CVE-2026-84378: HTTPX2: Quadratic SSE line buffering can cause CPU denial of service
Summary
HTTPX2'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.
Details
Before 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.
The behavior affects both httpx2.Client.sse() and httpx2.AsyncClient.sse(). Other response APIs do not use the SSE parsing path.
Impact
Applications 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.
Mitigation
Upgrade 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 maxeventsize.
If upgrading is not immediately possible, only consume SSE from trusted endpoints and enforce an external size or time budget on the stream.
Other sources
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.
— NVD
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/httpx2to a version that resolves this vulnerability.Fixed in 2.10.0 - Upgrade
Upgrade
httpx2to a version that resolves this vulnerability.Fixed in 2.10.0 - Compensating control
If upgrading to 2.10.0+ is not immediately possible, only consume SSE from trusted endpoints and enforce an external size or time budget on the SSE stream.
Event History
Frequently Asked Questions
Which applications are exposed to this issue?
Applications using httpx2.Client.sse() or httpx2.AsyncClient.sse() with affected HTTPX2 versions from 2.5.0 up to, but not including, 2.10.0 are exposed when they consume SSE data from an attacker-controlled or compromised endpoint.
What must an attacker do to trigger the CPU exhaustion?
The attacker must cause the SSE endpoint to send a single unterminated line split across many response chunks. Repeated buffering, copying, and rescanning of that line makes processing work grow quadratically as the line length increases.
What is the operational impact?
A crafted stream can consume excessive CPU. It can block a synchronous worker using Client.sse() or an asynchronous event loop using AsyncClient.sse().
What is the remediation?
Upgrade HTTPX2 to version 2.10.0, which fixes the issue.