CVE-2026-21891 Vulnerability Analysis & Exploit Details

CVE-2026-21891 Vulnerability Summary

CVE-2026-21891: ZimaOS is a fork of CasaOS, an operating system for Zima devices and x86-64 systems with UEFI. In versions up to and including 1.5.0, the application checks the validity of the username but appears to skip, misinterpret, or incorrectly validate the password when the provided username matches a known system service account. The application's login function fails to properly handle the password validation result for these users, effectively granting authenticated access to anyone who knows one of these common usernames and provides any password. As of time of publication, no known patched versions are available.

CVE-2026-21891 References

External References

• https://github.com/IceWhaleTech/ZimaOS/security/advisories/GHSA-xj93-qw9p-jxq4
• https://github.com/IceWhaleTech/ZimaOS/security/advisories/GHSA-xj93-qw9p-jxq4

CWE Common Weakness Enumeration

CAPEC Common Attack Pattern Enumeration and Classification

• Authentication Abuse CAPEC-114 An attacker obtains unauthorized access to an application, service or device either through knowledge of the inherent weaknesses of an authentication mechanism, or by exploiting a flaw in the authentication scheme's implementation. In such an attack an authentication mechanism is functioning but a carefully controlled sequence of events causes the mechanism to grant access to the attacker.
• Authentication Bypass CAPEC-115 An attacker gains access to application, service, or device with the privileges of an authorized or privileged user by evading or circumventing an authentication mechanism. The attacker is therefore able to access protected data without authentication ever having taken place.
• Identity Spoofing CAPEC-151 Identity Spoofing refers to the action of assuming (i.e., taking on) the identity of some other entity (human or non-human) and then using that identity to accomplish a goal. An adversary may craft messages that appear to come from a different principle or use stolen / spoofed authentication credentials.
• Fake the Source of Data CAPEC-194 An adversary takes advantage of improper authentication to provide data or services under a falsified identity. The purpose of using the falsified identity may be to prevent traceability of the provided data or to assume the rights granted to another individual. One of the simplest forms of this attack would be the creation of an email message with a modified "From" field in order to appear that the message was sent from someone other than the actual sender. The root of the attack (in this case the email system) fails to properly authenticate the source and this results in the reader incorrectly performing the instructed action. Results of the attack vary depending on the details of the attack, but common results include privilege escalation, obfuscation of other attacks, and data corruption/manipulation.
• Exploiting Trust in Client CAPEC-22 An attack of this type exploits vulnerabilities in client/server communication channel authentication and data integrity. It leverages the implicit trust a server places in the client, or more importantly, that which the server believes is the client. An attacker executes this type of attack by communicating directly with the server where the server believes it is communicating only with a valid client. There are numerous variations of this type of attack.
• Utilizing REST's Trust in the System Resource to Obtain Sensitive Data CAPEC-57 This attack utilizes a REST(REpresentational State Transfer)-style applications' trust in the system resources and environment to obtain sensitive data once SSL is terminated.
• Session Hijacking CAPEC-593 This type of attack involves an adversary that exploits weaknesses in an application's use of sessions in performing authentication. The adversary is able to steal or manipulate an active session and use it to gain unathorized access to the application.
• Token Impersonation CAPEC-633 An adversary exploits a weakness in authentication to create an access token (or equivalent) that impersonates a different entity, and then associates a process/thread to that that impersonated token. This action causes a downstream user to make a decision or take action that is based on the assumed identity, and not the response that blocks the adversary.
• Upload a Web Shell to a Web Server CAPEC-650 By exploiting insufficient permissions, it is possible to upload a web shell to a web server in such a way that it can be executed remotely. This shell can have various capabilities, thereby acting as a "gateway" to the underlying web server. The shell might execute at the higher permission level of the web server, providing the ability the execute malicious code at elevated levels.
• Adversary in the Middle (AiTM) CAPEC-94 An adversary targets the communication between two components (typically client and server), in order to alter or obtain data from transactions. A general approach entails the adversary placing themself within the communication channel between the two components.

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• CVE-2025-64093 – Remote Code Execution vulnerability that allows unauthenticated attackers to inject arbitrary commands into the hostname of the device.
• CVE-2025-64092 – This vulnerability allows unauthenticated attackers to inject an SQL request into GET request parameters and directly query the underlying database.
• CVE-2025-64091 – This vulnerability allows authenticated attackers to execute commands via the NTP-configuration of the device.
• CVE-2025-64090 – This vulnerability allows authenticated attackers to execute commands via the hostname of the device.