How severe is CVE-2026-21871?

CVE-2026-21871 has a CVSS 3.x base score of 6.1, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is low, integrity low, and availability none.

Is CVE-2026-21871 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (15th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-21871?

CVE-2026-21871 affects Zauberzeug Nicegui. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-21871?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

Does CVE-2026-21871 have an EU (EUVD) identifier?

Yes. CVE-2026-21871 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-1478.

When was CVE-2026-21871 published?

CVE-2026-21871 was published on 2026-01-08 and last updated on 2026-06-17.

CVE-2026-21871

CVE-2026-21871 is a medium-severity vulnerability in Zauberzeug Nicegui with a CVSS 3.x base score of 6.1. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-79.

Key facts

Severity: Medium (CVSS 3.x base score 6.1)
EPSS exploit prediction: 0% (15th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-1478
Weakness: CWE-79
Affected product: Zauberzeug Nicegui
Published: 2026-01-08
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2026-21871?: 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.
How severe is CVE-2026-21871?: CVE-2026-21871 has a CVSS 3.x base score of 6.1, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is low, integrity low, and availability none.
Is CVE-2026-21871 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (15th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-21871?: CVE-2026-21871 affects Zauberzeug Nicegui. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-21871?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-21871 have an EU (EUVD) identifier?: Yes. CVE-2026-21871 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-1478.
When was CVE-2026-21871 published?: CVE-2026-21871 was published on 2026-01-08 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:zauberzeug:nicegui:*:*:*:*:*:*:*:*

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