CPython [CVE-2026-3446] Base64 decoding stops at first padded quad by default

[Alan Coopersmith <alan.coopersmith () oracle com>]

-------- Forwarded Message --------
Subject: 	[Security-announce]Title: [CVE-2026-3446] Base64 decoding stops at 
first padded quad by default
Date:   Fri, 10 Apr 2026 18:19:00 +0000
From:   Seth Larson <seth () python org>
Reply-To:       security-sig () python org
To:     security-announce () python org



There is a MEDIUM severity vulnerability affecting {project}.

When calling base64.b64decode() or related functions the decoding process would 
stop after encountering the first padded quad regardless of whether there was 
more information to be processed. This can lead to data being accepted which may 
be processed differently by other implementations. Use "strict=True" to enable 
stricter processing of base64 data.

Please see the linked CVE ID for the latest information on affected versions:

* https://www.cve.org/CVERecord?id=CVE-2026-34460
* https://github.com/python/cpython/pull/145267

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