CVE-2026-61399 in CloudStack
Summary
by MITRE • 08/21/2026
Improper Encoding or Escaping of Output vulnerability in Apache CloudStack's UI while using Lock User Functionality.
This issue affects Apache CloudStack: from 4.20.0.0 through 4.20.3.0 and from 4.21.0.0 through 4.22.1.0.
Users are recommended to upgrade to version 4.20.3.1 or 4.22.1.1 or later, which fixes the issue.
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Analysis
by VulDB Data Team • 08/21/2026
The vulnerability identified in Apache CloudStack represents a critical failure in input validation and output encoding mechanisms within the user interface layer of the platform's administrative console. Specifically, this flaw manifests when administrators utilize the Lock User functionality to restrict access for specific accounts. The core technical deficiency lies in the improper handling of special characters or potentially malicious payloads embedded within usernames or associated metadata during the rendering process on the client side. Because the application fails to correctly encode these outputs before injecting them into HTML contexts, it creates a pathway for Cross-Site Scripting attacks. This type of vulnerability is classically categorized under CWE-79, which addresses Improper Neutralization of Input During Web Page Generation known as XSS. The severity of this issue is amplified by the fact that Apache CloudStack serves as an infrastructure-as-a-service platform where administrative actions carry significant weight and visibility across virtualized environments.
From a technical perspective, the exploitation vector relies on the attacker possessing valid credentials for an account with sufficient privileges to trigger the lock user action or manipulate the UI state in a way that triggers the vulnerable code path. When the application processes the request to lock a user, it retrieves data related to that user and renders it within the web interface without applying adequate sanitization rules. If an attacker has previously controlled a username field containing script tags or event handlers such as onerror or onload attributes, these payloads are stored in the backend database and subsequently reflected back into the DOM during subsequent administrative interactions. This allows for Stored Cross-Site Scripting if the payload is saved, or Reflected XSS depending on how the UI dynamically updates based on user input. The lack of context-aware encoding means that characters like less-than greater-than ampersands and quotes are not converted to their safe HTML entity equivalents before being displayed in browser-rendered elements.
The operational impact of this vulnerability extends beyond simple script execution within a victim's browser session. In the context of Apache CloudStack, an attacker who successfully exploits this flaw can perform actions on behalf of the administrative user whose session is compromised. This includes viewing sensitive configuration details, modifying network settings, creating or destroying virtual machines, and accessing other users' data stored within the cloud environment. Such capabilities effectively grant the attacker full control over the affected tenant's resources, leading to potential data exfiltration, service disruption, and lateral movement into deeper parts of the infrastructure. The trust boundary between the administrative UI and the backend API is breached because the frontend acts as an untrusted channel that injects malicious code which executes with the privileges of the logged-in administrator. This undermines the integrity and confidentiality guarantees provided by the platform's authentication and authorization models.
Industry frameworks such as MITRE ATT&CK classify this type of vulnerability under techniques related to Client-Side Scripting Injection, specifically T1059 if it leads to command execution via browser APIs or T1204 for user execution through malicious content delivery. The attack chain typically involves the attacker crafting a specific URL or manipulating UI elements to trigger the rendering bug, followed by social engineering or session hijacking to ensure an administrator interacts with the compromised interface element. Once executed, the script can steal cookies containing authentication tokens, capture keystrokes entered into the admin console, or redirect the user to phishing pages designed to harvest further credentials. The persistence of this risk is high because it affects multiple major versions of Apache CloudStack, indicating a systemic issue in how the UI framework handles dynamic content generation across different release cycles from 4.20.0 through 4.22.1.
Mitigation strategies must prioritize immediate version upgrades as recommended by the vendor. Administrators should upgrade to Apache CloudStack version 4.20.3.1 or 4.22.1.1 and later, where these encoding flaws have been patched with robust input validation libraries and strict output encoding policies. For environments where upgrading is not immediately feasible due to operational constraints, compensating controls include implementing a Web Application Firewall that can detect and block patterns associated with XSS payloads in HTTP requests and responses. Additionally, enforcing Content Security Policy headers within the application configuration can significantly reduce the impact by restricting the sources from which scripts are allowed to execute. Regular security audits of custom UI extensions or plugins integrated into CloudStack are also essential to ensure no third-party components introduce similar encoding weaknesses. Continuous monitoring for anomalous browser behavior and session anomalies in administrative logs provides an additional layer of defense against exploitation attempts targeting this specific vulnerability class.