CVE-2026-21873VulnerabilityPublished 8 Jan 2026

CVE-2026-21873: zauberzeug nicegui · Cross-site Scripting

Affects zauberzeug niceguiSource CVE Database V5

Not known to be exploited · CVSS 7.2 High : upgrade to 3.5.0 soon.

Data as of 26 Oct 2026, 07:57 UTC

CVSS 3.1 base score

7.2High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.2% chance of exploitation in 30 days – higher than 17% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 3.5.0

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Changed
Confidentiality: Low
Integrity: Low
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 8 Jan 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 2026-10-26.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade zauberzeug nicegui to 3.5.0.

Affected software

Product	Affected	Fixed in
zauberzeugnicegui	Affected >=2.22.0 <3.5.0	Fixed in 3.5.0
PyPInicegui`pkg:pypi/nicegui`Other threats	Affected >=2.22.0 <=3.4.1	Fixed in–

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

1. Upgrade all NiceGUI deployments to version 3.5.0 or later immediately to apply the official patch addressing the XSS vulnerability. 2. Implement strict Content Security Policies (CSP) that restrict script sources and disallow inline scripts to reduce the risk of script injection exploitation. 3. Review and sanitize any user-controllable inputs that may influence URL fragments or client-side routing logic beyond the framework's default handling. 4. Avoid embedding NiceGUI-based applications within iframes on untrusted domains or implement frame-ancestors directives to control framing. 5. Conduct security testing, including penetration tests and code reviews, focusing on client-side input handling and URL manipulation. 6. Monitor web application logs and user reports for suspicious activity related to URL fragment manipulation or unexpected script execution. 7. Educate developers about secure handling of client-side routing and fragment identifiers to prevent similar issues in future development.

Description

CVE-2026-21873 is a high-severity cross-site scripting (XSS) vulnerability in the Python UI framework NiceGUI versions 2.22.0 through 3.4.1. The flaw arises from unsafe handling of the URL fragment identifier in the pushstate event listener used by ui.sub_pages, allowing attackers to manipulate the URL fragment via cross-site iframes without requiring user interaction or privileges. This can lead to partial compromise of confidentiality and integrity by injecting malicious scripts. The vulnerability has been patched in version 3.5.

Impact

For European organizations, this vulnerability poses a significant risk to web applications built with the affected versions of NiceGUI. The ability to inject scripts via manipulated URL fragments can lead to theft of sensitive user data such as authentication tokens, personal information, or session cookies, undermining confidentiality. It can also allow attackers to alter the user interface or application logic, impacting integrity. Although availability is not directly affected, the loss of trust and potential regulatory consequences under GDPR due to data breaches could be severe. Organizations with public-facing dashboards, internal tools accessible via browsers, or embedded iframe content are particularly vulnerable. The cross-origin exploit vector increases the attack surface, as attackers can host malicious pages that trigger the vulnerability without direct access to the target domain. This could facilitate phishing, session hijacking, or further lateral attacks within corporate networks. The high CVSS score (7.2) reflects the ease of exploitation and the potential for impactful data compromise, making timely remediation critical.

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #1OWASP Top 10 A05:2025: Injection (opens in a new tab)
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.
Architecture and Design · Libraries or Frameworks
Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
Implementation · Architecture and Design
Understand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messages.
Architecture and Design · Implementation · Attack Surface Reduction
Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide…
OWASP Cheat Sheets
Cross Site Scripting Prevention (opens in a new tab)
DOM based XSS Prevention (opens in a new tab)
Injection Prevention (opens in a new tab)
Abuse Case (opens in a new tab)
Deserialization (opens in a new tab)
OWASP ASVS 4.0.3
V5.3.3 (opens in a new tab)
V1.5.1 (opens in a new tab)
Attack patterns
XSS Using MIME Type Mismatch (opens in a new tab)
DOM-Based XSS (opens in a new tab)
Reflected XSS (opens in a new tab)
Stored XSS (opens in a new tab)
Cross-Site Scripting (XSS) (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 15 Jan 2026, 12:41 UTC

WeaknessesCWE-79

Technical details

References

Community reviews

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