Client-authentication bypass via pre-handshake buffered data
Published Aug 25, 2023 · Updated Nov 3, 2025
TLS authentication bypass in affected Python 3.8 through 3.11 releases allows remote attackers to inject plaintext into server streams. SSLSocket skips the TLS handshake when a peer preloads the socket buffer and immediately closes the connection, after which recv returns that buffered plaintext as authenticated application data. Exploitation requires a TLS server that relies on client-certificate authentication and the narrow pre-close timing; injected data is buffer-limited, and this path cannot directly exfiltrate server data.
Summary
What happened
TLS authentication bypass in affected Python 3.8 through 3.11 releases allows remote attackers to inject plaintext into server streams. SSLSocket skips the TLS handshake when a peer preloads the socket buffer and immediately closes the connection, after which recv returns that buffered plaintext as authenticated application data. Exploitation requires a TLS server that relies on client-certificate authentication and the narrow pre-close timing; injected data is buffer-limited, and this path cannot directly exfiltrate server data.
The record
- CVE
- CVE-2023-40217
- Published
- Aug 25, 2023
- Updated
- Nov 3, 2025
- Vendor
- Unknown vendor
- Product
- Unknown product
- Classifications
- T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 25, 2023CVE publishedPublication date reported by the CVE source.
- Nov 3, 2025Record updatedLatest update available in the CVE record.
Exploitability
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What is affected?
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Published CVSS scores
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Attacks
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Technologies
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