CPU denial of service via quadratic SSE buffering
Published Sep 2, 2026 · Updated Sep 2, 2026
Inefficient SSE parsing in Pydantic HTTPX2 2.5.0 through 2.9.1 allows remote attackers to interrupt service via a crafted stream. The _SSELineDecoder.decode() path appends each fragment to the complete pending line and rescans it for separators, repeatedly copying prior text and producing quadratic work. Exploitation requires an application to consume SSE from an attacker-controlled or compromised endpoint; the stream can block one synchronous worker or the consuming asynchronous event loop.
Summary
What happened
Inefficient SSE parsing in Pydantic HTTPX2 2.5.0 through 2.9.1 allows remote attackers to interrupt service via a crafted stream. The _SSELineDecoder.decode() path appends each fragment to the complete pending line and rescans it for separators, repeatedly copying prior text and producing quadratic work. Exploitation requires an application to consume SSE from an attacker-controlled or compromised endpoint; the stream can block one synchronous worker or the consuming asynchronous event loop.
The record
- CVE
- CVE-2026-84378
- Published
- Sep 2, 2026
- Updated
- Sep 2, 2026
- Vendor
- Pydantic
- Product
- HTTPX2
- Classifications
- CWE-407, T1499.004
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Sep 2, 2026CVE publishedPublication date reported by the CVE source.
- Sep 2, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=>= 2.5.0, < 2.10.0
What conditions does exploitation require?
What is affected?
Published CVSS scores
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Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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