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

Vulnerability summary

CVE-2026-84378 is a medium (CVSS 5.9) Inefficient Algorithmic Complexity vulnerability in pydantic httpx2. EPSS estimates a 0.32% chance of exploitation in the next 30 days, higher than 24.5% of scored vulnerabilities.

Timeline

1. 02-09-2026
   PublishedVulnerability first disclosed.
2. 02-09-2026
   Last modifiedVulnerability information updated.

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.

• CWE-407Inefficient Algorithmic ComplexityAn algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.