CVE-2026-84378: HTTPX2 Denial of Service Vulnerability

MODERATE
Advisory Identifier: GHSA-f2fp-rgf2-35cp
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CVSS 5.9
CWE-407
Published
Advisory Summary

CVE-2026-84378 is a MODERATE (CVSS 5.9) denial of service vulnerability (CWE-407: Inefficient Algorithmic Complexity) affecting HTTPX2 libraries in the PyPI ecosystem. The issue involves quadratic sse line buffering can cause cpu denial of service. Remediation is available by upgrading affected packages to version 2.10.0.

CVE-2026-84378 at a Glance

CVE Identifier CVE-2026-84378
GHSA Identifier GHSA-f2fp-rgf2-35cp
Severity MODERATE
CVSS v3.1 Base Score 5.9
CWE Classification CWE-407: Inefficient Algorithmic Complexity
Affected Product HTTPX2
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 5.9

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H).

Attack Vector Network (AV:N)
Attack Complexity High (AC:H)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2026-84378

To remediate this vulnerability, upgrade all affected HTTPX2 dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

httpx2 fixed: 2.10.0 
pip install httpx2>=2.10.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show httpx2

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-84378, inspect your installed packages using pip:

pip show httpx2

Analyze the terminal output to locate any version of HTTPX2 in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 2.10.0), your application is affected.

Note: The pip show httpx2 command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-407: Inefficient Algorithmic Complexity, where an 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. In HTTPX2, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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 `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.

CVE-2026-84378 Affected HTTPX2 Versions

1 component(s)
Fixed Versions:
2.10.0
Product / Package Ecosystem Affected Versions Fixed Version
httpx2
pypi/httpx2
PyPI
≥ 2.5.0
< 2.10.0
2.10.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/pydantic/httpx2/security/advisories/GHSA-f2fp-rgf2-35cp
NVD — CVE-2026-84378 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-84378
HTTPX2 Source Repository REPOSITORY
https://github.com/pydantic/httpx2/pull/1071
HTTPX2 Source Repository REPOSITORY
https://github.com/pydantic/httpx2/pull/1117
Security Fix Commit FIX COMMIT
https://github.com/pydantic/httpx2/commit/cbfc0e04ef6507da29ccbb3b9c2e5b23dd693414
HTTPX2 Source Repository REPOSITORY
https://github.com/pydantic/httpx2
HTTPX2 Release RELEASE
https://github.com/pydantic/httpx2/releases/tag/v2.10.0

Frequently Asked Questions

What is CVE-2026-84378?
CVE-2026-84378 is a security vulnerability in HTTPX2 involving Quadratic SSE line buffering can cause CPU denial of service under CWE-407: Inefficient Algorithmic Complexity.
Which HTTPX2 versions are affected?
Affected versions include releases prior to 2.10.0 across affected packages: httpx2.
What version fixes CVE-2026-84378?
Upgrading to 2.10.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is httpx2 affected?
Yes, httpx2 is affected in releases prior to 2.10.0. Upgrade your dependency to 2.10.0 or later.
What is the CVSS score of CVE-2026-84378?
CVE-2026-84378 has a CVSS v3.1 base score of 5.9 (MODERATE) with vector string CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2026-84378?
CVE-2026-84378 is classified under CWE-407: Inefficient Algorithmic Complexity (An 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.).
How can I check PyPI dependencies for CVE-2026-84378?
Run the dependency tree command: 'pip show httpx2'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-f2fp-rgf2-35cp",
  "modified" : "2026-09-08T20:45:29Z",
  "published" : "2026-09-08T20:45:29Z",
  "aliases" : [ "CVE-2026-84378" ],
  "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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "httpx2"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.5.0"
      }, {
        "fixed" : "2.10.0"
      } ]
    } ]
  } ],
  "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-407" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-08T20:45:29Z",
    "nvd_published_at" : "2026-09-02T18:21:29Z"
  }
}