What is CVE-2020-0796?
CVE-2020-0796, also known as “SMBleed” or “EternalDarkness,” is a critical vulnerability in the Microsoft Server Message Block 3.1.1 (SMBv3) protocol. This vulnerability allows remote attackers to execute arbitrary code on vulnerable systems by sending specially crafted packets to a targeted SMBv3 server or client. It affects Windows 10, Windows Server versions 1903, and 1909, making it a significant threat to any organization using these versions of Windows.
CVSS Score and Severity
- CVSS Score: 10.0 (Critical)
- Severity: The CVSS score of 10.0 out of 10 indicates the highest level of severity, reflecting the critical impact of this vulnerability. It is classified as “Critical” due to its potential for remote code execution without authentication, which could lead to complete system compromise.
So what’s the problem?
CVE-2020-0796 is a major issue because it allows attackers to remotely execute code on vulnerable systems without any user interaction. The flaw exists in the SMBv3 protocol, which is widely used for file sharing and network communication in Windows environments. If exploited, this vulnerability could allow attackers to gain control of affected systems, spread malware, or launch further attacks within the network. Given its ease of exploitation and the severity of its impact, CVE-2020-0796 is a highly dangerous vulnerability for organizations that have not applied the necessary patches.
Background and Context
Background on the vulnerability
CVE-2020-0796 was discovered in March 2020 and is related to the handling of compressed data in the SMBv3 protocol. SMB is a network file-sharing protocol that allows applications to read and write to files and request services from server programs in a computer network. The vulnerability is caused by a buffer overflow when the server or client processes a specially crafted packet, which can be exploited to execute arbitrary code on the target system. The vulnerability is similar in its potential impact to the “EternalBlue” exploit used in the WannaCry ransomware attacks.
Description of the Vulnerability (CVE-2020-0796)
The vulnerability occurs due to a flaw in how the SMBv3 protocol handles certain requests. Specifically, when the SMBv3 server or client processes a maliciously crafted packet, it fails to properly handle compressed data, leading to a buffer overflow. This buffer overflow can then be exploited by an attacker to execute arbitrary code on the target system. The flaw can be triggered remotely, without requiring authentication, making it an extremely dangerous vulnerability for unpatched systems.
Root Cause Analysis
The root cause of CVE-2020-0796 is the improper handling of compressed data by the SMBv3 protocol in Windows. The vulnerability arises because the protocol does not correctly validate the size of the buffer allocated for compressed data, leading to a buffer overflow condition. This allows an attacker to overwrite memory and execute arbitrary code on the affected system, potentially leading to full system compromise.
Impact and Exploitation
The impact of CVE-2020-0796
Exploiting CVE-2020-0796 can have several devastating impacts:
- Remote Code Execution: The most severe impact is the ability for an attacker to execute arbitrary code remotely, potentially leading to full control over the affected system.
- Wormable Exploits: Like the EternalBlue vulnerability, CVE-2020-0796 has the potential to be used in wormable attacks, where malware can spread automatically from one vulnerable system to another, rapidly infecting an entire network.
- System and Network Compromise: A successful exploitation could lead to the complete compromise of the targeted system and potentially the entire network, allowing attackers to steal data, deploy ransomware, or disrupt operations.
Exploit
To exploit CVE-2020-0796, an attacker needs to send a specially crafted packet to a vulnerable SMBv3 server or client. The exploitation process involves:
- Identifying a target system running a vulnerable version of Windows with SMBv3 enabled.
- Crafting a packet that includes malicious data designed to trigger the buffer overflow in the SMBv3 protocol.
- Sending the packet to the target system, which processes the compressed data and executes the attacker’s code, leading to system compromise.
In-the-Wild Attacks
Since its disclosure, CVE-2020-0796 has been a target for exploitation in the wild. Attackers have attempted to leverage this vulnerability to gain unauthorized access to systems, deploy malware, and execute remote code. Although widespread exploitation akin to the WannaCry outbreak has been mitigated by timely patching and awareness, the vulnerability remains a significant threat to unpatched systems.
Vulnerable code/package in the application
The vulnerable code is located within the SMBv3 protocol implementation in Windows 10, Windows Server versions 1903, and 1909. The flaw specifically relates to the handling of compressed data, where a buffer overflow can occur if the data is not correctly validated before being processed.
Statistics on vulnerability
While specific exploitation statistics are not widely available, the potential impact of CVE-2020-0796 is significant, especially for organizations that use SMBv3 for file sharing and network communication. The vulnerability is particularly concerning for environments where SMBv3 is exposed to the internet or used extensively within internal networks.
Mitigation and Remediation
Mitigating CVE-2020-0796
Organizations can mitigate the risks associated with CVE-2020-0796 by:
- Applying Patches: Microsoft has released patches to address this vulnerability. It is critical to update all affected systems to the latest software versions to mitigate this issue.
- Disabling SMBv3 Compression: As a temporary workaround, SMBv3 compression can be disabled to prevent the exploitation of this vulnerability. However, this should be combined with applying the patch for full protection.
- Network Segmentation: Limiting the exposure of SMBv3 to trusted networks and implementing network segmentation can reduce the risk of exploitation.
Patch and Bypass: Fixes Added for CVE-2020-0796
Microsoft’s patches for CVE-2020-0796 address the buffer overflow issue in SMBv3 by ensuring that the protocol correctly handles compressed data. The patch also includes additional security checks to prevent similar vulnerabilities. Organizations should ensure that all affected systems are updated to the latest version to avoid potential exploitation.
Proactive response
A proactive security approach includes regularly updating software, disabling unnecessary services (such as SMBv3 if not needed), and conducting security assessments to identify and mitigate potential vulnerabilities before they can be exploited. Organizations should also monitor for any signs of attempted exploitation.
Proof of Concept (POC)
A POC for CVE-2020-0796 involves sending a specially crafted SMB packet to the target server using a tool like smbclient or a custom script:
bash –
smbclient //target_ip/share -U% -p 445If vulnerable, the server will process the malformed packet, potentially leading to memory disclosure or remote code execution.
Real-world Impact and Response
Timeline/changelog
- March 2020: Discovery and public disclosure of CVE-2020-0796.
- March 2020: Microsoft releases patches to address the vulnerability.
- April 2020: Increased awareness and guidance provided by security organizations to apply patches and secure networks.
- May 2020: Continued monitoring for potential exploitation and providing additional security updates.
Observed Activity
Since its disclosure, CVE-2020-0796 has been observed being targeted by attackers in various campaigns. While large-scale outbreaks like WannaCry have been avoided, isolated incidents of exploitation have occurred, particularly targeting organizations that have not applied the necessary patches.
Mass Scanning
Following the disclosure of CVE-2020-0796, mass scanning for vulnerable systems increased significantly. Attackers used automated tools to identify systems running vulnerable versions of SMBv3, which were then targeted for exploitation.
Vulnerable Server Discovery
Vulnerable systems can be discovered through targeted scanning or by analyzing network traffic for signs of SMBv3 usage. Systems that expose SMBv3 to the internet or are not properly segmented are at high risk of exploitation.
Reasoning and Scoring
Corporate networks impacted globally
CVE-2020-0796 has the potential to impact corporate networks globally, particularly in industries that rely heavily on Windows-based file sharing and network services. The vulnerability can be exploited to gain unauthorized access to sensitive information, disrupt operations, or deploy malware across an entire network.
Corporate numbers impacted by countries
- United States: Widespread use of Windows 10 and Windows Server in enterprise environments, with many organizations at risk.
- Europe: Significant adoption of Windows in technology, finance, and healthcare sectors, leading to potential exposure.
- Asia: Extensive use of Windows in various industries, where secure network operations are critical.
Conclusion
Who should be paying attention to this?
System administrators, cybersecurity professionals, and organizations that use Windows 10 or Windows Server versions 1903 and 1909 should prioritize attention to CVE-2020-0796. Ensuring that systems are patched and secure is critical for maintaining the overall security of network services and preventing potential exploitation.
Who is exploiting it and how?
CVE-2020-0796 has been exploited by attackers who identify systems running vulnerable versions of SMBv3. These attackers craft malicious packets designed to trigger the buffer overflow in the SMBv3 protocol, leading to remote code execution and system compromise.
How are things likely to develop?
As more organizations apply patches and secure their systems, the risk of widespread exploitation decreases. However, systems that remain unpatched are still vulnerable to attack, and the potential for wormable exploits similar to WannaCry remains a concern. Continuous vigilance and adherence to security best practices are essential to prevent exploitation.
How long has it been around?
CVE-2020-0796 was discovered and disclosed in March 2020, but the underlying flaw in SMBv3 may have existed for some time before its discovery. This highlights the importance of regular security assessments and the timely application of patches to address potential vulnerabilities.
