Cross-Site Cookie Injection in Eclipse Vert.x Web Client

Executive Summary

CVE-2026-15076 is a vulnerability in the Eclipse Vert.x Web Client, specifically in the WebClientSession component. It affects versions up to and including 4.5.29 (4.x branch) and 5.1.4 (5.x branch).

The vulnerability occurs because the WebClientSession component does not validate that the Domain attribute of a Set-Cookie response header matches the originating server's domain. This violates RFC 6265 section 5.3, which specifies how cookies should be handled across domains.

An attacker who controls any server that the victim application contacts can exploit this flaw. They can inject a cookie scoped to an arbitrary third-party domain. Since the session store does not perform a cross-domain ownership check, it stores this cookie and later transmits it to the targeted domain.

When the victim application sends a subsequent request to the targeted domain using the same WebClientSession, it includes the attacker-injected cookie. This allows the attacker to access sensitive data, such as payment details or API payloads, through their own account on the targeted service.

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Detection Guidance

Detecting this vulnerability requires checking if your application uses Eclipse Vert.x Web Client versions up to and including 4.5.29 (4.x branch) or 5.1.4 (5.x branch). Since the issue involves improper validation of the Domain attribute in Set-Cookie headers, you can monitor network traffic for unexpected cookies being set or transmitted to third-party domains.

• Check the version of Eclipse Vert.x Web Client in your project dependencies. For Maven projects, inspect the pom.xml file for the version tag under the vertx-web-client dependency.
• Use network monitoring tools like Wireshark or tcpdump to capture HTTP/HTTPS traffic and inspect Set-Cookie headers for mismatched Domain attributes. Example command for tcpdump: tcpdump -i any -A 'port 80 or port 443' | grep -i 'Set-Cookie'.
• Review application logs for unexpected cookie behavior, such as cookies being sent to domains that do not match the originating server.
• If your application interacts with untrusted servers, verify that no cookies are being injected into requests to trusted third-party domains.

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Impact Analysis

This vulnerability can impact you in several ways if you are using an affected version of Eclipse Vert.x Web Client.

• An attacker could inject malicious cookies into your application's session, leading to unauthorized access to sensitive data.
• If your application interacts with third-party services (e.g., payment gateways, APIs), the attacker could intercept or manipulate requests sent to those services under your session.
• Sensitive information included in your requests, such as payment amounts, card details, or other API payloads, could be exposed to the attacker.
• The attacker could gain access to your account on the targeted third-party service, potentially leading to financial loss or data breaches.

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Compliance Impact

This vulnerability can have significant implications for compliance with common standards and regulations.

• GDPR (General Data Protection Regulation): If the vulnerability leads to unauthorized access to personal data of EU citizens, it could result in a data breach. GDPR requires organizations to implement appropriate security measures to protect personal data. Failure to do so could lead to fines and legal consequences.
• HIPAA (Health Insurance Portability and Accountability Act): If the affected application handles protected health information (PHI), this vulnerability could lead to unauthorized access to PHI. HIPAA requires strict controls over access to PHI, and a breach could result in penalties and legal action.
• PCI DSS (Payment Card Industry Data Security Standard): If the application processes payment card information, this vulnerability could lead to unauthorized access to cardholder data. PCI DSS requires organizations to protect cardholder data, and a breach could result in fines, increased transaction fees, or loss of the ability to process payments.

In summary, this vulnerability could lead to non-compliance with these regulations, resulting in legal, financial, and reputational consequences for affected organizations.

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Mitigation Strategies

To mitigate this vulnerability, follow these steps:

• Upgrade Eclipse Vert.x Web Client to a patched version as soon as it becomes available. The fix is planned for release early next week, according to the provided resource.
• If upgrading is not immediately possible, avoid using the WebClientSession component for requests to untrusted servers or domains where cookie injection could occur.
• Implement strict validation of Set-Cookie headers in your application to ensure the Domain attribute matches the originating server's domain, as per RFC 6265 section 5.3.
• Use network-level protections, such as firewalls or web application firewalls (WAFs), to block or alert on suspicious Set-Cookie headers that violate domain matching rules.
• Monitor outgoing requests from your application to ensure no attacker-injected cookies are being transmitted to third-party domains.
• Consider isolating sensitive operations or data from requests that involve untrusted servers to minimize the risk of exposure.

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