CVE-2026-85547 in MISP
Summary
by MITRE • 09/04/2026
A cross-site request forgery (CSRF) vulnerability exists in MISP due to form-security and CSRF protections being disabled based on whether an incoming request was identified as a REST request.
MISP's REST detection can be influenced by request properties such as the URL suffix or the HTTP Accept header. Because Accept: application/json can be supplied by a cross-origin page without requiring a CORS preflight, an attacker could cause a request originating from another website to be treated as REST traffic. MISP would consequently disable its normal form-security and CSRF validation even though the request was authenticated using the victim's existing browser session.
An unauthenticated remote attacker could exploit this behavior by convincing an authenticated MISP user to visit or interact with a malicious web page. The attacker's page could then issue crafted requests to susceptible state-changing MISP endpoints using the victim's privileges. Depending on the permissions of the victim and the targeted endpoint, this could allow unauthorized modification, creation, publication, or removal of data and other state changes.
The vulnerability originates from granting the form-security exemption based on _isRest() rather than on the authentication mechanism used by the request. The patch changes this behavior so that CSRF and form-security exemptions are granted only when the request actually carries a MISP API key. Session-authenticated REST-style requests remain subject to CSRF protection. The fix also introduces support for transmitting CSRF tokens through the X-CSRF-Token header for legitimate same-origin AJAX requests. Such a header cannot normally be attached by a cross-origin page without triggering a CORS preflight, preventing it from being used to reproduce the original attack.
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Analysis
by VulDB Data Team • 09/04/2026
The vulnerability identified in MISP represents a critical flaw in how the application distinguishes between standard web form submissions and REST API requests for security validation purposes. Cross-site request forgery attacks rely on tricking an authenticated user into submitting a malicious request that they did not intend to make, exploiting the fact that browsers automatically include credentials such as session cookies with every request to a given domain. In this specific instance, MISP’s defense mechanism against CSRF was inadvertently bypassed because the system disabled form-security and CSRF protections whenever it identified an incoming request as a REST API call rather than a traditional HTML form submission. This logic flaw created a significant gap in security posture for any user interacting with the platform through web-based interfaces that could be manipulated to mimic REST traffic characteristics.
The root cause of this vulnerability lies in the method used by MISP to detect whether a request is part of its REST API interface. The application relied on superficial indicators such as URL suffixes or, more critically, the HTTP Accept header to make this determination. Specifically, if an incoming request included the header value Accept: application/json, the system assumed it was a legitimate API call and consequently disabled CSRF checks. This assumption is flawed because web browsers allow cross-origin pages to set custom headers like Accept without triggering Cross-Origin Resource Sharing preflight requests in many scenarios. Consequently, an attacker hosting a malicious webpage could craft a request that includes this specific header value, causing the victim’s browser to send it to MISP while appearing as a valid REST API call from the server's perspective.
When an authenticated user visits the attacker-controlled page, their browser automatically attaches their active session cookies to any requests made to the MISP domain due to same-origin policy exceptions for certain headers and methods. Because the malicious request included the Accept: application/json header, MISP’s backend logic classified it as a REST request and stripped away CSRF validation protections. This allowed the attacker to execute state-changing actions on behalf of the victim without their knowledge or consent. The impact is severe because it grants an unauthenticated remote actor the ability to perform any action that the compromised user has permissions for, including modifying threat intelligence data, creating new indicators, publishing information, or removing existing records from the platform.
This vulnerability aligns with CWE-352, which describes Cross-Site Request Forgery (CSRF), specifically highlighting a failure in verifying the intent of state-changing requests by relying on insufficient request attributes rather than robust authentication mechanisms for security exemptions. From an offensive perspective, this technique is consistent with ATT&CK tactic T1078, Valid Accounts, where attackers leverage existing user credentials to perform unauthorized actions within a system. The exploitation does not require complex injection techniques or server-side vulnerabilities but instead exploits the browser’s native behavior of sending cookies and allowing certain headers in cross-origin contexts, making it difficult for users to detect while the attack is occurring.
The remediation strategy implemented addresses this flaw by fundamentally changing how MISP determines when CSRF protections should be bypassed. Instead of relying on potentially spoofable request properties like URL suffixes or Accept headers, the updated logic requires that a request must carry a valid MISP API key to qualify for form-security exemptions. This ensures that only requests explicitly authenticated via the dedicated API mechanism are exempt from CSRF checks, while session-authenticated REST-style requests remain subject to standard protections. Additionally, the fix introduces support for transmitting CSRF tokens through the X-CSRF-Token header for legitimate same-origin AJAX requests. Since browsers generally block cross-origin pages from setting custom headers like X-CSRF-Token without triggering a CORS preflight request, this measure effectively neutralizes the attack vector by ensuring that forged requests cannot include the necessary token to bypass validation.