Security readout for executives and security teams
Plain-English summary
CVE-2026-21873 is a high-severity cross-site scripting flaw in NiceGUI applications using ui.sub_pages. A malicious site can manipulate a victim’s browser URL fragment through an iframe, triggering script execution without user interaction. Organizations running affected NiceGUI versions should prioritize upgrading to the patched release.
Executive priority
Treat as a near-term remediation item for any internet-facing NiceGUI application using ui.sub_pages. The issue requires no user interaction and can affect confidentiality and integrity, but availability impact is not indicated.
Technical view
NiceGUI versions 2.22.0 through 3.4.1 have an unsafe pushstate event listener implementation used by ui.sub_pages. The flaw enables zero-click XSS via URL fragment manipulation, including cross-site iframe scenarios. It is classified as CWE-79 with CVSS 3.1 score 7.2 and scope changed.
Likely exposure
Exposure is limited to applications built with NiceGUI versions >=2.22.0 and <3.5.0 that use ui.sub_pages. Publicly accessible applications increase business risk because attacks can originate from another site visited by users.
Exploitation context
The source bundle describes zero-click XSS and cross-site iframe-based URL fragment manipulation. It does not state active exploitation, and KEV is false. No public exploit status is established from the provided sources.
Researcher notes
Evidence comes from the CVE record and GitHub advisory. The vulnerable range and patched version are clear. The provided material does not include exploit code, reports of exploitation in the wild, or compensating controls beyond upgrading.
Mitigation direction
- Upgrade NiceGUI to version 3.5.0 or later.
- Identify applications using ui.sub_pages before prioritizing rollout.
- Review vendor advisory and release notes for any additional guidance.
- If immediate upgrade is delayed, assess exposure of affected applications.
Validation and detection
- Inventory NiceGUI package versions across deployed applications.
- Confirm affected applications use ui.sub_pages.
- Verify NiceGUI is upgraded to 3.5.0 or later.
- Review application security tests for reflected or DOM-based XSS coverage.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-79: User-session and phishing behavior lookup
Client-side and session-facing weaknesses should be reviewed alongside initial-access and user-execution behaviors. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2026-21873 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 7.2 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N3.92.7Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
7.2HighVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
Source materials
- CVE List V5 sourceCVE List V5
- https://github.com/zauberzeug/nicegui/security/advisories/GHSA-mhpg-c27v-6mxrCVE reference · x_refsource_CONFIRM
- https://github.com/zauberzeug/nicegui/releases/tag/v3.5.0CVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.