{"id":"PYSEC-2026-3847","summary":"HTTPX2: Quadratic SSE line buffering can cause CPU denial of service","details":"### Summary\n\nHTTPX2's Server-Sent Events (SSE) parser repeatedly copied and rescanned buffered text when a server split one unterminated line across many response chunks. The total work grows quadratically with the length of the line. An attacker-controlled or compromised SSE endpoint can exploit this behavior to consume excessive client CPU.\n\n### Details\n\nBefore version 2.10.0, HTTPX2 combined the complete pending SSE line with each newly received chunk and then scanned the combined text for line separators. If an SSE server sends a long line as many small chunks without a line separator, every chunk causes all previously received text to be copied and scanned again. For `n` fixed-size chunks, this results in O(n²) processing.\n\nThe behavior affects both `httpx2.Client.sse()` and `httpx2.AsyncClient.sse()`. Other response APIs do not use the SSE parsing path.\n\n### Impact\n\nApplications that consume SSE from an attacker-controlled or compromised endpoint can experience excessive CPU usage. A crafted stream can block a synchronous worker or the asynchronous event loop that is consuming it, degrading availability for other work in that process. Confidentiality and integrity are not affected.\n\n### Mitigation\n\nUpgrade to HTTPX2 2.10.0 or later. SSE parsing now accumulates incomplete line fragments and combines them only when necessary, making processing linear in the amount of received data. HTTPX2 2.10.0 also limits buffered SSE events to 1 MiB by default through `max_event_size`.\n\nIf upgrading is not immediately possible, only consume SSE from trusted endpoints and enforce an external size or time budget on the stream.","aliases":["CVE-2026-84378","GHSA-f2fp-rgf2-35cp"],"modified":"2026-09-10T12:15:05.087047573Z","published":"2026-09-10T09:45:00.972470Z","references":[{"type":"WEB","url":"https://github.com/pydantic/httpx2/security/advisories/GHSA-f2fp-rgf2-35cp"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-84378"},{"type":"WEB","url":"https://github.com/pydantic/httpx2/pull/1071"},{"type":"WEB","url":"https://github.com/pydantic/httpx2/pull/1117"},{"type":"WEB","url":"https://github.com/pydantic/httpx2/commit/cbfc0e04ef6507da29ccbb3b9c2e5b23dd693414"},{"type":"PACKAGE","url":"https://github.com/pydantic/httpx2"},{"type":"WEB","url":"https://github.com/pydantic/httpx2/releases/tag/v2.10.0"},{"type":"PACKAGE","url":"https://pypi.org/project/httpx2"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-f2fp-rgf2-35cp"}],"affected":[{"package":{"name":"httpx2","ecosystem":"PyPI","purl":"pkg:pypi/httpx2"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"2.5.0"},{"fixed":"2.10.0"}]}],"versions":["2.5.0","2.6.0","2.7.0","2.8.0","2.9.0","2.9.1"],"database_specific":{"source":"https://github.com/pypa/advisory-database/blob/main/vulns/httpx2/PYSEC-2026-3847.yaml"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H"}]}