2025-12-11
7 min read
On December 3, 2025, immediately following the public disclosure of the critical, maximum-severity React2Shell vulnerability (CVE-2025-55182), the Cloudforce One Threat Intelligence team began monitoring for early signs of exploitation. Within hours, we observed scanning and active exploitation attempts, including traffic originating from infrastructure associated with Asian-nexus threat groups.
Early activity indicates that threat actors quickly integrated this vulnerability into their scanning and reconnaissance routines. We observed systematic probing of exposed …
2025-12-11
7 min read
On December 3, 2025, immediately following the public disclosure of the critical, maximum-severity React2Shell vulnerability (CVE-2025-55182), the Cloudforce One Threat Intelligence team began monitoring for early signs of exploitation. Within hours, we observed scanning and active exploitation attempts, including traffic originating from infrastructure associated with Asian-nexus threat groups.
Early activity indicates that threat actors quickly integrated this vulnerability into their scanning and reconnaissance routines. We observed systematic probing of exposed systems, testing for the flaw at scale, and incorporating it into broader sweeps of Internet‑facing assets. The identified behavior reveals the actors relied on a combination of tools, such as standard vulnerability scanners and publicly accessible Internet asset discovery platforms, to find potentially vulnerable React Server Components (RSC) deployments exposed to the Internet.
Patterns in observed threat activity also suggest that the actors focused on identifying specific application metadata — such as icon hashes, SSL certificate details, or geographic region identifiers — to refine their candidate target lists before attempting exploitation.
In addition to React2Shell, two additional vulnerabilities affecting specific RSC implementations were disclosed: CVE-2025-55183 and CVE-2025-55184. Both vulnerabilities, while distinct from React2Shell, also relate to RSC payload handling and Server Function semantics, and are described in more detail below.
Background: React2Shell vulnerability (CVE-2025-55182)
On December 3, 2025, the React Team disclosed a Remote Code Execution (RCE) vulnerability affecting servers using the React Server Components (RSC) Flight protocol. The vulnerability, CVE-2025-55182, received a CVSS score of 10.0 and has been informally referred to as React2Shell.
The underlying cause of the vulnerability is an unsafe deserialization flaw in the RSC Flight data-handling logic. When a server processes attacker-controlled payloads without proper validation, it becomes possible to influence server-side execution flow. In this case, crafted input allows an attacker to inject logic that the server interprets in a privileged context.
Exploitation is straightforward. A single, specially crafted HTTP request is sufficient; there is no authentication requirement, user interaction, or elevated permissions involved. Once successful, the attacker can execute arbitrary, privileged JavaScript on the affected server.
This combination of authenticated access, trivial exploitation, and full code execution is what places CVE-2025-55182 at the highest severity level and makes it significant for organizations relying on vulnerable versions of React Server Components.
In response, Cloudflare has deployed new rules across its network, with the default action set to Block. These new protections are included in both the Cloudflare Free Managed Ruleset (available to all Free customers) and the standard Cloudflare Managed Ruleset (available to all paying customers), as detailed below. More information about the different rulesets can be found in our documentation.
| CVE | Description | Cloudflare WAF Rule ID |
|---|---|---|
| CVE-2025-55182React - RCE | Rules to mitigate React2Shell Exploit | Paid: 33aa8a8a948b48b28d40450c5fb92fbaFree: 2b5d06e34a814a889bee9a0699702280 |
| CVE-2025-55182 - 2React - RCE Bypass | Additional rules to mitigate exploit bypass | Paid: bc1aee59731c488ca8b5314615fce168Free: cbdd3f48396e4b7389d6efd174746aff |
| CVE-2025-55182Scanner Detection | Additional paid WAF rule to catch React2Shell scanning attempts | Paid: 1d54691cb822465183cb49e2f562cf5c |
Recently disclosed RSC vulnerabilities
In addition to React2Shell, two additional vulnerabilities affecting specific RSC implementations were disclosed. The two vulnerabilities, while distinct from React2Shell, also relate to RSC payload handling and Server Function semantics, with corresponding Cloudflare protections noted below:
| CVE | Description | Cloudflare WAF Rule ID |
|---|---|---|
| CVE-2025-55183Leaking Server Functions | In deployments where Server Function identifiers are insufficiently validated, an attacker may force the server into returning the source body of a referenced function | Paid: 17c5123f1ac049818765ebf2fefb4e9b Free: 3114709a3c3b4e3685052c7b251e86aa |
| CVE-2025-55184React Function DoS | A crafted RSC Flight Payload containing cyclical Promise references can trigger unbounded recursion or event-loop lockups under certain server configurations, resulting in denial-of-service conditions | Paid: 2694f1610c0b471393b21aef102ec699 |
Investigation of early scanning and exploitation
The following analysis details the initial wave of activity observed by Cloudforce One, focusing on threat actor attempts to scan for and exploit the React2Shell vulnerability. While these findings represent activity immediately following the vulnerability’s release, and were focused on known threat actors, it is critical to note that the volume and scope of related threat activity have expanded dramatically since these first observations.
Tactics
Unsurprisingly, the threat actors were relying heavily on publicly available, commercial, and a variety of other tools to identify vulnerable servers:
Vulnerability intelligence: The actors leveraged vulnerability intelligence databases that aggregated CVEs, advisories, and exploits for tracking and prioritization.
Vulnerability reconnaissance: The actors conducted searches using large-scale reconnaissance services, indicating they are relying on Internet-wide scanning and asset discovery platforms to find exposed systems running React App or RSC components. They also made use of tools that identify the software stack and technologies used by websites.
Vulnerability scanning: Activity included use of Nuclei (User-Agent: Nuclei - CVE-2025-55182), a popular rapid scanning tool used to deploy YAML-based templates to check for vulnerabilities. The actors were also observed using a highly likely React2Shell scanner associated with the User-Agent "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36 React2ShellScanner/1.0.0".
Vulnerability exploitation: The actors made use of Burp Suite, a web application security testing platform for identifying and exploiting vulnerabilities in HTTP/S traffic.
Techniques
Recon via Internet-wide scanning and asset discovery platform To enumerate potential React2Shell targets, the actors leveraged an Internet-wide scanning and asset-discovery platform commonly used to fingerprint web technologies at scale. Their queries demonstrated a targeted effort to isolate React and Next.js applications — two frameworks directly relevant to the vulnerability — by searching for React-specific icon hashes, framework-associated metadata, and page titles containing React-related keywords. This approach likely allowed them to rapidly build an inventory of exploitable hosts before initiating more direct probing.
**Targeting enumeration and filtering ** During their reconnaissance phase, the operators applied additional filtering logic to refine their target set and minimize noise. Notably, they excluded Chinese IP space from their searches, indicating that their enumeration workflow intentionally avoided collecting data on possibly domestic infrastructure. They also constrained scanning to specific geographic regions and national networks to identify likely high-value hosts. Beyond basic fingerprinting, the actors leveraged SSL certificate attributes — including issuer details, subject fields, and top-level domains — to surface entities of interest, such as government or critical-infrastructure systems using .gov or other restricted TLDs. This combination of geographic filtering and certificate-based pivoting enabled a more precise enumeration process that prioritized strategically relevant and potentially vulnerable high-value targets.
Preliminary target analysis Observed activity reflected a clear focus on strategically significant organizations across multiple regions. Their highest-density probing occurred against networks in Taiwan, Xinjiang Uygur, Vietnam, Japan, and New Zealand — regions frequently associated with geopolitical intelligence collection priorities. Other selective targeting was also observed against entities across the globe, including government (.gov) websites, academic research institutions, and critical‑infrastructure operators. These infrastructure operators specifically included a national authority responsible for the import and export of uranium, rare metals, and nuclear fuel.
The actors also prioritized high‑sensitivity technology targets such as enterprise password managers and secure‑vault services, likely due to their potential to provide downstream access to broader organizational credentials and secrets.
Additionally, the campaign targeted edge‑facing SSL VPN appliances whose administrative interfaces may incorporate React-based components, suggesting the actor sought to exploit React2Shell against both traditional web applications and embedded web management frameworks in order to maximize access opportunities.
Early threat actor observations Cloudforce One analysis confirms that early scanning and exploitation attempts originated from IP addresses previously associated with multiple Asia-affiliated threat actor clusters. While not all observed IP addresses belong to a single operator, the simultaneous activity suggests shared tooling, infrastructure, or experimentation in parallel among groups with a common purpose and shared targeting objectives. Observed targeting enumeration and filtering (e.g. a focus on Taiwan and Xinjiang Uygur, but exclusion of China), as well as heavy use of certain scanning and asset discovery platforms, suggest general attribution to Asia-linked threat actors.
Overall trends
Cloudflare’s Managed Rulesets for React2Shell began detecting significant activity within hours of the vulnerability’s disclosure. The graph below shows the daily hit count across the two exploit-related React2Shell WAF rules.
Aggregate rule hit volume over time
The React2Shell disclosure triggered a surge of opportunistic scanning and exploit behavior. In total, from 2025-12-03 00:00 UTC to 2025-12-11 17:00UTC, we received 582.10M hits. That equates to an average of 3.49M hits per hour, with a maximum number of hits in a single hour reaching 12.72M. The average unique IP count per hour was 3,598, with the maximum number of IPs in an hour being 16,585.
Hourly count of unique IPs sending React2Shell-related probes
Our data also shows distinct peaks above 6,387 User-Agents per hour, indicating a heterogeneous mix of tools and frameworks in use, with the average number of unique User-Agents per hour being 2,255. The below graph shows exploit attempts based on WAF rules (Free and Managed) triggering on matching payloads:
Unique User-Agent strings used in React2Shell-related requests
To better understand the types of automated tools probing for React2Shell exposure, Cloudflare analyzed the User-Agent strings associated with React2Shell-related requests since December 3, 2025. The data shows a wide variety of scanning tools suggesting broad Internet-wide reconnaissance:
| Top 10 User Agent strings by exploit attempts |
|---|
| Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36 Assetnote/1.0.0 |
| Block Security Team/Assetnote-HjJacErLyq2xFe01qaCM1yyzs |
| Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36 (GIS - AppSec Team - Project Vision) |
| Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 |
| python-requests/2.32.5 |
| Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36 Assetnote/1.0.0 (ExposureScan) |
| Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.124 Safari/537.36 |
| Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/142.0.0.0 Safari/537.36 |
| Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36 |
| Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/18.1 Safari/605.1.1 |
Payload variation and experimentation
Cloudflare analyzed the payload sizes associated with requests triggering React2Shell-related detection rules. The long-tailed distribution — dominated by sub-kilobyte probes, but punctured by extremely large outliers — suggest actors are testing a wide range of payload sizes:
| Metric | Value |
|---|---|
| Maximum payload size | 375 MB |
| Average payload size | 3.2 KB |
| p25 (25th Percentile) | 703 B |
| p75 (75th Percentile) | 818 B |
| p90 (90th Percentile) | 2.7 KB |
| p99 (99th Percentile) | 66.5 KB |
| Standard deviation | 330 KB |
Additional React vulnerabilities identified
In parallel with our ongoing analysis of the React2Shell vulnerability, two additional vulnerabilities affecting React Server Components (RSC) implementations have been identified:
1. React function DoS
The vulnerability CVE-2025-55184 was recently disclosed, revealing that React Server Component frameworks can be forced into a Node.js state where the runtime unwraps an infinite recursion of nested Promises.
This behavior:
Freezes the server indefinitely
Prevents yielding back to the event loop
Effectively takes the server offline
Does not require any specific Server Action usage — merely the presence of a server capable of processing an RSC Server Action payload
The trigger condition is a cyclic promise reference inside the RSC payload.
2. Leaking server functions
Another vulnerability, CVE-2025-55183, was also recently disclosed, revealing that certain React Server Component frameworks can leak server-only source code under specific conditions.
If an attacker gains access to a Server Function that:
Accepts an argument that undergoes string coercion, and
Does not validate that the argument is of an expected primitive type
then the attacker can coerce that argument into a reference to a different Server Function. The coerced value’s toString() output causes the server to return the source code of the referenced Server Function.
How Cloudflare is protecting customers
Cloudflare’s protection strategy is multi-layered, relying on both the inherent security model of its platform and immediate, proactive updates to its Web Application Firewall (WAF).
Cloudflare Workers: React-based applications and frameworks deployed on Cloudflare Workers are inherently immune. The Workers security model prevents exploits from succeeding at the runtime layer, regardless of the malicious payload.
Proactive WAF deployment: Cloudflare urgently deployed WAF rules to detect and block traffic proxied through its network related to React2Shell and the recently disclosed RSC vulnerabilities.
The Cloudflare security team continues to monitor for additional attack variations and will update protections as necessary to maintain continuous security for all proxied traffic.
Continuous monitoring
While Cloudflare’s emergency actions — the WAF limit increase and immediate rule deployment — have successfully mitigated the current wave of exploitation attempts, this vulnerability represents a persistent and evolving threat. The immediate weaponization of CVE-2025-55182 by sophisticated threat actors underscores the need for continuous defense.
Cloudflare remains committed to continuous surveillance for emerging exploit variants and refinement of WAF rules to detect evasive techniques. However, network-level protection is not a substitute for remediation at the source. Organizations must prioritize immediate patching of all affected React and Next.js assets. This combination of platform-level WAF defense and immediate application patching remains the only reliable strategy against this critical threat.
Indicators of Compromise
| Tool/Scanner | User Agent String | Observation/Purpose |
|---|---|---|
| Nuclei | Nuclei - CVE-2025-55182 | User-Agent for rapid, template-based scanning for React2Shell vulnerability |
| React2ShellScanner | Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36 React2ShellScanner/1.0.0 | User-Agent for a likely custom React2Shell vulnerability scanner |
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