OXFORD, U.K. — 十二月 21, 2021 —

[Update: 12-24-2021, 7:35 am EST - Sophos originally tested the CAB-less exploit on an unpatched system for the research. Sophos later tested the CAB-less exploit on a testbed that had the September 2021 Cumulative Update. In the later test, the malicious Word document tried to connect to the external website hosting the exploit code, but the website was shut down, and we could not complete the test. The research article and press release have been updated accordingly. The updated press release copy is marked in bold.]

Sophos, a global leader in next-generation cybersecurity, today released details of a novel exploit that attempted to bypass a patch for a critical vulnerability (CVE-2021-40444) affecting the Microsoft Office file format. The findings are reported in a new SophosLabs Uncut article, “Attackers test ‘CAB-less 40444’ exploit in a dry run,” that shows how the attackers took a publicly available proof-of-concept Office exploit and weaponized it to deliver Formbook malware. The attackers then distributed it through spam emails for approximately 36 hours before it disappeared, according to Sophos researchers.

From CAB to “CAB-less” Exploit

In September 2021, Microsoft released a patch to prevent attackers from executing malicious code embedded in a Word document that downloads a Microsoft Cabinet (CAB) archive, which, in turn, contains a malicious executable. Sophos researchers discovered that attackers have reworked the original exploit by placing the malicious Word document inside a specially crafted RAR archive. The newer, “CAB-less” form of the exploit attempts to evade the original patch.

Sophos data shows that the amended exploit was used in the wild for around 36 hours. According to Sophos researchers, the limited lifespan of the updated attack could mean it was a “dry run” experiment that might return in future incidents.

“In theory, this attack approach shouldn't have worked, but it did,” said Andrew Brandt, principal threat researcher at Sophos. “The pre-patch versions of the attack involved malicious code packaged into a Microsoft Cabinet file. When Microsoft’s patch closed that loophole, attackers discovered a proof-of-concept that showed how you could bundle the malware into a different compressed file format, a RAR archive. RAR archives have been used before to distribute malicious code, but the process used here was unusually complicated. It likely succeeded only because the patch’s remit was very narrowly defined and because the WinRAR program that users need to open the RAR is very fault tolerant and doesn’t appear to mind if the archive is malformed, for example, because it’s been tampered with.”

The Infection Chain

Sophos researchers found that the attackers had created an abnormal RAR archive that had a PowerShell script prepending a malicious Word document stored inside the archive.

The attackers created and distributed spam emails that included the malformed RAR file as an attachment. The emails invited recipients to uncompress the RAR file to access the Word document.  Opening the Word document triggered a process that, on unpatched systems, ran the front-end script, leading eventually to an infection with Formbook malware.

“This research is a reminder that patching alone cannot protect against all vulnerabilities in all cases,” said Brandt. “Setting restrictions that prevent a user from accidentally triggering a malicious document can help to protect against such exploits, but people can still be lured into clicking the ‘enable content’ button. It is therefore vitally important to educate employees and remind them to be suspicious of emailed documents, especially when they arrive in unusual or unfamiliar compressed file formats from people or companies they don't know. If in doubt, always check with the sender or someone in IT.”

The CVE-2021-40444 vulnerability is a critical remote code execution (RCE) vulnerability that attackers can exploit to execute any code or commands on a target machine without the owner’s knowledge. Microsoft released an urgent mitigation followed by a patch in September. Sophos researchers found the 36 hours-campaign featuring the new exploit in late October.

Sophos endpoint products will detect the document files that contain the “CAB-less -40444” exploit as Troj/DocDL-AEOL.

For further information read the article on SophosLabs Uncut.

关于 Sophos

Sophos 是先进网络安全解决方案的全球领导者和创新者,包括托管式侦测与响应 (MDR) 和事件响应服务,各种端点、网络、电子邮件和云安全技术,帮助企业防御网络攻击。作为最大的纯网络安全供应商之一,Sophos 帮助全球超过 500,000 家企业和超过 1 亿用户抵御主动攻击对手、勒索软件、网络钓鱼、恶意软件等。Sophos 的服务和产品通过 Sophos Central 云管理控制台连接,并得到内部跨领域威胁情报部门 Sophos X-Ops 的支持。Sophos X-Ops 情报优化整个 Sophos Adaptive Cybersecurity Ecosystem 自适应网络安全生态体系,包括一个中央数据湖,为客户、合作伙伴、开发人员和其他网络安全与信息技术供应商提供一组丰富的开放 API。Sophos 为需要全托管并即使可用的安全解决方案的企业提供网络安全即服务,客户还可以直接利用 Sophos 的安全运行平台管理其网络安全,或者采用混合方法,为内部团队补充 Sophos 服务(包括威胁追踪与修复)。Sophos 通过世界各地的经销商合作伙伴和托管服务供应商 (MSP) 销售。Sophos 总部位于英国牛津。如欲了解更多信息,请访问 www.sophos.com.