PythonApplication
CVE-2025-12781
A fix is available.
Upgrade to 3.13.10 / 3.14.1 or later.
See remediation →
62/100
Remediation priority · Elevated
Public exploit
Remotely reachable
No privileges
Zero-click
Patch available
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedWhen passing data to the b64decode(), standard_b64decode(), and urlsafe_b64decode() functions in the "base64" module the characters "+/" will always be accepted, regardless of the value of "altchars" parameter, typically used to establish an "alternative base64 alphabet" such as the URL safe alphabet. This behavior matches what is recommended in earlier base64 RFCs, but newer RFCs now recommend either dropping characters outside the specified base64 alphabet or raising an error. The old behavior has the possibility of causing data integrity issues. This behavior can only be insecure if your application uses an alternate base64 alphabet (without "+/"). If your application does not use the "altchars" parameter or the urlsafe_b64decode() function, then your application does not use an alternative base64 alphabet. The attached patches DOES NOT make the base64-decode behavior raise an error, as this would be a change in behavior and break existing programs. Instead, the patch deprecates the behavior which will be replaced with the newly recommended behavior in a future version of Python. Users are recommended to mitigate by verifying user-controlled inputs match the base64 alphabet they are expecting or verify that their application would not be affected if the b64decode() functions accepted "+" or "/" outside of altchars.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysisA detailed technical summary for this CVE is being prepared.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
PythonApplication
Affected:
< 3.13.10>= 3.14.0, < 3.14.1= 3.15.0CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped
Upgrade available
Upgrade to 3.13.10 / 3.14.1 or later
Fixed in
3.13.103.14.1
Vendor patch
github.com →
Recommended fix
High confidence
Python 3.13.10+, 3.14.1+, or latest 3.x release
- 1. Identify the current Python version in use by running 'python --version' or 'python3 --version'
- 2. If using Python < 3.13.10, upgrade to Python 3.13.10 or later
- 3. If using Python 3.14.0, upgrade to Python 3.14.1 or later
- 4. If using Python 3.15.0, upgrade to the latest 3.15.x release when available
- 5. After upgrading, run your application and monitor for DeprecationWarning related to base64 b64decode - this will indicate code using altchars that may be affected
- 6. Review code that uses b64decode(), standard_b64decode(), or urlsafe_b64decode() with the altchars parameter
- 7. Verify that any user-controlled base64 input matches the expected alphabet, or that acceptance of '+' or '/' outside altchars does not cause data integrity issues in your application
Caveat This is a deprecation warning, not a breaking change - the old behavior still works but will emit warnings. A future version will change the behavior to reject '+'/'/' outside altchars.
Generated from the published advisory — verify against the referenced sources before acting.
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2025-12781 in production — separate from our analysis above.
Know something about CVE-2025-12781?
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
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What belongs here
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- Version or environment caveats, and links to real fixes
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