Validation bypass via truncated Base64 decoding
Published Apr 10, 2026 · Updated Apr 10, 2026
CPython before 3.13.13, 3.14.0 through 3.14.3, and 3.15.0a1 through 3.15.0a7 allows attackers to bypass Base64 validation. The non-strict binascii.a2b_base64 path used by base64.b64decode stops at the first valid padded quad and silently discards subsequent encoded data instead of continuing past the padding. Reachability requires a consuming application to decode attacker-controlled Base64 and trust the truncated result; another implementation can process the discarded suffix and obtain different data.
Summary
What happened
CPython before 3.13.13, 3.14.0 through 3.14.3, and 3.15.0a1 through 3.15.0a7 allows attackers to bypass Base64 validation. The non-strict binascii.a2b_base64 path used by base64.b64decode stops at the first valid padded quad and silently discards subsequent encoded data instead of continuing past the padding. Reachability requires a consuming application to decode attacker-controlled Base64 and trust the truncated result; another implementation can process the discarded suffix and obtain different data.
The record
- CVE
- CVE-2026-3446
- Published
- Apr 10, 2026
- Updated
- Apr 10, 2026
- Vendor
- Python Software Foundation
- Product
- CPython
- Classifications
- CWE-345
- Attack vector
- network
- Privileges
- authenticated
Timeline
How it unfolded
- Apr 10, 2026CVE publishedPublication date reported by the CVE source.
- Apr 10, 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 <3.13.13
- Affected versionversion=3.14.0 <3.14.4
- Affected versionversion=3.15.0a1 <3.15.0a8
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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