medium CVSS 6.1

CVE-2026-21871·Python vulnerability

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.

Severity
medium
Software
Python
Fixed in
3.5.0
Published
2026-01-08

Affected versions

From: 2.13.0

Until: 3.5.0

Fixed in: 3.5.0

How to fix this CVE

Upgrade NiceGUI to version 3.5.0 or later to patch the XSS vulnerability in the history navigation helpers. If your application passes user-controlled input to ui.navigate.history.push() or ui.navigate.history.replace(), this update is critical. Ensure all Python dependencies are updated after patching.

sudo dnf update python3-nicegui

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST|GET request with URL parameters containing encoded JavaScript payloads (e.g., %27onclick=%27|javascript:|<script) passed to endpoints that process ui.navigate.history navigation calls

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Block or sanitize URL parameters passed to backend endpoints that trigger ui.navigate.history.push() or ui.navigate.history.replace(). Implement output encoding rules to escape single quotes, angle brackets, and JavaScript protocol handlers in URL values before embedding them in generated JavaScript.

How to check if you are affected

  1. Step 1: Check the installed NiceGUI version with: pip show nicegui | grep Version
  2. Step 2: Review your application code for calls to ui.navigate.history.push() or ui.navigate.history.replace() that accept external input: grep -r 'ui.navigate.history' /path/to/app --include='*.py'
  3. Step 3: Search web server logs for anomalous JavaScript payloads in URL parameters or POST data, particularly strings containing javascript: or script tags in navigated URLs
  4. Step 4: Verify the patch by re-running 'pip show nicegui' and confirming the version is 3.5.0 or higher

FAQ

What is CVE-2026-21871?

CVE-2026-21871 is a cross-site scripting (XSS) vulnerability in NiceGUI's history navigation API where unsafe URL arguments are embedded into JavaScript without proper escaping, allowing attackers to inject malicious code that executes in the user's browser.

Is CVE-2026-21871 being actively exploited?

No, CVE-2026-21871 is not currently listed in the CISA Known Exploited Vulnerabilities catalog and no public exploits have been released.

What versions of NiceGUI are affected by CVE-2026-21871?

NiceGUI versions 2.13.0 through 3.5.0 (exclusive) are vulnerable. Version 3.5.0 and later contain the patch.

How do I check if my server is vulnerable to CVE-2026-21871?

Run 'pip show nicegui' to check your installed version. If it is between 2.13.0 and 3.4.1 inclusive, and your application uses ui.navigate.history.push() or ui.navigate.history.replace() with untrusted input, you are vulnerable.

Does Defensia detect CVE-2026-21871?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If NiceGUI is installed on a monitored server, CVE-2026-21871 will appear in your dashboard with remediation steps.

Related Python CVEs

CVE-2026-25632CVSS 10EPyT-Flow is a Python package designed for the easy generation of hydraulic and water quality scenario data of water distribution networks. Prior to 0.16.1, EPyT-Flow’s REST API parses attacker-controlled JSON request bodies using a custom deserializer (my_load_from_json) that supports a type field. When type is present, the deserializer dynamically imports an attacker-specified module/class and instantiates it with attacker-supplied arguments. This allows invoking dangerous classes such as subprocess.Popen, which can lead to OS command execution during JSON parsing. This also affects the loading of JSON files. This vulnerability is fixed in 0.16.1.
CVE-2026-34938CVSS 10PraisonAI is a multi-agent teams system. Prior to version 1.5.90, execute_code() in praisonai-agents runs attacker-controlled Python inside a three-layer sandbox that can be fully bypassed by passing a str subclass with an overridden startswith() method to the _safe_getattr wrapper, achieving arbitrary OS command execution on the host. This issue has been patched in version 1.5.90.
CVE-2026-33054CVSS 10Mesop is a Python-based UI framework that allows users to build web applications. Versions 1.2.2 and below contain a Path Traversal vulnerability that allows any user supplying an untrusted state_token through the UI stream payload to arbitrarily target files on the disk under the standard file-based runtime backend. This can result in application denial of service (via crash loops when reading non-msgpack target files as configurations), or arbitrary file manipulation. This vulnerability heavily exposes systems hosted utilizing FileStateSessionBackend. Unauthorized malicious actors could interact with arbitrary payloads overwriting or explicitly removing underlying service resources natively outside the application bounds. This issue has been fixed in version 1.2.3.
CVE-2026-28505CVSS 10Tautulli is a Python based monitoring and tracking tool for Plex Media Server. Prior to version 2.17.0, the str_eval() function in notification_handler.py implements a sandboxed eval() for notification text templates. The sandbox attempts to restrict callable names by inspecting code.co_names of the compiled code object. However, co_names only contains names from the outer code object. When a lambda expression is used, it creates a nested code object whose attribute accesses are stored in code.co_consts, NOT in code.co_names. The sandbox never inspects nested code objects. This issue has been patched in version 2.17.0.
CVE-2026-26030CVSS 9.9Semantic Kernel, Microsoft's semantic kernel Python SDK, has a remote code execution vulnerability in versions prior to 1.39.4, specifically within the `InMemoryVectorStore` filter functionality. The problem has been fixed in version `python-1.39.4`. Users should upgrade this version or higher. As a workaround, avoid using `InMemoryVectorStore` for production scenarios.

References

Track CVEs across your fleet automatically

Defensia scans your Linux servers and tells you exactly which ones are running vulnerable versions — including CVE-2026-21871. Free for 1 server.

Get started free