CVE-2026-42551

Request::getMethod() unconditionally honors the X-HTTP-Method-Override header and the $_REQUEST['_method'] parameter on any HTTP verb (including safe verbs such as GET), with no opt-in and no whitelist of permitted target methods. A GET request can silently become a DELETE or PUT, enabling CSRF escalation against destructive endpoints, bypass of middleware gated on unsafe verbs, and cache poisoning between CDN and origin.

Affected code

flight/net/Request.php (≈ lines 281-292):

public static function getMethod(): string
{
    $method = self::getVar('REQUEST_METHOD', 'GET');
    if (self::getVar('HTTP_X_HTTP_METHOD_OVERRIDE') !== '') {
        $method = self::getVar('HTTP_X_HTTP_METHOD_OVERRIDE');
    } elseif (isset($_REQUEST['_method']) === true) {
        $method = $_REQUEST['_method'];
    }
    return strtoupper($method);
}

$_REQUEST aggregates $_GET and $_POST; on PHP runtimes with request_order=GPC it also includes $_COOKIE.

Proof of concept

GET /item/42?_method=DELETE        HTTP/1.1

is dispatched as DELETE /item/42.

GET /item/42                       HTTP/1.1
X-HTTP-Method-Override: DELETE

is also dispatched as DELETE /item/42.

Trivial CSRF vector (no JavaScript required):

<img src="https://victim.tld/item/42?_method=DELETE">

loaded on any attacker-controlled page triggers the destructive DELETE on page load, bypassing Same-Origin Policy (image loads are not blocked).

Reproduced against /poc4/item/42.

Patch (fixed in 3.18.1, commit b8dd23a)

A new flight.allow_method_override setting controls both override vectors. Operators can set it to false to disable X-HTTP-Method-Override and _method entirely.

Credit

Discovered by @Rootingg.

• GET → DELETE / PUT CSRF on any route registered for unsafe verbs.
• Bypass of authentication, CSRF token, or rate-limiting middleware that is gated only on POST/DELETE.
• CDN cache poisoning: the CDN caches the GET response body while the origin executed a DELETE.

CVE-2026-42551 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.18.1); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-42551? CVE-2026-42551 is a high-severity security vulnerability in flightphp/core (composer), affecting versions < 3.18.1. It is fixed in 3.18.1.
2. How severe is CVE-2026-42551? CVE-2026-42551 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of flightphp/core are affected by CVE-2026-42551? flightphp/core (composer) versions < 3.18.1 is affected.
4. Is there a fix for CVE-2026-42551? Yes. CVE-2026-42551 is fixed in 3.18.1. Upgrade to this version or later.
5. Is CVE-2026-42551 exploitable, and should I be worried? Whether CVE-2026-42551 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-42551 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-42551? Upgrade flightphp/core to 3.18.1 or later.