CVE-2022-31108 is a security vulnerability classified under CWE-74, which pertains to improper neutralization of special elements in output used by a downstream component, specifically an injection flaw in the mermaid-js 'mermaid' library. Mermaid is a JavaScript-based diagramming and charting tool that allows users to define complex diagrams using Markdown-inspired text definitions. The vulnerability exists in versions >= 8.0.0 and < 9.1.3 of mermaid. An attacker can exploit this flaw by injecting arbitrary CSS into the generated graph output. This CSS injection allows the attacker to manipulate the styling of elements outside the intended graph area, potentially affecting the entire document's appearance and behavior. More critically, the injected CSS can be crafted to exfiltrate sensitive information from the user's environment. For example, by using CSS selectors and functions that trigger HTTP requests (such as loading background images), an attacker can brute force the value of input fields one character at a time. When a character matches, the browser issues an HTTP request, which the attacker can detect, thereby leaking confidential data. This technique leverages the browser's CSS processing and network request capabilities to bypass traditional same-origin protections. Additionally, the vulnerability enables an attacker to alter the document in ways that may cause users to perform unintended actions, such as clicking malicious links or triggering other UI elements. The issue has been addressed in mermaid version 9.1.3, and users are strongly advised to upgrade. For those unable to upgrade immediately, it is critical to ensure that any user input embedded in CSS blocks is properly escaped to prevent injection. No known exploits have been reported in the wild to date, but the potential for information disclosure and UI manipulation makes this a significant concern for applications relying on vulnerable versions of mermaid for rendering diagrams, especially in web environments where untrusted input may be processed.

For European organizations, the impact of CVE-2022-31108 can be significant, particularly for those that integrate mermaid-js into their web applications, documentation portals, or internal tools that render user-generated diagrams. The vulnerability primarily threatens confidentiality by enabling attackers to exfiltrate sensitive information from users through CSS-based side-channel attacks. This could include credentials, personal data, or other confidential inputs entered into web forms. Integrity and availability impacts are secondary but still relevant, as attackers can manipulate the document's appearance and behavior, potentially misleading users into performing harmful actions such as clicking malicious links or triggering unintended workflows. Organizations in sectors with high regulatory requirements for data protection, such as finance, healthcare, and government, may face compliance risks if sensitive data is leaked. The attack vector requires that malicious or crafted input be processed by the vulnerable mermaid component, so environments where users can submit or share diagram definitions are at higher risk. Since exploitation does not require authentication or complex user interaction beyond viewing the maliciously crafted diagram, the attack surface includes public-facing web applications and internal collaboration platforms. The absence of known exploits in the wild suggests limited active targeting so far, but the ease of exploitation and potential for stealthy data exfiltration warrant prompt remediation to prevent future incidents.

1. Upgrade mermaid-js to version 9.1.3 or later immediately to apply the official fix addressing this vulnerability. 2. For environments where upgrading is not immediately feasible, implement strict input validation and sanitization on all user-supplied diagram definitions before passing them to mermaid. Specifically, escape or remove any CSS-related characters or constructs that could be injected into the output. 3. Employ Content Security Policy (CSP) headers that restrict the loading of external resources and disallow inline styles where possible, limiting the ability of injected CSS to trigger HTTP requests or manipulate the DOM. 4. Monitor web application logs and network traffic for unusual HTTP requests that may indicate CSS-based exfiltration attempts, such as repeated background image loads with suspicious query parameters. 5. Educate developers and content creators about the risks of embedding untrusted input in diagram definitions and encourage the use of trusted sources only. 6. Where feasible, isolate the rendering of mermaid diagrams within sandboxed iframes or separate contexts to limit the scope of CSS injection impact on the broader document. 7. Conduct regular security reviews and penetration testing focused on injection vectors in web components, including third-party libraries like mermaid, to detect similar issues proactively.