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
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-25640 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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' |
Detection & 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
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-25640 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-25640. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-25640 in your dependencies?
O3 Security finds CVE-2026-25640 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.