CVE-2026-87940 in Key Auth
Summary
by MITRE • 10/22/2026
Use of Web Browser Cache Containing Sensitive Information vulnerability in Drupal Key auth allows Exploiting Incorrectly Configured Access Control Security Levels. This issue affects Key auth versions: from 0.0.0 to 2.2.4.
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Analysis
by VulDB Data Team • 10/22/2026
The vulnerability identified within the Drupal Key authentication module, specifically affecting versions ranging from zero point zero point zero up to two point two point four, represents a critical intersection of client-side caching mechanisms and server-side access control logic. This flaw stems from the improper handling of sensitive information stored in web browser caches when interacting with endpoints managed by the key-based authentication system. In typical web application architectures, browsers cache responses to improve performance and reduce latency for subsequent requests. However, when these cached responses contain authenticated data or tokens that were intended only for specific authorized users, they can be inadvertently exposed to unauthorized parties through shared devices, network proxies, or malicious scripts with access to browser storage. The core technical flaw lies in the absence of appropriate cache control directives on HTTP responses associated with sensitive authentication operations. Without explicit instructions such as Cache-Control: no-store or private headers, modern browsers and intermediate caching servers may store these responses in public caches, making them retrievable by any user who gains access to that specific browser session or cached resource location.
From a technical perspective, this vulnerability exploits the discrepancy between how Drupal handles key-based authentication tokens and how standard web infrastructure treats HTTP response caching. When an administrator or privileged user authenticates using their API keys, the resulting session data or token validation responses may include sensitive metadata about access levels, permissions, or internal identifiers. If these responses are cached publicly, a subsequent request from any other user on the same network or device can retrieve this information without proper authentication checks by the application layer. This effectively bypasses the intended security controls because the attacker does not need to crack credentials or exploit logic flaws in the authentication algorithm itself; instead, they leverage the infrastructure's default behavior of caching content for efficiency. The vulnerability is particularly dangerous because it relies on environmental factors and user behavior rather than a direct code execution flaw, making it harder to detect through traditional static analysis tools that focus solely on server-side code paths.
The operational impact of this vulnerability allows an attacker to exploit incorrectly configured access control security levels by gaining unauthorized visibility into privileged operations or potentially hijacking authentication sessions if session identifiers are cached alongside other sensitive data. An adversary could use this information to map out the administrative structure of a Drupal site, identify high-privilege accounts, and plan further attacks such as privilege escalation or targeted phishing campaigns based on the revealed user roles and permissions. In severe cases, if the cached content includes active authentication tokens that are not properly invalidated upon logout or session expiration, an attacker could replay these tokens to maintain persistent unauthorized access to the system. This undermines the fundamental principle of least privilege and compromises the integrity of the application's security model, potentially leading to data breaches, defacement, or complete compromise of the underlying server depending on the permissions associated with the exposed keys.
To mitigate this vulnerability, developers must ensure that all HTTP responses containing sensitive authentication information are explicitly marked as non-cacheable by implementing strict Cache-Control headers such as no-store, no-cache, and private directives in the application's response configuration for key-related endpoints. Additionally, Drupal core and contributed modules should enforce these standards across all API routes to prevent leakage through intermediate proxies or browser caches. System administrators running affected versions of the Key module are strongly advised to upgrade immediately to version two point two five or later where this issue has been addressed in the codebase. For environments that cannot be patched instantly, implementing reverse proxy configurations to strip caching headers from sensitive responses and enforcing strict access controls on shared devices can provide temporary relief. Regular security audits focusing on HTTP header configuration and browser cache behavior are essential to maintain robust defense-in-depth strategies against information disclosure vulnerabilities of this nature. This issue aligns with CWE-524 which categorizes the exposure of caches containing sensitive information, and it maps to ATT&CK techniques related to collection via local system imaging or screen capture if cached data is accessed locally by an attacker with physical access, highlighting the broad risk surface associated with improper cache management in web applications.