What is CVE-2020-15778?
CVE-2020-15778 is a security vulnerability discovered in OpenSSH, a widely used tool for secure communication over a network using the SSH protocol. This vulnerability allows an attacker to execute arbitrary commands on a server when the “scp” command is used to copy files from a malicious server. It affects OpenSSH versions before 8.3p1, making it a notable risk for systems that rely on SSH for secure file transfers and remote administration.
CVSS Score and Severity
- CVSS Score: 7.5 (High)
- Severity: The CVSS score of 7.5 out of 10 indicates a high severity level, primarily due to the potential for remote code execution if a user is tricked into copying files from a malicious server using the vulnerable scp command.
So what’s the problem?
The core issue with CVE-2020-15778 is that it allows a malicious server to execute arbitrary commands on a client’s machine during an SCP (Secure Copy Protocol) operation. This can lead to remote code execution, enabling the attacker to take control of the client’s machine or perform other malicious activities. The vulnerability is particularly concerning because SCP is widely used for transferring files between systems in many automated scripts and administrative tasks, increasing the risk of exploitation.
Background and Context
Background on the vulnerability
CVE-2020-15778 was identified in mid-2020 during security reviews of the OpenSSH codebase. OpenSSH is a critical tool in the Unix/Linux ecosystem, providing encrypted communication between computers. The vulnerability arises from the way SCP handles filenames when copying files from a remote server. By crafting a malicious filename, an attacker can inject commands that are executed on the client’s machine when the file is copied, potentially leading to a full system compromise.
Description of the Vulnerability (CVE-2020-15778)
The vulnerability occurs in the SCP command of OpenSSH, which is used to securely copy files between hosts. The SCP client does not properly sanitize filenames received from the server, allowing a malicious server to include shell metacharacters in filenames. These characters can be interpreted as commands by the client’s shell, leading to the execution of arbitrary commands on the client machine. This flaw is a result of insufficient input validation and escaping of special characters in filenames.
Root Cause Analysis
The root cause of CVE-2020-15778 is the failure of the SCP command to properly sanitize filenames when copying files from a remote server. SCP relies on the shell to interpret filenames, and without proper escaping of special characters, these filenames can be interpreted as commands. This leads to a scenario where a malicious server can trick the client into executing arbitrary commands during the file transfer process.
Impact and Exploitation
The impact of CVE-2020-15778
Exploiting CVE-2020-15778 can have several serious impacts:
- Remote Code Execution: The most significant impact is the ability for an attacker to execute arbitrary commands on the client’s machine. This can lead to a complete system compromise, allowing the attacker to install malware, steal data, or further propagate the attack.
- Data Breach: If exploited, this vulnerability can be used to exfiltrate sensitive information from the client machine or to inject malicious files that can compromise the integrity of the system.
- Disruption of Services: Automated scripts and services that rely on SCP for file transfers can be disrupted or hijacked by an attacker, leading to broader operational impacts.
Exploit
To exploit CVE-2020-15778, an attacker must control or compromise a server that a client connects to using SCP. The exploitation process involves:
- Crafting a filename that includes shell metacharacters or commands.
- Hosting this malicious filename on a compromised or malicious server.
- Tricking the victim into using SCP to copy files from the malicious server.
- The malicious filename is interpreted as a command by the client’s shell, leading to the execution of the attacker’s payload on the client machine.
In-the-Wild Attacks
As of the last update, there have been no widely reported instances of CVE-2020-15778 being exploited in the wild. However, given the popularity of OpenSSH and the SCP command, the potential for exploitation remains high, especially in targeted attacks where attackers can control or spoof trusted servers.
Vulnerable code/package in the application
The vulnerability is found within the SCP command implementation in OpenSSH versions prior to 8.3p1. The affected code is responsible for handling filenames received from the server during a file transfer operation. The lack of proper input validation and escaping in this process is what makes the vulnerability exploitable.
Statistics on vulnerability
While exact statistics on the exploitation of CVE-2020-15778 are not available, it is known that a large number of systems worldwide rely on OpenSSH for secure communications. Systems that frequently use SCP to transfer files are particularly at risk if they have not been updated to a version of OpenSSH that includes the fix for this vulnerability.
Mitigation and Remediation
Mitigating CVE-2020-15778
Organizations and users can mitigate the risks associated with CVE-2020-15778 by:
- Updating OpenSSH: The most effective mitigation is to update OpenSSH to version 8.3p1 or later, where this vulnerability has been patched. This update ensures that SCP properly sanitizes filenames and prevents command injection.
- Avoiding Untrusted Servers: Users should avoid using SCP to copy files from untrusted or suspicious servers. If SCP must be used, carefully inspect filenames and consider using other methods for file transfer that do not rely on shell interpretation of filenames.
- Using Alternative Tools: Consider using more secure file transfer methods, such as SFTP (Secure File Transfer Protocol), which do not rely on shell commands and are less susceptible to such vulnerabilities.
Patch and Bypass: Fixes Added for CVE-2020-15778
The patch for CVE-2020-15778 involves changes to how SCP handles filenames received from remote servers. Specifically, the patch ensures that filenames are properly escaped and sanitized before being interpreted by the shell, thus preventing the execution of arbitrary commands. Organizations should ensure that all systems using OpenSSH are updated to the latest version to prevent exploitation.
Proactive response
A proactive approach includes regularly updating software, conducting security audits of remote access tools, and educating users about the risks of interacting with untrusted servers. Additionally, organizations should review and improve their use of SSH and SCP, potentially replacing them with more secure alternatives where appropriate.
Proof of Concept (POC)
A POC for CVE-2020-15778 can be demonstrated by crafting a malicious filename on a server to which an SCP client connects. For example, create a file with a name containing shell metacharacters:
echo "malicious content" > '`echo vulnerable`'When the SCP client copies this file, the shell command echo vulnerable is executed on the client’s system, demonstrating the vulnerability.
Real-world Impact and Response
Timeline/changelog
- June 2020: Discovery of CVE-2020-15778 during a security audit of OpenSSH.
- July 2020: Public disclosure of the vulnerability and release of OpenSSH version 8.3p1, which includes the fix.
- August 2020: Continued advisories and recommendations for users to update their OpenSSH installations.
- September 2020: Monitoring for potential exploitation and providing additional guidance on secure file transfer practices.
Observed Activity
While no significant in-the-wild exploitation of CVE-2020-15778 has been reported, security researchers and organizations are advised to remain vigilant. The vulnerability is particularly concerning for environments where SCP is used frequently, such as automated scripts and system administration tasks.
Mass Scanning
There have been no widespread reports of mass scanning specifically targeting CVE-2020-15778. However, the nature of the vulnerability makes it a potential target for targeted attacks against specific organizations or individuals who use SCP to interact with potentially compromised servers.
Vulnerable Server Discovery
Vulnerable systems can be discovered by attackers who control or compromise a server that users connect to with SCP. These servers can then serve malicious filenames to exploit the vulnerability. Users and administrators should be cautious when connecting to unknown or untrusted servers.
Reasoning and Scoring
Corporate networks impacted globally
CVE-2020-15778 has the potential to impact corporate networks globally, especially in environments where OpenSSH is heavily relied upon for secure communications. The vulnerability can be exploited to gain unauthorized access to systems, potentially leading to broader network compromises.
Corporate numbers impacted by countries
- United States: Widespread use of OpenSSH in enterprise environments, with many organizations at risk.
- Europe: Significant adoption of OpenSSH in technology and finance sectors, leading to potential exposure.
- Asia: Extensive use of OpenSSH in telecommunications and IT services, where secure file transfers are critical.
Conclusion
Who should be paying attention to this?
System administrators, cybersecurity professionals, and organizations that use OpenSSH and SCP for file transfers should prioritize attention to CVE-2020-15778. Ensuring that these tools are secure is critical for maintaining the overall security of remote communications and file transfer operations.
Who is exploiting it and how?
While no widespread exploitation has been reported, attackers could exploit CVE-2020-15778 by controlling or compromising a server that the victim connects to with SCP. The attacker would then deliver malicious filenames designed to execute arbitrary commands on the victim’s machine.
How are things likely to develop?
As organizations apply patches, the likelihood of widespread exploitation diminishes. However, environments that remain unpatched are at significant risk. It is essential for users and administrators to stay vigilant and maintain up-to-date software to prevent potential exploitation.
How long has it been around?
CVE-2020-15778 was discovered and disclosed in mid-2020, but the underlying issue in SCP’s handling of filenames may have existed for years before its discovery. This highlights the importance of ongoing security reviews and the need for robust input validation practices in software development.
