TLS policy bypass via fail-open SNI callback
Published Mar 17, 2026 · Updated Mar 17, 2026
Fail-open callback handling in pyOpenSSL 0.14.0 through 25.3.0 allows remote attackers to bypass SNI-based TLS policy checks. The Context.set_tlsext_servername_callback wrapper let exceptions escape its CFFI callback boundary, so the handshake continued as if the callback succeeded. Exploitation requires an application to use that callback for security-sensitive SNI decisions and an attacker-controlled handshake to trigger an exception.
Summary
What happened
Fail-open callback handling in pyOpenSSL 0.14.0 through 25.3.0 allows remote attackers to bypass SNI-based TLS policy checks. The Context.set_tlsext_servername_callback wrapper let exceptions escape its CFFI callback boundary, so the handshake continued as if the callback succeeded. Exploitation requires an application to use that callback for security-sensitive SNI decisions and an attacker-controlled handshake to trigger an exception.
The record
- CVE
- CVE-2026-27448
- Published
- Mar 17, 2026
- Updated
- Mar 17, 2026
- Vendor
- Python Cryptographic Authority
- Product
- pyOpenSSL
- Classifications
- CWE-636, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Mar 17, 2026CVE publishedPublication date reported by the CVE source.
- Mar 17, 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=>= 0.14.0, < 26.0.0
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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