Introduction
The exploitation of ZIP file concatenation by attackers poses a significant threat to cybersecurity. This technique allows malicious actors to bypass traditional security measures by combining multiple ZIP files into a single archive, exploiting the varying interpretations of these files by different software [7]. This method is particularly effective in delivering malware, such as the SmokeLoader Trojan, through phishing attacks [1] [2] [4] [6].
Description
Attackers are exploiting the flexible structure of ZIP archives through a technique known as ZIP file concatenation, which allows them to combine multiple ZIP files into a single file. This anti-detection tactic enables them to bypass traditional security software by taking advantage of the varying interpretations of concatenated files by different ZIP readers and archive managers. As a result, malicious content can remain undetected by standard security tools [7], particularly on Windows machines.
This method is frequently employed to deliver various strains of Trojan malware, prominently including the SmokeLoader Trojan, which utilizes the AutoIt scripting language to facilitate the execution of additional threats. In a recent phishing attack [3], an email purportedly from a shipping company leveraged a sense of urgency to lure users into opening a ZIP file disguised as a legitimate RAR file. The attachment [2] [5] [7], named SHIPPINGINVPLBLpdf.rar [5] [7], appeared harmless when opened with 7zip [5], which only read the first ZIP archive [2]. However, when accessed through tools like WinRAR or Windows File Explorer [7], it revealed a malicious executable [7], SHIPPINGINVPLBLpdf.exe [4] [5] [7], designed to automate malicious tasks such as downloading and executing additional payloads [4], which may include other types of malware like banking trojans or ransomware [4].
Different archiver programs handle these concatenated files in various ways: 7zip reads only the first file, while WinRAR displays all files, including the malicious ones, and Windows File Explorer may not open the file correctly. This variability enables threat actors to effectively distribute malware while evading detection. Victims are typically lured into downloading and executing these attachments through phishing emails that often contain warnings about pending invoices or undelivered parcels. The use of modified ZIP files raises significant cybersecurity concerns, as a single infected ZIP file can compromise the privacy and integrity of sensitive data, particularly for less experienced users.
The effectiveness of ZIP file concatenation lies in its ability to exploit the limitations of common ZIP tools [7], which may not fully scan concatenated archives for hidden threats [7]. Traditional detection tools frequently struggle to unpack these complex ZIP files [2], underscoring the importance of employing security solutions capable of recursive unpacking and enforcing strict email filtering policies for ZIP and RAR files. The rise of remote work has further increased vulnerabilities, making robust protective measures essential to detect and neutralize emerging threats. Security researchers have reported these issues to developers [5], confirming that the exploitation method remains viable and highlighting the need for enhanced protective measures against these sophisticated attacks. Recent analyses have further emphasized the prevalence of this technique in delivering malicious payloads, particularly through deceptive phishing tactics.
Conclusion
The use of ZIP file concatenation by cybercriminals underscores the need for advanced security measures to protect against evolving threats. Organizations must adopt comprehensive security solutions that can effectively scan and unpack complex archives. Additionally, implementing strict email filtering policies and educating users about the risks of phishing attacks are crucial steps in mitigating these threats. As remote work continues to expand, the importance of robust cybersecurity practices becomes even more critical to safeguard sensitive information and maintain data integrity.
References
[1] https://lifeboat.com/blog/2024/11/hackers-now-use-zip-file-concatenation-to-evade-detection
[2] https://www.techradar.com/pro/security/windows-machines-are-being-targeted-with-zip-file-workaround
[3] https://www.techepages.com/hackers-now-use-zip-file-concatenation-to-evade-detection/
[4] https://zephyrnet.com/el/%CE%B5%CF%85%CE%AD%CE%BB%CE%B9%CE%BA%CF%84%CE%B7-%CE%B4%CE%BF%CE%BC%CE%AE-%CE%B1%CF%81%CF%87%CE%B5%CE%AF%CF%89%CE%BD-zip-%CF%80%CE%BF%CF%85-%CF%87%CF%81%CE%B7%CF%83%CE%B9%CE%BC%CE%BF%CF%80%CE%BF%CE%B9%CE%B5%CE%AF%CF%84%CE%B1%CE%B9-%CE%B3%CE%B9%CE%B1-%CF%84%CE%B7%CE%BD-%CE%B1%CF%80%CF%8C%CE%BA%CF%81%CF%85%CF%88%CE%B7-%CE%BA%CE%B1%CE%BA%CF%8C%CE%B2%CE%BF%CF%85%CE%BB%CE%BF%CF%85-%CE%BB%CE%BF%CE%B3%CE%B9%CF%83%CE%BC%CE%B9%CE%BA%CE%BF%CF%8D-%CF%80%CE%BF%CF%85-%CE%B4%CE%B5%CE%BD-%CE%AD%CF%87%CE%B5%CE%B9-%CE%B5%CE%BD%CF%84%CE%BF%CF%80%CE%B9%CF%83%CF%84%CE%B5%CE%AF/
[5] https://www.techmonitor.ai/technology/cybersecurity/perception-point-new-zip-file-concatenation-exploit
[6] https://www.darkreading.com/threat-intelligence/flexible-structure-zip-archives-exploited-hide-malware-undetected
[7] https://cybermaterial.com/zip-concatenation-evades-windows-security/
