi installers. It employs advanced techniques to evade detection, including DLL side-loading and the removal of file headers from its core components. By stripping these identifiers, UULoader’s executables become nearly invisible to static analysis tools, allowing the malware to masquerade as harmless data and evade scrutiny until it is executed.
The UULoader malware operates through a complex multi-phase delivery mechanism. It packages its malicious components within a .cab archive and uses heavily obfuscated payloads. During execution, UULoader employs a legitimate Realtek executable as a side-loader for another stripped DLL, complicating detection and analysis. Additionally, the malware includes tiny files with “M” and “Z” characters to repair stripped headers and ensure successful execution of the payloads. This layered obfuscation technique significantly hampers static detection and classification efforts.
To further avoid detection, UULoader often uses a decoy tactic, including legitimate files alongside malicious components. For instance, it might disguise itself as a “Chrome update,” embedding an authentic Chrome updater to divert attention from its malicious activities. The malware also creates directories and deploys obfuscated payloads through a .vbs script, which excludes the newly created directory from Windows Defender protection and processes extracted files. This script incorporates irrelevant arithmetic to obscure the malicious code further, enhancing its stealth capabilities.
The UULoader threat is notable for its exceptionally low detection rates on security platforms like VirusTotal, underscoring its effective evasion strategies. The malware’s ability to deploy payloads such as Gh0stRat and Mimikatz indicates a potential risk of remote access and credential theft. Although the exact origin of UULoader remains undetermined, its characteristics suggest a possible link to Chinese threat actors. This sophisticated malware highlights the ongoing challenges in cybersecurity, particularly in combating advanced evasion techniques and ensuring effective threat detection and response.
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