Header injection via unchecked folded newlines
Published Jan 23, 2026 · Updated Jan 23, 2026
Header injection in CPython before 3.10.20 and in affected 3.11–3.15 releases allows remote authenticated users to alter serialized email headers. BytesGenerator writes LiteralHeader output from fold_binary() without rejecting embedded CRLF sequences that start another header field. Exploitation requires an application to let a remote authenticated user influence a LiteralHeader value; the resulting message can carry attacker-chosen headers but does not itself execute code.
Summary
What happened
Header injection in CPython before 3.10.20 and in affected 3.11–3.15 releases allows remote authenticated users to alter serialized email headers. BytesGenerator writes LiteralHeader output from fold_binary() without rejecting embedded CRLF sequences that start another header field. Exploitation requires an application to let a remote authenticated user influence a LiteralHeader value; the resulting message can carry attacker-chosen headers but does not itself execute code.
The record
- CVE
- CVE-2026-1299
- Published
- Jan 23, 2026
- Updated
- Jan 23, 2026
- Vendor
- Python Software Foundation
- Product
- CPython
- Classifications
- CWE-93
- Attack vector
- network
- Privileges
- authenticated
Timeline
How it unfolded
- Jan 23, 2026CVE publishedPublication date reported by the CVE source.
- Jan 23, 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?
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- Affected versionversion=0 <3.15.0
What conditions does exploitation require?
What is affected?
Published CVSS scores
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Attacks
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Weakness, pattern, technique
Public exploit references
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Technologies
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Your stack
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