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Vulnerability · Published Oct 8, 2026 · Assigned by GitHub_M

CVE-2026-107290Pydantic AI: Event loop blocked by quadratic title extraction in `web_fetch`

Medium 6.5

In plain English

A medium-severity regular-expression denial of service in pydantic-ai from pydantic. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 44% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)6.5Medium
Exploit odds (30 days)0.5%top 56.2% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
PyPIpydantic-ai1.107.6GHSA-fpf4-vwcp-v4hp
PyPIpydantic-ai2.44.0GHSA-fpf4-vwcp-v4hp
PyPIpydantic-ai1.107.6, 2.44.0PYSEC-2026-4277
PyPIpydantic-ai-slim1.107.6GHSA-fpf4-vwcp-v4hp
PyPIpydantic-ai-slim2.44.0GHSA-fpf4-vwcp-v4hp
PyPIpydantic-ai-slim1.107.6, 2.44.0PYSEC-2026-4286

Affected products (as reported)

VendorProductVersions
pydantic pydantic-ai >= 1.77.0, < 1.107.6>= 2.0.0b1, < 2.44.0
pydantic pydantic-ai-slim >= 1.77.0, < 1.107.6>= 2.0.0b1, < 2.44.0

Weakness: CWE-1333 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

References