CVE-2026-84378 HTTPX2: Quadratic SSE line buffering can cause CPU denial of service
A crafted Server-Sent Events stream can cause quadratic CPU consumption in the HTTPX2 library, leading to denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-84378?
CVE-2026-84378 is a medium-severity denial of service vulnerability in pydantic httpx2, published on 2 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-407
Which versions are affected by CVE-2026-84378?
3 open source packages ship an affected version, including httpx2.
| Package | Affected | Fixed in |
|---|---|---|
httpx2 PyPI | From 2.5.0 | 2.10.0 |
httpx2 PyPI | From 2.5.0 | 2.10.0 |
httpx2 PyPI | From 2.5.0 | 2.10.0 |
How do I fix CVE-2026-84378?
Upgrade httpx2 to 2.10.0 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-84378 being exploited?
There is no known exploitation of CVE-2026-84378 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-84378, ECHO-e305-29d2-dcb3, GHSA-f2fp-rgf2-35cp, MINI-485q-xq5h-m8rc, PYSEC-2026-3847, UBUNTU-CVE-2026-84378.