Summary
HTTPX2: Quadratic SSE line buffering can cause CPU denial of service
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.
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 max_event_size.
If upgrading is not immediately possible, only consume SSE from trusted endpoints and enforce an external size or time budget on the stream.
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.
CVE-2026-84378 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.10.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-84378? CVE-2026-84378 is a medium-severity security vulnerability in httpx2 (pip), affecting versions >= 2.5.0, < 2.10.0. It is fixed in 2.10.0.
- How severe is CVE-2026-84378? CVE-2026-84378 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of httpx2 are affected by CVE-2026-84378? httpx2 (pip) versions >= 2.5.0, < 2.10.0 is affected.
- Is there a fix for CVE-2026-84378? Yes. CVE-2026-84378 is fixed in 2.10.0. Upgrade to this version or later.
- Is CVE-2026-84378 exploitable, and should I be worried? Whether CVE-2026-84378 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-84378 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-84378? Upgrade
httpx2to 2.10.0 or later.