CVE-2026-107290 Pydantic AI: Event loop blocked by quadratic title extraction in `web_fetch`
An attacker can cause a denial of service by blocking the event loop via quadratic processing in the web_fetch tool of Pydantic AI.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-107290?
CVE-2026-107290 is a medium-severity denial of service vulnerability in pydantic-ai, published on 8 Oct 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Pydantic AI is a Python agent framework for building applications and workflows with Generative AI. From 1.77.0 until 1.107.6 and 2.44.0, the local web_fetch_tool and the WebFetch local fallback process server-controlled responses with quadratic title extraction, whitespace normalization, and ordered-list numbering. An attacker-controlled page of modest size can therefore block the event loop for an extended period, stalling other agent runs and requests, while unsupported codecs or excessive HTML or JSON nesting can abort an individual run. This issue is fixed in versions 1.107.6 and 2.44.0.
Weakness: CWE-1333
Which versions are affected by CVE-2026-107290?
10 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 1.77.0 | 1.107.6, 2.44.0 |
pydantic-ai PyPI | From 1.77.0 | 1.107.6 |
pydantic-ai PyPI | From 1.77.0 | 1.107.6 |
pydantic-ai PyPI | From 2.0.0b1 | 2.44.0 |
pydantic-ai PyPI | From 2.0.0b1 | 2.44.0 |
pydantic-ai-slim PyPI | From 1.77.0 | 1.107.6, 2.44.0 |
4 more packages
pydantic-ai-slim PyPI | From 1.77.0 | 1.107.6 |
pydantic-ai-slim PyPI | From 1.77.0 | 1.107.6 |
pydantic-ai-slim PyPI | From 2.0.0b1 | 2.44.0 |
pydantic-ai-slim PyPI | From 2.0.0b1 | 2.44.0 |
Products include Pydantic Pydantic Ai.
How do I fix CVE-2026-107290?
Upgrade pydantic-ai to a fixed version for your release line: 1.107.6 or 2.44.0.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-107290 being exploited?
There is no known exploitation of CVE-2026-107290 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-2026-25640 Pydantic AI affected by Stored XSS via Path Traversal in Web UI CDN URL
- CVE-2026-107289 Pydantic AI: SSRF cloud-metadata blocklist bypass via IPv6 zone identifiers
Sources
Also known as CLEANSTART-2026-DI21626, GHSA-fpf4-vwcp-v4hp, PYSEC-2026-4277, PYSEC-2026-4286.