CVE-2026-25640 Pydantic AI affected by Stored XSS via Path Traversal in Web UI CDN URL
Attackers can execute arbitrary JavaScript in victims' browsers via path traversal in the web UI CDN URL, stealing chat history and client-side data.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-25640?
CVE-2026-25640 is a high-severity cross-site scripting vulnerability in pydantic-ai, published on 6 Feb 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
Pydantic AI is a Python agent framework for building applications and workflows with Generative AI. From 1.34.0 to before 1.51.0, 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. 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. 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_web to serve a chat interface and clai web to serve a chat interface from the CLI. These are typically run locally (on localhost), but may also be deployed on a remote server. This vulnerability is fixed in 1.51.0.
Which versions are affected by CVE-2026-25640?
6 open source packages ship an affected version, including pydantic-ai and pydantic-ai-slim. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
pydantic-ai PyPI | From 1.34.0 | 1.51.0 |
pydantic-ai PyPI | From 1.34.0 | 1.51.0 |
pydantic-ai PyPI | From 1.34.0 | 1.51.0 |
pydantic-ai-slim PyPI | From 1.34.0 | 1.51.0 |
pydantic-ai-slim PyPI | From 1.34.0 | 1.51.0 |
pydantic-ai-slim PyPI | From 1.34.0 | 1.51.0 |
Products include Pydantic Pydantic Ai.
How do I fix CVE-2026-25640?
Upgrade pydantic-ai to 1.51.0 or later.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-25640 being exploited?
There is no known exploitation of CVE-2026-25640 in the wild and no public exploit code.
Other vulnerabilities in pydantic-ai
- CVE-2026-25580 Pydantic AI Affected by Server-Side Request Forgery (SSRF) in URL Download Handling
- CVE-2026-107295 Pydantic AI Web chat UI (`Agent.to_web()`, `clai web`): a website visited by the developer can trigger agent runs and server-side tool execution on the local chat endpoint
- CVE-2026-107286 Pydantic AI: Concurrency-limited models can keep their slot when a streamed request ends early
- CVE-2026-107289 Pydantic AI: SSRF cloud-metadata blocklist bypass via IPv6 zone identifiers
- CVE-2026-46678 Pydantic AI: SSRF cloud-metadata blocklist bypass via IPv4-mapped IPv6 (Incomplete fix of CVE-2026-25580)
Sources
Also known as GHSA-wjp5-868j-wqv7, PYSEC-2026-2979, PYSEC-2026-2983.