Impacted organisations include financial services and critical infrastructure providers

On 29 March 2022, security researchers posted a now-removed screenshot to Twitter purporting to show a trivially-exploited unauthenticated remote code execution (RCE) vulnerability in the Spring Framework, one of the most popular Java frameworks in use globally.[1] While the screenshot did not include a proof of concept or public details, Proof of Concepts dubbed “SpringShell” or “Spring4Shell” have since emerged since 30 March 2022 and have been validated by DarkLab to be working exploits.[2]

The Spring Framework is among the most widely used lightweight open source framework for Java, as a result of its design philosophy that enables developers to focus on business logic, while simplifying the development cycle of Java enterprise applications.[3] Given its widespread use globally, the nature of the vulnerability being more general such that there may be unknown and additional ways of exploiting it, the impact of this vulnerability is compounded significantly and would be in excess of the impact observed for infamous vulnerabilities such as Log4Shell (CVE-2021-44228).

Technical Analysis

Based on analysis on consolidated data source and technical analysis, DarkLab has been able to recreate the attack in a simulated environment. In order to exploit this vulnerability, an unauthenticated attacker must send a crafted HTTP request to trigger the mechanisms through parameter binding functions of the framework to achieve arbitrary file write, with calls to specific Java ‘classLoader’/’pipeline’ functions. It is likely that the Spring Framework does not handle these calls properly, allowing for arbitrary writing of the JSP web shell to the root directory of the server, which can then be interacted with for unauthenticated remote code execution.

DarkLab has been actively performing discovery using our proprietary PoC since 30 March 2022.  As a result of conducting the scan across all external facing applications in Hong Kong and Macau, we observed that over thousands of organisations – including financial services and critical infrastructure providers – are potentially vulnerable to the unauthenticated RCE vulnerability. At the time of writing, the scope of impacted organisations and the broader implications of exploitation are still being estimated and not fully known, as it depends on whether particular functions are used within the Spring application.[4] The general nature of the vulnerability implies there may be other still undiscovered methods to exploit it.

Probability of Exploitation by Threat Actors

Given that this is an unauthenticated RCE vulnerability in the widely-adopted Spring Framework, it is likely that it will present an attractive exploit for a variety of threat actors to weaponize and add to their arsenal for the purpose of obtaining initial access to unsuspecting victims’ systems.

Per DarkLab’s Deep and Dark Web monitoring, we observed on 29 March 2022 that English-speaking threat actors had exchanged messages via Telegram requesting for a working exploit code. While we are unable to ascertain with confidence whether they obtained this information through communication exchange, we observed clear intent from these threat actors to leverage the unauthenticated RCE vulnerability to perform malicious activities against a specific range of targets. This includes exfiltrating sensitive personally identifiable information from South Asian state-owned enterprises, which suggests that these threat actors have a more targeted mindset and are capable of directing their attention to the observed vulnerable organisations in Hong Kong and Macau should it align with their objectives.

While there has not been active exploitation in the wild for Spring4Shell, we posit that threat actors of various objectives – ranging from espionage to financial motivation – will continue to invest resources to explore how best to weaponise the vulnerability to achieve their goals. DarkLab will continue to monitor the Deep and Dark Web for more insights on their innovations and targeting and provide updates as necessary.

Conclusion

In summary, this unauthenticated RCE vulnerability in the widely-adopted Spring Core makes it an attractive proposition for threat actors of all profiles and motivations. In particular, the general nature of the vulnerability implies there may be other ways to exploit it. As a result, we expect threat actors of all motivations will invest resources to innovate new techniques; until then, detection opportunities will remain limited. This implies that teams should first rely on their defense-in-depth security controls to mitigate the known risks, while continuing to track the status of this vulnerability regarding preventive and detective controls as they become publicly available.

Recommendations

Organisations using affected versions 5.3.x should upgrade to 5.3.18+, while versions 5.2.x should upgrade to 5.2.20+. However, there are other workaround solutions for applications that cannot upgrade to the above versions as listed on the Spring blog post.[5]

From a detection perspective, exploitation attempts will require HTTP requests making use of Java classes. As such, filtering for strings such as “class.“, “Class.“, “.class.“, and “.Class.” may detect exploitation attempts.

In addition, we strongly urge our clients to consider the following:

• Review their application stack to ascertain the scope of impact in preparation for the impending patch to be released.
• Monitor the official Spring vulnerability report [6] or Git repository for further updates to the patch releases and apply accordingly.[7]
• Leverage cyber threat intelligence to monitor for further updates to the threat landscape as a result of new information pertaining to the unauthenticated RCE vulnerability.

MITRE ATT&CK TTPs Leveraged

• Initial Access: Exploit Public-Facing Application (T1190)
• Execution: Exploitation for Client Execution (T1203)
• Persistence: Server Software Component – Web Shell (T1505.003)
• Command and Control: Application Layer Protocol – Web Protocols (T1071.001)

Feel free to contact us at [darklab dot cti at hk dot pwc dot com] for any further information.