Is CVE-2026-21871 in the CISA KEV catalog?

No. CVE-2026-21871 is not currently on the CISA Known Exploited Vulnerabilities list.

What is the CVSS severity of CVE-2026-21871?

CVE-2026-21871 has a CVSS v3 base score of 6.1 (MEDIUM).

What is the EPSS score for CVE-2026-21871?

CVE-2026-21871 has an EPSS score of 0.24% (15th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2026-21871?

Upgrade nicegui to 3.5.0 or later.

CVE & CISA-KEV Catalog

CVE-2026-21871

MEDIUM

6.1

CVSS v3

NVD

Description

NiceGUI is a Python-based UI framework. From versions 2.13.0 to 3.4.1, there is a XSS risk in NiceGUI when developers pass attacker-controlled strings into ui.navigate.history.push() or ui.navigate.history.replace(). These helpers are documented as History API wrappers for updating the browser URL without page reload. However, if the URL argument is embedded into generated JavaScript without proper escaping, a crafted payload can break out of the intended string context and execute arbitrary JavaScript in the victim’s browser. Applications that do not pass untrusted input into ui.navigate.history.push/replace are not affected. This issue has been patched in version 3.5.0.

CWE-79Cross-Site Scripting (XSS)

How to fix

Remediation Available

niceguiNVD

Affected:>= 2.13.0, < 3.5.0Fixed in:3.5.0CVE-2026-21871derived from NVD

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityLow

Privileges RequiredNone

User InteractionRequired

ScopeChanged

Impact

ConfidentialityLow

IntegrityLow

AvailabilityNone

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Exploit Intelligence

0.24%probability of exploitation in 30 days

15thpercentile

Low risk: more likely to be exploited than 15% of all known CVEs.

References

Exploit1

github.com

Release Notes1

github.com

Related Vulnerabilities

Other CWE-79 (Cross-Site Scripting (XSS)) vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2020-11022	Medium	6.9	99%	-	Fix
CVE-2019-3929	Critical	9.8	99%	KEV	Fix
CVE-2020-9496	Medium	6.1	99%	-	-
CVE-2023-28341	Medium	6.1	99%	-	Fix
CVE-2018-12998	Medium	6.1	98%	-	-
CVE-2025-4123	High	7.6	98%	-	Fix

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-01-15.