CVE-2026-74997 in Roundcube
Summary
by MITRE • 08/17/2026
In Roundcube Webmail before 1.6.18 and 1.7.x before 1.7.3, the cmd_learn driver of the markasjunk plugin is subject to remote code execution via crafted placeholder replacement values. This issue only affects Roundcube instances using the markasjunk plugin with its cmd_learn driver.
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Analysis
by VulDB Data Team • 08/17/2026
The vulnerability identified in Roundcube Webmail versions prior to 1.6.18 and 1.7.x prior to 1.7.3 represents a critical remote code execution flaw within the markasjunk plugin, specifically affecting instances that utilize its cmd_learn driver. This security issue stems from improper validation of user-supplied input during placeholder replacement operations. When an attacker crafts specific malicious values for these placeholders, they can bypass existing sanitization mechanisms and inject arbitrary commands into the application context. The vulnerability is not universal across all Roundcube installations but is strictly limited to environments where the markasjunk plugin is active and configured with the cmd_learn driver enabled, which allows interaction with external spam learning services such as SpamAssassin or similar tools that process email content for classification purposes.
From a technical perspective, the core of this vulnerability lies in how the application handles dynamic string substitution within the command execution pipeline. The placeholder replacement mechanism fails to adequately escape special characters or restrict the scope of allowed inputs before passing them to system-level commands. This lack of strict input validation allows an attacker to inject shell metacharacters that alter the intended command structure. By manipulating these values, a remote user can execute arbitrary operating system commands with the privileges of the web server process. This type of flaw is characteristic of CWE-78 Improper Neutralization of Special Elements used in an OS Command, commonly known as OS Command Injection. The attacker does not need to be authenticated if the plugin exposes endpoints that accept such inputs without sufficient access controls or session validation checks during the learning phase.
The operational impact of this vulnerability is severe due to its potential for full system compromise. Since Roundcube typically runs under a web server user account, successful exploitation could allow an attacker to read sensitive configuration files, exfiltrate email data stored on the disk, pivot into internal network segments if the mail server has broader access, or establish persistent backdoors. The ability to execute commands remotely means that even unauthenticated users who can interact with the spam learning feature might leverage this flaw. This undermines the integrity and confidentiality of the entire messaging infrastructure hosted by Roundcube. In enterprise environments where email is a primary communication channel, such a breach could lead to significant data loss, regulatory non-compliance under standards like GDPR or HIPAA depending on the content involved, and reputational damage due to compromised user accounts.
Mitigation strategies must focus primarily on immediate patching and configuration hardening. Administrators should upgrade Roundcube Webmail to version 1.6.18 or later for the stable branch, or version 1.7.3 and above for the development branch, as these releases contain fixes that properly sanitize placeholder inputs in the cmd_learn driver. For organizations unable to patch immediately due to operational constraints, disabling the markasjunk plugin entirely is a viable temporary workaround if spam learning functionality is not critical. Alternatively, restricting access to Roundcube via IP whitelisting or strong authentication mechanisms can reduce the attack surface by preventing unauthenticated users from triggering the vulnerable code paths. Additionally, deploying Web Application Firewalls with rules specifically targeting OS command injection patterns may provide an additional layer of defense against exploitation attempts while permanent fixes are implemented.
This vulnerability aligns with MITRE ATT&CK techniques related to Command and Scripting Interpreter abuse, particularly T1059 which covers execution through various scripting languages or shell commands. The attack vector is classified as Network-based because it can be exploited remotely over the internet without requiring prior access to the system. Defense-in-depth measures such as running web applications in restricted containers with minimal file system permissions and using application-level sandboxing can further limit the blast radius of any successful exploitation attempt. Regular security audits focusing on third-party plugins are essential, as extensions often introduce complex code paths that may not receive the same level of scrutiny as core framework components. Ensuring that all plugin dependencies are up to date and verifying their configuration settings against secure baseline standards remains a critical practice for maintaining robust email infrastructure security posture.