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LOW severity

CVE-2021-31404 — flow-server

LOWFix: vaadin/flow#9875

CVE-2021-31404 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 UIDL request handler in Vaadin 10, 11-14, and 15-18

Also known asGHSA-xwg3-qrcg-w9x6
Published
Updated
Affected
3 pkgs
Patched
3 / 3
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%0.0%0.2%Apr 26Aug 26Oct 26

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

How urgent is this, really

CVE-2021-31404 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

3 pkgs affected
☕com.vaadin:flow-server☕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 UIDL request handler in com.vaadin:flow-server versions 1.0.0 through 1.0.13 (Vaadin 10.0.0 through 10.0.16), 1.1.0 prior to 2.0.0 (Vaadin 11 through 13), 2.0.0 through 2.4.6 (Vaadin 14.0.0 through 14.4.6), 3.0.0 prior to 5.0.0 (Vaadin 15 prior to 18), and 5.0.0 through 5.0.2 (Vaadin 18.0.0 through 18.0.5) allows attacker to guess a security token via timing attack.

Affected Packages

3 total 3 fixed
EcosystemPackageVulnerable rangeFix
☕Mavencom.vaadin:flow-server≥ 1.0.0&&< 1.0.141.0.14com.vaadin:flow-server:1.0.14
☕Mavencom.vaadin:flow-server≥ 1.1.0&&< 2.4.72.4.7com.vaadin:flow-server:2.4.7
☕Mavencom.vaadin:flow-server≥ 3.0.0&&< 5.0.35.0.3com.vaadin:flow-server:5.0.3

Affected Products

2 products · 10 configurations
Application
flowvaadin
≥ 5.0.0 && < 5.0.3
range
Application
vaadinvaadin
≥ 18.0.0 && < 18.0.6
range

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 1.0.14 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-31404 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 UIDL request handler in `com.vaadin:flow-server` versions 1.0.0 through 1.0.13 (Vaadin 10.0.0 through 10.0.16), 1.1.0 prior to 2.0.0 (Vaadin 11 through 13), 2.0.0 through 2.4.6 (Vaadin 14.0.0 through 14.4.6), 3.0.0 prior to 5.0.0 (Vaadin 15 prior to 18), and 5.0.0 through 5.0.2 (Vaadin 18.0.0 through 18.0.5) allows attacker to guess a security token via timing attack. - https://vaadin.com/security/cve-2021-31404
O3 Security · Impact-Aware SCA

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CVE-2021-31404: flow-server CSRF — Fixed in 1.0.14