CVE-2025-55182 (React2Shell)

Today we witnessed defense in depth working exactly as intended. Within hours of CVE-2025-55182 being disclosed, a critical remote code execution vulnerability in React Server Components, we received alerts from two different sources:

1. Vercel’s dashboard, warning us that WAF rules were already protecting our production deployment
2. Dependabot PR #87, automatically upgrading Next.js from 16.0.6 to 16.0.7

This is what modern security infrastructure looks like: multiple layers working in concert. Vercel’s WAF provided immediate protection before we could even review the PR, while Dependabot delivered the actual patch for permanent remediation.

In this post, we’ll break down what CVE-2025-55182 is, why it matters, how these layered defenses protected our app, and what you should do to secure your own projects.

Vulnerability Overview: CVE-2025-55182 (React2Shell)

CVE-2025-55182, also known as “React2Shell” in the security community, is a critical-severity vulnerability affecting React Server Components (RSC). Under certain conditions, specially crafted requests can lead to unintended remote code execution (RCE) on the server.

Technical Details:

• Affected Packages: React 19’s server-side rendering packages (react-server-dom-parcel, react-server-dom-webpack, react-server-dom-turbopack)
• Related CVE: CVE-2025-66478 for Next.js specifically
• Impact: Remote code execution, environment access, infrastructure pivoting, data exfiltration

Affected Versions & Patches

Other affected frameworks: Vite with RSC, Parcel, React Router (RSC), RedwoodSDK, Waku

Quick Version Check:

npm ls next react  # or check package.json

Complete CVE Family (December 2025 Discoveries)

Following React2Shell’s disclosure, researchers discovered three additional vulnerabilities⁴:

Critical: CVE-2025-55184’s original fix was incomplete⁴. Upgrade directly to React 19.2.3+ to address all four vulnerabilities in one update⁸.

Timeline & Platform Response

Attack Chain Context: Exploitation is fully automated, progressing from scanner identification to payload deployment in under 10 seconds. Attackers use PowerShell arithmetic validation (powershell -c "40138*41979"), encoded payloads, AMSI bypasses, then deploy cryptocurrency miners, backdoors, and credential harvesters.

Here’s what makes this disclosure interesting from a DevSecOps perspective: Vercel’s platform detected and mitigated this vulnerability before Dependabot could even create a PR.

Both systems worked, but with different roles and limitations:

• Vercel’s WAF: Defense-in-depth layer (mitigation, not complete protection)
• Dependabot: Same-day patch delivery (complete remediation)

Exploitation Tool Maturity

As of December 8, 2025:

• Multiple working weaponized exploits publicly available
• Metasploit module integrated into standard toolkit
• Automated scanning tools widely adopted by threat actors
• Payload variations observed as attackers refine techniques against real targets
• High-fidelity exploitation with near 100% success rate reported by researchers

Vercel’s Approach

Vercel coordinated with the React team pre-disclosure, deployed WAF rules automatically to all projects, and continuously improved protections as bypass variants emerged. They released automated fix tooling (npx fix-react2shell-next) and began blocking new vulnerable deployments.

“We have created new WAF rules to address this vulnerability and deployed them to Vercel WAF that will automatically and at no cost protect all projects hosted on Vercel.” — Jimour Lai, Vercel Security Engineer

Important WAF Limitations: Vercel acknowledges that “WAF rules are one layer of defense but can never guarantee 100% coverage.” As new exploit variants emerged (evidenced by payload variations observed on December 8), Vercel identified and patched WAF bypasses, but upgrading remains the only complete fix.

CISA Recognition: The addition of CVE-2025-55182 to CISA’s Known Exploited Vulnerabilities list on December 5 signals that this is now considered a priority threat by federal agencies, underscoring the urgency of remediation.

This means that while the WAF provided valuable defense-in-depth, our production deployment wasn’t truly secure until we merged the upgrade. The WAF bought us time and reduced risk, but it wasn’t a substitute for patching.

The Broader Impact

Vercel also worked with the React team to provide recommendations to major WAF and CDN providers. This collaborative approach helps protect the broader ecosystem, not just Vercel customers.

Vercel’s $1 Million Bug Bounty Program

In an unprecedented security initiative, Vercel launched a $1 million bug bounty program specifically targeting WAF bypass techniques:

Program Results:

• Engaged 116 security researchers to find every possible WAF bypass
• Paid out over $1 million in bounties
• Shipped 20 unique WAF updates within 48 hours as new bypass techniques were reported
• All discovered bypass techniques are now permanent firewall additions

Dual-Layer Defense Architecture:

Vercel deployed a two-layer protection system:

Layer 1: Seawall WAF

• Deep request inspection
• Pattern-based exploit detection
• Continuous updates from bug bounty findings

Layer 2: Runtime Mitigation (First Public Disclosure)

• Operates directly on compute layer inside applications
• Does not rely on heuristics
• Directly eliminates code evaluation vector
• covers 96% of Vercel traffic
• Automatic alerting when triggered

This runtime layer provided visibility into actual WAF bypass attempts in production, allowing Vercel to state “with high confidence that the WAF was extraordinarily effective”—because they had ground truth data from the second layer.

Automated Remediation Tools:

• Vercel Agent: Automatically detects vulnerable projects and opens PRs
• npx fix-react2shell-next: One-command CLI fix tool

Threat Landscape: The Scale of React2Shell Exploitation

Attack Statistics (January 2026)

One month after disclosure, React2Shell has achieved unprecedented exploitation scale:

Volume Metrics:

• 8.1+ million attack sessions recorded since December 3, 2025¹
• 300,000-400,000 daily attacks currently sustained (not declining)¹
• 8,163 unique attacking IPs across 1,071 ASNs in 101 countries¹
• 644,000+ vulnerable domains still exposed (Shadowserver Foundation)²

Infrastructure Analysis:

• 50% of attacking IPs first observed after July 2025 (recent allocation)¹
• AWS alone accounts for over one-third of exploitation traffic¹
• Sophisticated coordination indicates organized threat actor campaigns⁵
• Attack volume has stabilized but not declined—permanent fixture in attacker toolkits¹¹

Nation-State & APT Activity

Five China-nexus threat clusters have been specifically attributed (AWS + Google Threat Intelligence)⁵:

• UNC6600 (Espionage): MINOCAT tunneler deployment with FRP client integration⁵
• UNC6586, UNC6588, UNC6603, UNC6595: Multiple distinct clusters with custom tooling⁵
• Earth Lamia (UNC5454) & Jackpot Panda: Active within hours of disclosure⁹
• RondoDx botnet: Steadily expanding N-day vulnerability arsenal¹²

Advanced Tactics:

• Automated scanning using commercial vulnerability scanners¹³
• Metadata targeting (icon hashes, SSL certificates, geographic identifiers)¹³
• Large-scale anonymization networks masking attribution⁵
• Rapid weaponization of public PoCs (within hours)⁹
• Simultaneous exploitation of multiple N-day vulnerabilities⁵

New Malware Families (Exclusive to React2Shell)

Security researchers have identified seven sophisticated malware families deployed specifically through CVE-2025-55182⁶:

Defense & Response: Multi-Layered Protection Strategy

Immediate Action Required: Upgrade Now

Even with cloud provider WAF protections, you must upgrade. WAF rules provide interim protection but are not a permanent fix.

For Next.js Projects:

# Option 1: Use Vercel's automated fix tool (recommended)
npx fix-react2shell-next
 
# Option 2: Manual upgrade to latest patched versions
npm install next@latest react@latest react-dom@latest
 
# Option 3: Specify exact versions (January 2026 recommendations)
npm install next@16.0.10 react@19.2.3 react-dom@19.2.3

Verify Your Upgrade:

npm ls next react react-dom

For Other RSC Frameworks: Check framework documentation and ensure React 19.2.3+ (addresses all 4 CVEs).

Vercel’s Unprecedented Defense Response

$1 Million Bug Bounty Program Results:

• Engaged 116 security researchers to find WAF bypasses
• Paid out over $1 million in bounties
• Shipped 20 unique WAF updates within 48 hours
• All bypass techniques now permanent firewall additions

Dual-Layer Defense Architecture:

Layer 1: Seawall WAF

• Deep request inspection and pattern-based detection
• Continuous updates from bug bounty findings
• Automatic deployment across all Vercel projects

Layer 2: Runtime Mitigation (First Public Disclosure)

• Operates directly inside applications on compute layer
• Directly eliminates code evaluation vector
• covers 96% of Vercel traffic with automatic alerting
• Provides ground truth data on actual bypass attempts

Automated Remediation Tools:

• Vercel Agent: Dashboard detection + automatic PR creation
• npx fix-react2shell-next: One-command CLI fix tool

Industry-Wide WAF Protection

Automatically Deployed (No Action Required):

• Vercel WAF: All Vercel-hosted projects
• AWS WAF: AWSManagedRulesKnownBadInputsRuleSet v1.24+
• Cloudflare WAF: React2Shell rules (December 3-4, 2025)
• Google Cloud Armor: App Engine, Cloud Functions, Cloud Run

Available for Deployment:

• Palo Alto Networks NGFW, Akamai WAF, Fastly WAF

Detection Tools & Enterprise Platforms

Open Source Scanners:

1. react2shell-scanner (Assetnote): github.com/assetnote/react2shell-scanner
2. npx fix-react2shell-next: Official Next.js remediation tool
3. Vercel Agent: Dashboard-based detection with automated PRs

Enterprise Security Platforms:

• Microsoft Defender Vulnerability Management (MDVM)¹⁴
• Microsoft Defender for Cloud (agentless scanning)¹⁴
• Dynatrace Runtime Vulnerability Analytics¹⁵
• JFrog Advanced Security¹⁶, Mondoo Security Platform⁸

Compromise Detection & Response

Temporary Mitigations (If Upgrade Delayed)

For Vercel Customers:

• WAF provides partial protection (not complete)
• Monitor dashboard for exploitation attempts
• Plan upgrade as the highest priority

For Other Hosting:

• Deploy available WAF (Cloudflare, AWS WAF, etc.)
• Contact hosting provider for protection options
• Consider taking applications offline until patched

Defense-in-Depth (NOT Complete Protection):

• Block suspicious patterns at reverse proxy/load balancer
• Implement strict input validation
• Isolate affected applications from critical infrastructure
• Monitor aggressively using detection patterns

Critical Reality Check: Active botnet exploitation means constant probing. Every hour of delay increases risk exponentially.

Long-Term Impact: The “Log4Shell of Frontend”

Security researchers are now comparing React2Shell to Log4Shell in terms of long-term impact and exploitation trajectory⁷:

Industry Impact Statistics

Wiz Research has provided comprehensive statistics on React2Shell’s impact¹⁰:

Vulnerability Exposure:

• 45% of cloud environments contain vulnerable React instances¹⁰
• 61% of Next.js deployments are publicly exposed to the Internet¹⁰
• 44% of all cloud environments have publicly exposed Next.js instances¹⁰
• 12% of cloud environments expose vulnerable React/Next.js directly to the Internet¹⁰

Key Parallels with Log4Shell

Similar Exploitation Velocity:

• Weaponization within hours of public disclosure
• Integration into standard attacker toolkits (Metasploit modules available December 5)
• Automated scanning at massive scale (8.1M+ attack sessions)
• Sustained attack volumes months after patches available

Ubiquitous Framework Creates Massive Attack Surface:

• React is used by millions of applications globally
• Next.js is the dominant React framework for production deployments
• Cloud-native deployment patterns amplify exposure
• Microservices architectures multiply vulnerable endpoints

Long-Tail Exploitation Expected:

• VulnCheck assessment: “Likely to have a long tail” of exploitation¹¹
• Daily attack volumes stabilized at 300k-400k (not declining as of January 2026)¹
• Vulnerability permanently added to attacker reconnaissance playbooks¹¹
• Organizations with slow patch cycles remain at risk indefinitely

Sustained Threat Trajectory

Unlike typical vulnerabilities that see exploitation decline after initial waves, React2Shell exhibits characteristics of permanent infrastructure risk:

1. Attack volume stabilization (not reduction) indicates systematic integration into attacker toolkits¹
2. New malware families continue to emerge (PeerBlight, CowTunnel, KSwapDoor identified in January 2026)⁶
3. Ransomware group adoption signals high-value target assessment and commercial exploitation⁷
4. APT group sustained interest suggests strategic intelligence value beyond opportunistic attacks⁵
5. 644,000+ unpatched domains (as of January 2026) provide ongoing opportunities for exploitation²

Organizations should treat React2Shell as a persistent threat requiring ongoing vigilance, not a one-time patch event. The vulnerability’s integration into criminal infrastructure, nation-state operations, and automated exploitation frameworks ensures it will remain a significant risk factor for years to come.

What This Means for RSC Security

React Server Components represent a significant architectural shift. They blur the line between client and server code, which creates new attack surfaces that didn’t exist in traditional client-side React.

This vulnerability is a reminder that:

1. Server-side code needs server-side security thinking. RSC isn’t “just React”, it’s React running in a privileged environment.

2. Framework-level protections matter. Vercel’s ability to deploy WAF rules across their platform demonstrates the value of managed infrastructure.

3. The disclosure process is evolving. Coordinated disclosure with platform providers can protect users faster than traditional CVE → Dependabot → CI workflows.

Frequently Asked Questions

Key Takeaways

1. Upgrade immediately—this is under active exploitation: Botnet integration confirmed, mass scanning ongoing, post-exploitation includes cryptomining and ransomware staging
2. Use Vercel’s fix tool for fastest remediation: npx fix-react2shell-next automates the upgrade process
3. WAF protection is defense-in-depth, not a complete fix: WAF cannot guarantee 100% coverage; bypasses exist and new variants emerge
4. Monitor for compromise using the detection patterns: PowerShell Event ID 4104, encoded commands, AMSI bypasses, unusual network connections
5. Enable Dependabot security updates: Same-day PRs for critical vulnerabilities are invaluable (when you actually merge them)
6. RSC introduces server-side attack surfaces: React Server Components blur client/server boundaries, creating new security considerations
7. Expect long-term exploitation: This vulnerability will be part of the threat landscape indefinitely—unpatched systems will be found
8. Don’t test POCs on production: Use version checks and log analysis instead of running exploits

This incident highlights several important lessons for DevSecOps teams:

• Defense in depth works: Vercel’s WAF and Dependabot provided layered protection
• Automated tooling is essential: Automated fix tools and dependency management speed remediation
• Proactive monitoring is critical: Detection patterns help identify potential compromises quickly
• Collaboration matters: Coordinated disclosure with platform providers enhances ecosystem security

Acknowledgments

Credit: This vulnerability was responsibly disclosed by Lachlan Davidson. Thanks to Jimour Lai, Luba Kravchenko, and Andy Riancho at Vercel for the coordinated response.

Resources

Post-Exploitation Artifacts:

Credential Harvesting Targets:

Persistence Mechanisms:

• systemd service creation (system-updates-service, system-update-service)
• cron job injection, authorized_keys manipulation
• Shell configuration modification (~/.bashrc, ~/.bash_profile)

• RMM/RAT

  deployment: MeshAgent, Cobalt Strike, AnyDesk