Timing side channel vulnerability in endpoint request handler in Vaadin 15-19

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

4.0 · CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

EPSS

• CVE-2021-31406: 0.21% estimated exploitation activity in the next 30 days; percentile 11.36 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Non-constant-time comparison of CSRF tokens in endpoint request handler in com.vaadin:flow-server versions 3.0.0 through 5.0.3 (Vaadin 15.0.0 through 18.0.6), and com.vaadin:fusion-endpoint version 6.0.0 (Vaadin 19.0.0) allows attacker to guess a security token for Fusion endpoints via timing attack. - https://vaadin.com/security/cve-2021-31406

Published by the source

19 Apr 2021, 14:50 UTC

Last modified by the source

1 Feb 2023, 05:05 UTC

Source last checked

26 Oct 2026, 16:06 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-p7jq-v8jp-j424

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# Patch Next advisory brief: CVE-2021-31406

Advisory: Timing side channel vulnerability in endpoint request handler in Vaadin 15-19
IDs: CVE-2021-31406, GHSA-p7jq-v8jp-j424
Record: https://patchnext.com/advisories/cve-2021-31406 (revision 1, source checked 26 Oct 2026, 16:06 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.0)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: com.vaadin:flow-server 6.0.1, 5.0.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven com.vaadin:flow-server: affected >= 3.0.0, < 5.0.4; >= 6.0.0, <= 6.0.0; fixed in 5.0.4, 6.0.1

## Suggested inspection (read-only)
- Maven com.vaadin:flow-server: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on com.vaadin:flow-server.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-p7jq-v8jp-j424 (GitHub-reviewed advisory GHSA-p7jq-v8jp-j424 lists affected ranges: Maven com.vaadin:flow-server >=3.0.0 =6.0.0 <=6.0.0)

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