CVE-2026-34959 in Adminer
Summary
by MITRE • 08/25/2026
Adminer 4.6.0 before 5.5.0 prepends the client-supplied X-Forwarded-Prefix header to $_SERVER["REQUEST_URI"] with no trusted-proxy check and no validation of the prefix value. An attacker can supply an absolute URL (e.g. X-Forwarded-Prefix: https://evil.example) that flows into Location redirect headers, the Set-Cookie path attribute, and self-referential links. This enables an authenticated open redirect after state-changing POSTs, unauthenticated control of the session cookie path attribute, and poisoning of self-referential links; CR/LF cannot be injected, so header splitting/XSS is not possible.
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Analysis
by VulDB Data Team • 08/25/2026
The vulnerability identified in Adminer versions prior to 5.6.0 stems from an improper validation of the X-Forwarded-Prefix HTTP header within the application's request handling logic. In modern web architectures utilizing reverse proxies or load balancers, this header is commonly used to convey the original scheme and host requested by the client before it was modified during proxying. However, Adminer versions 4.6.0 through 5.5.0 exhibit a critical flaw where they blindly prepend the value of this client-supplied header directly to the $_SERVER["REQUEST_URI"] superglobal without performing any validation against a trusted proxy list or checking for absolute URL formats. This lack of sanitization allows an attacker who can control the X-Forwarded-Prefix header, typically through direct HTTP requests bypassing internal proxies, to inject arbitrary prefixes into the application's perceived base path.
The technical consequence of this flaw is significant because the manipulated prefix flows directly into several critical components of the web server response and client-side state management. Specifically, when Adminer generates redirects or constructs self-referential links for subsequent actions, it utilizes the corrupted REQUEST_URI value which now contains an attacker-controlled absolute URL scheme such as https://evil.example. This leads to three distinct attack vectors depending on the context in which the header is processed. First, during state-changing POST requests that result in a redirect response, the application issues a Location header containing the malicious prefix, resulting in an open redirect vulnerability. Second, when setting session cookies, the path attribute of the Set-Cookie header is derived from this unvalidated input, allowing an attacker to control where the session cookie is sent by specifying arbitrary paths or domains if the structure permits it, although the primary impact here is often related to scope manipulation rather than full domain takeover due to browser security policies. Third, self-referential links embedded in HTML responses are poisoned with the malicious prefix, potentially leading users to unintended destinations when they interact with interface elements that rely on these relative paths being resolved correctly against a trusted base URL.
The operational impact of this vulnerability is multifaceted and poses serious risks to both authenticated and unauthenticated users. For authenticated administrators or database operators using Adminer, an open redirect following state-changing operations can be leveraged in phishing attacks where the user is redirected from what appears to be a legitimate Adminer interface to a malicious site designed to steal credentials or deliver malware. Although CR/LF characters cannot be injected into this specific header value due to HTTP protocol constraints and application filtering, which effectively mitigates risks associated with header splitting and cross-site scripting via response injection, the ability to control redirect targets remains a severe security issue under CWE-601 (Open Redirect). Furthermore, for unauthenticated users or in contexts where session management is involved, the manipulation of the Set-Cookie path attribute can potentially affect cookie scope. While browsers enforce strict rules on domain attributes, controlling the path can influence which URLs transmit the session token, potentially aiding in session hijacking scenarios if combined with other vulnerabilities or misconfigurations, aligning with aspects of CWE-614 (Sensitive Cookie in Session Token Without Secure Attribute) depending on deployment specifics.
From a threat intelligence perspective, this vulnerability maps to MITRE ATT&CK technique T1071.001 (Application Layer Protocol: Web Protocols), specifically involving the manipulation of HTTP headers for malicious redirection or session control. The exploitation requires the attacker to send crafted requests with the X-Forwarded-Prefix header set to an absolute URL, which is feasible in environments where external traffic can directly reach the Adminer instance without strict proxy validation rules that strip or validate such headers before they reach the application backend. This makes it particularly dangerous for deployments exposed directly to the internet or within compromised internal networks where lateral movement involves crafting HTTP requests with custom headers.
Mitigation strategies should prioritize immediate patching as this issue is resolved in Adminer version 5.6.0 and later, which implements proper validation logic. Administrators must ensure that their reverse proxies are configured to either strip untrusted X-Forwarded-Prefix headers from external connections or validate them against a whitelist of trusted origins before passing them to the application backend. Additionally, implementing strict Content Security Policy (CSP) directives can help mitigate some downstream effects by restricting where redirects and form submissions are allowed to navigate, although this is not a substitute for fixing the root cause in the application code. Regular security audits focusing on header handling practices and adherence to CWE-20 (Improper Input Validation) principles will further reduce the risk of similar vulnerabilities arising from other HTTP headers or input parameters that influence URL construction logic.