CVE-2026-59809 in SiYuan
Summary
by MITRE • 08/22/2026
SiYuan before v3.8.0 interpolates secret placeholders into the destination URL parameter of the http_request MCP tool, allowing attackers to exfiltrate stored secrets. An MCP client can craft a request with an attacker-controlled URL containing secret placeholders to send plaintext secret values to any public host without confirmation.
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Analysis
by VulDB Data Team • 08/22/2026
The vulnerability identified in SiYuan versions prior to 3.8.0 represents a critical security flaw within the Model Context Protocol implementation, specifically affecting the http_request tool functionality. This issue stems from an insecure handling of variable interpolation where secret placeholders are resolved and embedded directly into the destination URL parameter before the HTTP request is dispatched. In typical secure architectures, sensitive data such as API keys, authentication tokens, or user credentials should be isolated from network transmission parameters unless explicitly required by a trusted endpoint with proper encryption controls. However, in this flawed implementation, the system treats secret placeholders as standard variables that can be injected into any URL provided by an MCP client application. This design oversight allows for the unintended leakage of sensitive information through standard HTTP GET requests or other methods where query strings are logged and visible to intermediaries.
From a technical perspective, the core flaw lies in the lack of validation regarding the destination host when secret interpolation is detected. The software does not restrict which domains can receive interpolated secrets, nor does it require explicit user confirmation before transmitting data containing sensitive placeholders to external servers. An attacker who has gained access to an MCP client environment or can influence the configuration of such a client can craft malicious requests that include these secret placeholders within the URL structure. When executed, the SiYuan application resolves these placeholders into their actual plaintext values and appends them to the specified URL. This results in the transmission of clear-text secrets over potentially unencrypted channels, making them susceptible to interception by network eavesdroppers or storage on remote server logs that may not have adequate security controls.
The operational impact of this vulnerability is severe, as it effectively neutralizes the confidentiality protections intended for stored secrets within the SiYuan ecosystem. Attackers can exfiltrate high-value credentials such as database passwords, third-party API keys, and session tokens without triggering any warnings or requiring additional authentication steps beyond those already present in the MCP client interaction. This capability facilitates unauthorized access to downstream services that rely on these stolen credentials, potentially leading to broader system compromise, data breaches, and loss of integrity for applications integrated with SiYuan. The ease of exploitation means that even users with limited technical expertise could inadvertently trigger this exfiltration if they interact with a compromised or malicious MCP client configuration.
This vulnerability aligns closely with CWE-209, which describes the generation of error messages containing sensitive information, and more accurately with CWE-798, concerning the use of hardcoded credentials that are transmitted in cleartext. In terms of offensive security frameworks, this behavior maps to ATT&CK technique T1530, Data from Local System Exfiltration, specifically through methods involving unencrypted network protocols. The lack of confirmation dialogs further exacerbates the risk by removing a critical layer of user awareness and consent that might otherwise prevent accidental data leakage.
To mitigate this vulnerability, organizations must immediately upgrade SiYuan to version 3.8.0 or later, where these interpolation issues have been addressed with stricter controls on secret handling. Until an update is applied, administrators should restrict the use of MCP clients in untrusted environments and avoid configuring tools that allow arbitrary URL destinations when secrets are involved. Implementing network-level monitoring can also help detect unusual outbound traffic patterns containing known secret formats or high-entropy strings typical of API keys. Additionally, adopting a zero-trust approach to internal tool configurations ensures that only verified and audited MCP clients have access to sensitive data stores, reducing the attack surface for such exfiltration attempts.