CVE-2026-25640 — pydantic-ai
HIGHCVE-2026-25640 is a high-severity (CVSS 7.1) Path Traversal vulnerability in pydantic-ai. A fix is available for pydantic-ai — see the affected versions and patch details below.
Pydantic AI affected by Stored XSS via Path Traversal in Web UI CDN URL
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-25640.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
pydantic-ai🐍pydantic-ai-slimReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Summary
A Path Traversal vulnerability in the Pydantic AI web UI allows an attacker to serve arbitrary JavaScript in the context of the application by crafting a malicious URL. If a victim clicks the link or visits it via an iframe, attacker-controlled code executes in their browser, enabling theft of chat history and other client-side data.
This vulnerability only affects applications that use:
Agent.to_webto serve a chat interfaceclai webto serve a chat interface from the CLI
These are typically run locally (on localhost), but may also be deployed on a remote server.
Description
The web UI serves its frontend HTML by fetching it from a CDN. In affected versions, the CDN URL is constructed using a version query parameter from the request URL. This parameter is not validated, allowing path traversal sequences that cause the server to fetch and serve attacker-controlled HTML/JavaScript from an arbitrary source on the same CDN, instead of the legitimate chat UI package.
Who Is Affected
Projects are affected if your application uses Agent.to_web or clai web to serve the Pydantic AI chat interface.
Attack Scenario
-
An attacker crafts a URL pointing to the victim's Pydantic AI web UI instance (either
localhostwith the known port, or a remote server endpoint) with a maliciousversionquery parameter containing path traversal sequences. -
The attacker gets the victim to visit this URL — directly via a link, through a redirect, or by embedding it in an iframe.
-
When the victim's browser loads the page, the server fetches and serves attacker-controlled HTML/JavaScript instead of the legitimate chat UI.
-
The attacker's JavaScript executes in the victim's browser in the context of the Pydantic AI web application, with access to:
- Chat history stored in
localStorage(all user messages and AI responses) - Session cookies that are not set as
HttpOnly, if authentication middleware is configured
- Chat history stored in
Remediation
Upgrade to Patched Version
Upgrade to the patched version or later. The fix removes the user-controllable version parameter entirely. The CDN URL is now hardcoded at startup and cannot be influenced by request parameters.
A new html_source parameter is available on Agent.to_web and create_web_app for applications that need to customize the UI source (e.g., for enterprise environments, offline usage, or custom UI builds). This parameter is only settable in application code, not via query parameters.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | pydantic-ai | ≥ 1.34.0&&< 1.51.0 | 1.51.0pip install --upgrade 'pydantic-ai==1.51.0' |
| 🐍PyPI | pydantic-ai-slim | ≥ 1.34.0&&< 1.51.0 | 1.51.0pip install --upgrade 'pydantic-ai-slim==1.51.0' |
Affected Products
pydantic aipydanticDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for pydantic-ai, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update pydantic-ai to 1.51.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-25640 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This IMPORTANT vulnerability in Pydantic AI, as included in Red Hat Enterprise Linux AI, involves a path traversal flaw in the web UI's CDN URL construction. An attacker can exploit this by crafting a malicious URL, leading to arbitrary JavaScript execution in a victim's browser if they click the link. This primarily…
To reduce exposure, restrict network access to the Pydantic AI web interface. If the application is deployed on a remote server, configure firewall rules to limit access to trusted networks or localhost. This prevents untrusted clients from reaching the vulnerable component. After applying firewall changes, ensure the rules are persistent and the firewall service is reloaded.Source: Red Hat security advisory for CVE-2026-25640 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-25640 in your dependencies?
Find it across PyPI, including transitive dependencies.