What’s Vulnerable › Vulnerabilities

CVE-2026-84378: HTTPX2: Quadratic SSE line buffering can cause CPU denial of service

• Severity: medium (CVSS 5.9 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-02 · updated 2026-09-09
• Weakness: CWE-407

Description

HTTPX2 is a next generation HTTP client for Python. From 2.5.0 until 2.10.0, the HTTPX2 Server-Sent Events parser in src/httpx2/httpx2/_sse.py repeatedly copies and rescans buffered text in _SSELineDecoder.decode() when an attacker-controlled or compromised SSE endpoint splits one unterminated line across many response chunks. The behavior affects httpx2.Client.sse() and httpx2.AsyncClient.sse(), and the total processing work grows quadratically with the line length, allowing a crafted stream to consume excessive CPU and block a synchronous worker or asynchronous event loop. This issue is fixed in version 2.10.0.

Affected products

• pydantic httpx2: from 2.5.0, < 2.10.0

References

• https://github.com/pydantic/httpx2/commit/cbfc0e04ef6507da29ccbb3b9c2e5b23dd693414
• https://github.com/pydantic/httpx2/pull/1071
• https://github.com/pydantic/httpx2/pull/1117
• https://github.com/pydantic/httpx2/releases/tag/v2.10.0
• https://github.com/pydantic/httpx2/security/advisories/GHSA-f2fp-rgf2-35cp

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