Summary
Server actions in rwsdk apply HTTP method enforcement but no origin validation. A request originating from a different origin that the browser treats as same-site can invoke a server action with the victim's session cookie attached.
Affected Configurations
Applications using rwsdk server actions (serverAction() or functions invoked via the RSC action protocol) in combination with cookie-based authentication. serverQuery() is not affected because it is designed to be idempotent and is invoked via GET.
Credits
Reported by @mthx.
Impact
An attacker who controls any origin the browser considers same-site with the deployed app can induce an authenticated victim's browser to invoke arbitrary server actions. The exposure depends on deployment shape:
- Apps deployed on custom domains (for example
app.example.com) are exposed whenever the attacker controls any sibling subdomain under the same registrable domain. Plausible vectors include subdomain takeover of stale DNS records pointing at third-party services, cross-site scripting on a sibling application, or content served from a user-content subdomain. - Apps deployed on platform-suffix domains on the Public Suffix List (
*.workers.dev,*.pages.dev) are not exposed to the sibling-subdomain vector, because sibling subdomains under those suffixes are treated as cross-site. - In local development,
localhoston any other port is treated as same-site with the app's dev server. A separate process running on the developer's machine can invoke server actions against the dev server.
The attacker cannot read action responses (mode: "no-cors" yields opaque responses). Impact is therefore limited to side effects of action invocation: writes, state changes, and any externally observable action the application performs in response.
Cross-site requests from unrelated origins (evil.com targeting app.com) are not affected because SameSite=Lax session cookies are not attached by default in that scenario.
A victim's authenticated browser session is used to submit forged requests to an application that cannot distinguish them from legitimate ones. Typical impact: state-changing actions performed as the victim without their consent.
CVE-2026-42190 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and user interaction required. 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 (1.2.3); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
The patched release enforces an Origin/Host match for non-GET action requests. Requests whose Origin header does not match the request's own origin are rejected with HTTP 403 unless the origin is listed in a new allowedOrigins configuration option.
No application code changes are required for apps that invoke server actions from their own origin. Apps that legitimately invoke server actions from another origin must add those origins to the allowedOrigins option on defineApp.
Frequently Asked Questions
- What is CVE-2026-42190? CVE-2026-42190 is a medium-severity cross-site request forgery (CSRF) vulnerability in rwsdk (npm), affecting versions >= 1.0.0-beta.50, <= 1.2.2. It is fixed in 1.2.3. A victim's authenticated browser session is used to submit forged requests to an application that cannot distinguish them from legitimate ones.
- How severe is CVE-2026-42190? CVE-2026-42190 has a CVSS score of 5.3 (Medium). 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.
- Which versions of rwsdk are affected by CVE-2026-42190? rwsdk (npm) versions >= 1.0.0-beta.50, <= 1.2.2 is affected.
- Is there a fix for CVE-2026-42190? Yes. CVE-2026-42190 is fixed in 1.2.3. Upgrade to this version or later.
- Is CVE-2026-42190 exploitable, and should I be worried? Whether CVE-2026-42190 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
- What actually determines whether CVE-2026-42190 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.
- How do I fix CVE-2026-42190? Upgrade
rwsdkto 1.2.3 or later.