CVE-2021-33604

Reflected XSS via unchecked dev-server URL forwarding

Published Jun 24, 2021 · Updated Sep 17, 2024

Reflected XSS in Vaadin Flow Server 2.0.0 through 2.6.1 and 3.0.0 through 6.0.9 allows local users to execute JavaScript. DevModeHandlerImpl forwards request paths containing an unescaped double quote to webpack dev-server, which does not escape the invalid URL character. Exploitation requires a developer to run the frontend development server and open an attacker-controlled page, exposing data available in the local application; deployed applications are unaffected.

CVSS severity2.5
Low
EPSS probability0.29%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Reflected XSS in Vaadin Flow Server 2.0.0 through 2.6.1 and 3.0.0 through 6.0.9 allows local users to execute JavaScript. DevModeHandlerImpl forwards request paths containing an unescaped double quote to webpack dev-server, which does not escape the invalid URL character. Exploitation requires a developer to run the frontend development server and open an attacker-controlled page, exposing data available in the local application; deployed applications are unaffected.

The record

CVE
CVE-2021-33604
Published
Jun 24, 2021
Updated
Sep 17, 2024
Vendor
Vaadin
Product
Flow Server
Classifications
CWE-172, T1189
Attack vector
local
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 24, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=2.0.0 <unspecified
  2. Affected versionversion=3.0.0 <unspecified
  3. Affected versionversion=unspecified <=2.6.1
  4. Affected versionversion=unspecified <=6.0.9

What conditions does exploitation require?

Attack vectorlocal
Required privilegesunauthenticated

What is affected?

Vaadin · Flow Serverversion=2.0.0 <unspecified; version=3.0.0 <unspecified; version=unspecified <=2.6.1; version=unspecified <=6.0.9
Vaadin · Vaadin Frameworkversion=14.0.0 <unspecified; version=15.0.0 <unspecified; version=unspecified <=14.6.1; version=unspecified <=19.0.8

Published CVSS scores

2.5Vaadin Ltd.CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:U/C:L/I:N/A:N
1.2NIST NVDAV:L/AC:H/Au:N/C:P/I:N/A:N
2.5NIST NVDCVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:U/C:L/I:N/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-172Encoding Error
T1189Drive-by Compromise

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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