CVE-2026-15076 in Eclipse Vert.x
In versions up to and including 4.5.29 (4.x branch) and 5.1.4 (5.x branch), the WebClientSession component of Eclipse Vert.x Web Client does not validate that the Domain attribute of a Set-Cookie response header matches the originating server's domain, in violation of RFC 6265 section 5.3. An attacker who controls any server that the victim application contacts can inject a cookie scoped to an arbitrary third-party domain; because the session store performs no cross-domain ownership check, it stores and later transmits that cookie to the targeted domain. When the victim application subsequently sends a request to the targeted domain using the same WebClientSession, it presents the attacker-injected cookie, causing the receiving service to process the request under the attacker's account. Sensitive data included in the victim application's requests, such as payment amounts, card details, or other API payloads, may then be accessible to the attacker through their own account on that service.
Quick answer
Eclipse Foundation Eclipse Vert.x Web Client should be reviewed and updated if it matches the affected versions. The recommended fix is to apply the vendor-supported patched version or the mitigation steps below, then retest the public website with Fixnx.
Who is affected
Affected versions
- Eclipse Vert.x 4.x through 4.5.29
- Eclipse Vert.x 5.x through 5.1.4
Fixed versions
- Eclipse Vert.x 4.5.30 or later
- Eclipse Vert.x 5.1.5 or later
How to fix it
CVE-2026-15076 affects Eclipse Vert.x Web Client. In simple terms, private headers or cookies may be sent to the wrong place. Fix it by updating to Eclipse Vert.x 4.5.30 or later; Eclipse Vert.x 5.1.5 or later. If this system is online or used by admins, treat the fix as important.
- Find every place where Eclipse Vert.x Web Client is installed or used.
- Check if the version matches: Eclipse Vert.x 4.x through 4.5.29; Eclipse Vert.x 5.x through 5.1.4.
- Update or apply the vendor fix: Eclipse Vert.x 4.5.30 or later; Eclipse Vert.x 5.1.5 or later.
- If you cannot update today, allow only trusted users and trusted networks.
- Turn off the affected feature or plugin if the vendor says that is safe to do.
- Check logs for crashes, strange requests, new admin actions, or unexpected access.
- Change secrets or passwords if you think data or admin access may have been exposed.
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Verify the fix
- Confirm the installed version or mitigation matches: Eclipse Vert.x 4.5.30 or later; Eclipse Vert.x 5.1.5 or later.
- Confirm the affected feature is no longer exposed to untrusted users.
- Run the affected workflow again and make sure it still works.
- Review logs again after the fix for repeated suspicious activity.
- Run a new Fixnx scan if this software is part of a public website.
- Save the advisory link and proof of the fix in your change record.
Related categories
Trusted references
FAQ
What is affected by CVE-2026-15076?
Eclipse Foundation Eclipse Vert.x Web Client versions listed as affected should be reviewed: Eclipse Vert.x 4.x through 4.5.29, Eclipse Vert.x 5.x through 5.1.4.
What should I fix first?
Start with internet-facing sites, admin panels, login flows, plugins, themes, modules, packages, and systems that process user-controlled input or sensitive data.
How do I confirm the fix worked?
Apply the patched version or mitigation, clear caches where relevant, retest the affected workflow, and run a new Fixnx scan to verify public website exposure signals.
How are Fixnx security risk categories chosen?
Fixnx keeps one canonical risk page and assigns only broad, relevant categories such as ecosystem, technology area, or vulnerability class.