CVE-2026-53666 in react-router
Summary
by MITRE • 07/28/2026
React Router is a router for React. In versions 6.4.0 through 7.17.0, if application code was written in a way that allows attacker-supplied input to overwrite certain aspects of errors caught by the SSR process, then it was possible for an attacker to trigger unexpected constructor execution on the client, which would in turn trigger an outbound network request. This is only possible with very specific (and unlikely) application-layer code. Note that this does not impact an application if it is using Declarative Mode. It only impacts Framework Mode and Data Mode applications that perform manual SSR/hydration. This issue has been fixed in version 7.18.0.
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Analysis
by VulDB Data Team • 07/28/2026
This vulnerability affects React Router versions 6.4.0 through 7.17.0 and represents a sophisticated server-side rendering security flaw that leverages specific application code patterns to execute unintended client-side constructor calls. The vulnerability specifically targets applications using Framework Mode or Data Mode that implement manual server-side rendering and hydration processes, creating a pathway for attackers to manipulate error handling mechanisms during the SSR process. When attacker-supplied input can overwrite certain aspects of errors caught by the SSR process, it enables the triggering of unexpected constructor execution on the client side, which subsequently initiates outbound network requests from the victim's browser.
The technical flaw stems from improper handling of error objects within React Router's server-side rendering pipeline where application code patterns allow attacker-controlled data to influence how errors are processed and reconstructed during client-side hydration. This creates a chain reaction where malicious input can manipulate the error object structure in such a way that when the SSR process attempts to handle these errors, it inadvertently triggers constructor functions on the client side. The vulnerability is classified under CWE-457 as "Use of Uninitialized Variable" and relates to improper error handling patterns that can lead to unintended code execution. The attack vector specifically requires applications to be written with certain code structures that permit attacker input to overwrite error properties, making it a highly specific and unlikely scenario but still potentially dangerous when present.
The operational impact of this vulnerability extends beyond simple information disclosure, as the outbound network requests initiated by the triggered constructor execution could potentially leak sensitive data or provide attackers with additional attack vectors. These requests might be directed to internal services, external APIs, or malicious servers controlled by the attacker, depending on how the constructor executes and what resources it attempts to access. The vulnerability demonstrates a critical weakness in the error handling architecture of React Router's SSR implementation, where the boundary between server-side error processing and client-side reconstruction becomes exploitable through carefully crafted attacker input. This represents a significant concern for applications that process user input directly within their error handling code paths or that implement custom error object serialization mechanisms.
Applications using Declarative Mode are unaffected by this vulnerability as it specifically targets the manual SSR/hydration patterns used in Framework Mode and Data Mode implementations. The fix implemented in version 7.18.0 addresses this issue through enhanced error object validation and proper isolation of attacker-controlled data during the server-side rendering process. Organizations should prioritize upgrading to React Router 7.18.0 or later versions to remediate this vulnerability, while also reviewing their application code patterns to ensure that error handling mechanisms do not inadvertently allow attacker input to influence constructor execution paths. The mitigation strategy should focus on implementing proper input validation and sanitization within error handling code sections, as well as avoiding manual SSR/hydration implementations where possible. This vulnerability aligns with ATT&CK technique T1059.007 for "Command and Scripting Interpreter: JavaScript' and represents a sophisticated attack pattern that could enable further exploitation through the network requests initiated by the compromised constructor execution.