Mitigating the React Server Components RCE (CVE-2025-55182)

#nextjs #react #rce #security

React Server Components (RSC) were recently found to be affected by a high-severity remote code execution vulnerability (CVE-2025-55182).

Attackers may exploit this issue by crafting malicious serialized data in the Flight protocol, abusing features such as Next.js Server Actions, which can trigger a deserialization flaw and potentially lead to remote code execution on the server.

This post focuses how this vulnerability works, who is affected, and what you can realistically do to reduce risk—both at the application level and at the WAF layer.

What Is CVE-2025-55182?

React Server Components (RSC) were found to contain a high-risk deserialization flaw that can lead to remote code execution.

In affected setups, an attacker can:

Craft malicious serialized payloads in the React Flight protocol
Abuse Next.js Server Actions ("use server")
Trigger unsafe deserialization logic
Potentially execute arbitrary code on the server

This is not a theoretical issue. The attack surface exists anywhere RSC and Server Actions are exposed to untrusted input.

Priority 1: Identify and Patch (Primary Mitigation)

The most important mitigation is identification and upgrading. WAF rules can help, but they do not replace fixing the root cause.

1. How to Tell If You’re Affected

You are likely impacted if all or most of the following are true:

You are using Next.js, especially v13.4 or later
Your application uses React Server Components
Your codebase includes the "use server" directive (Server Actions)

⚠️ Important note

The vulnerable package (react-server-dom-webpack) is usually bundled internally by Next.js.

You will not necessarily see it in package.json, so don’t assume you’re safe just because it’s not listed as a direct dependency.

2. Upgrade Recommendation

Upgrade immediately to the latest security-patched version of Next.js.

At the time of writing, this means:

Patched releases in Next.js 14.x
Or newer fixed versions in Next.js 15.x

The React team has addressed the root issue by fixing the unsafe parsing of malicious Thenable objects, which was the core of the deserialization vulnerability.

This is the only complete fix.

Anything else should be treated as a temporary risk-reduction measure.

Priority 2: WAF-Level Mitigation with SafeLine

If immediate upgrading is not possible (for example, due to release freezes or legacy dependencies), you can add a defensive layer at the WAF level.

This does not eliminate the vulnerability, but it can help block known exploit patterns.

Example: SafeLine Custom Deny Rule

In SafeLine WAF, you can configure a custom deny rule with the following conditions:

Request Header
Content-Type matches (regex): multipart

AND

Request Body
Regex match: [’”]?then[’”]?\s*:\s*[’”]?$1:proto:then

This rule targets payloads attempting to exploit the RSC deserialization logic by abusing Thenable structures in serialized data.

What This Rule Does (and Does Not Do)

✔️ Helps block known exploit signatures

✔️ Reduces exposure during patching windows

❌ Does not guarantee protection against new or obfuscated payloads

❌ Does not replace upgrading Next.js

Think of this as a seatbelt, not a cure.

Conclusion

If you use Next.js + RSC + Server Actions, you should assume exposure until proven otherwise
Upgrading is mandatory, not optional
WAF rules (like those in SafeLine) are best used as defense-in-depth, especially during transition periods

If you’re running production workloads on modern React infrastructure, this is a good reminder that application-layer vulnerabilities can’t be fully solved at the perimeter—but a well-configured WAF can still buy you valuable time.