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LOW severity

CVE-2021-31406 — flow-server

LOWFix: vaadin/flow#10157

CVE-2021-31406 is a low-severity (CVSS 2.5) CWE-208 vulnerability in com.vaadin:flow-server. A fix is available for com.vaadin:flow-server — see the affected versions and patch details below.

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

Also known asGHSA-p7jq-v8jp-j424
Published
Updated
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
0.2%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs10th percentile — riskier than 10% of all scored CVEsHighest risk
0.00%0.24%0.47%0.71%0.1%0.2%Apr 26Sep 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-31406 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

2 pkgs affected
☕com.vaadin:flow-server☕com.vaadin:flow-server

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.

Description

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.

Affected Packages

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
☕Mavencom.vaadin:flow-server≥ 3.0.0&&< 5.0.45.0.4com.vaadin:flow-server:5.0.4
☕Mavencom.vaadin:flow-server≥ 6.0.0&&< 6.0.16.0.1com.vaadin:flow-server:6.0.1

Affected Products

2 products · 4 configurations
Application
flowvaadin
≥ 3.0.0 && < 5.0.4
1 version
6.0.0
Application
vaadinvaadin
≥ 15.0.0 && < 18.0.7
1 version
19.0.0

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for com.vaadin:flow-server, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update com.vaadin:flow-server to 5.0.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-31406 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

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
O3 Security · Impact-Aware SCA

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CVE-2021-31406: flow-server CSRF — Fixed in 5.0.4