What’s Vulnerable › Vulnerabilities

CVE-2026-3446: Base64 decoding stops at first padded quad by default

• Severity: medium (CVSS 6.0 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-04-10 · updated 2026-10-26
• Weakness: CWE-345

Description

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 "validate=True" to enable stricter processing of base64 data.

Affected products

• Python: before 3.13.13; from 3.14.0 before 3.14.4; from 3.15.0a1 before 3.15.0a8 — fixed in 3.13.13, 3.14.4, 3.15.0a8

How to fix

Upgrade to a fixed version: Python 3.13.13, 3.14.4, 3.15.0a8. Check the vendor advisory for mitigations.

References

• https://github.com/python/cpython/commit/1f9958f909c1b41a4ffc0b613ef8ec8fa5e7c474
• https://github.com/python/cpython/commit/4561f6418a691b3e89aef0901f53fe0dfb7f7c0e
• https://github.com/python/cpython/commit/e31c55121620189a0d1a07b689762d8ca9c1b7fa
• https://github.com/python/cpython/issues/145264
• https://github.com/python/cpython/pull/145267
• https://mail.python.org/archives/list/security-announce@python.org/thread/F5ZT5ICGJ6CKXVUJ34YBVY7WOZ5SHG53/

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