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NVD / CVE Program· CNA PSF

CVE-2025-12781: base64.b64decode() always accepts "+/" characters, despite setting altchars

Noroxi assessment

Priority: Monitor (action score 25/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.85% (FIRST EPSS); higher than 58.4% of all records.
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. Special conditions such as timing or configuration are required.
  • Fixed version: Python Software Foundation CPython 3.13.10, 3.14.1, 3.15.0a2; azl3 python-py 1.11.0-1 on Azure Linux 3.0 Release Notes.

Low priority for now. Upgrade to Python Software Foundation CPython 3.13.10, 3.14.1, 3.15.0a2; azl3 python-py 1.11.0-1 on Azure Linux 3.0 Release Notes or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

When 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.

MediumCVSS 6.3 · v4.0—No exploit Fix available
Published
Jan 21, 2026
Updated
Jun 17, 2026
EPSS
0.8% · 58th percentile

Report tools

JSON

Action score

25

Monitor

Low priority for now.

CVSS
25 / 40 · 6.3 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.8%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · PSF
6.3
NVD has not scored this yet; the score shown is the CNA’s.

CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

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When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
pythonpython

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • python pythonbefore 3.13.10
  • python python3.14.0 and later · before 3.14.1
  • python python3.15.0

Versions reported by the vendor

Affected version ranges reported by the assigning authority (PSF). Independent of NVD's CPE analysis and usually ahead of it.

  • Python Software Foundation CPython

    • before 3.13.10affected · python
    • 3.14.0 and later · before 3.14.1affected · python
    • 3.15.0a1 and later · before 3.15.0a2affected · python

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Debian:12pypy3from 0—
Debian:12python3.11from 0—
Debian:13python3.13from 0—
Debian:14python3.14before 3.14.2-13.14.2-1
openSUSE:Leap 15.6python311before 3.11.14-150600.3.44.13.11.14-150600.3.44.1
openSUSE:Leap 15.6python311-corebefore 3.11.14-150600.3.44.13.11.14-150600.3.44.1
openSUSE:Leap 15.6python311-documentationbefore 3.11.14-150600.3.44.13.11.14-150600.3.44.1
openSUSE:Tumbleweedpython311before 3.11.14-4.13.11.14-4.1
openSUSE:Tumbleweedpython312before 3.12.12-5.13.12.12-5.1
openSUSE:Tumbleweedpython314before 3.14.3-1.13.14.3-1.1
openSUSE:Tumbleweedpython315before 3.15.0~a3-4.13.15.0~a3-4.1
SUSE:Linux Enterprise High Performance Computing 15 SP4-ESPOSpython311-documentationbefore 3.11.14-150400.9.75.13.11.14-150400.9.75.1
SUSE:Linux Enterprise Module for Basesystem 15 SP7python311-corebefore 3.11.14-150600.3.44.13.11.14-150600.3.44.1
SUSE:Linux Enterprise Module for Public Cloud 15 SP4python311before 3.11.14-150400.9.75.13.11.14-150400.9.75.1
SUSE:Linux Enterprise Module for Public Cloud 15 SP4python311-corebefore 3.11.14-150400.9.75.13.11.14-150400.9.75.1
SUSE:Linux Enterprise Module for Python 3 15 SP7python311before 3.11.14-150600.3.44.13.11.14-150600.3.44.1
SUSE:Linux Micro 6.0python311before 3.11.14-3.13.11.14-3.1
SUSE:Linux Micro 6.0python311-corebefore 3.11.14-3.13.11.14-3.1
SUSE:Linux Micro 6.1python311before 3.11.14-slfo.1.1_3.13.11.14-slfo.1.1_3.1
SUSE:Linux Micro 6.1python311-corebefore 3.11.14-slfo.1.1_3.13.11.14-slfo.1.1_3.1

Same product

python: all records

Other highest-scoring records for the same primary product.

  • CVE-2016-5636Integer overflow in the get_data function in zipimport.c in CPython (aka Python) before 2.7.12, 3.x before 3.4.5, and 3.5.x before 3.5.2 all
    47Plan
  • CVE-2007-4559Directory traversal vulnerability in the (1) extract and (2) extractall functions in the tarfile module in Python allows user-assisted remot
    47Plan
  • CVE-2021-3177Python 3.x through 3.9.1 has a buffer overflow in PyCArg_repr in _ctypes/callproc.c, which may lead to remote code execution in certain Pyth
    46Plan

Published before and after this one in the same product

  • ← EarlierCVE-2025-13837Dec 1, 2025 · Out-of-memory when loading Plist
    8Monitor
  • ← EarlierCVE-2025-12084Dec 3, 2025 · Quadratic complexity in node ID cache clearing
    25Monitor
  • Later →CVE-2025-13462Mar 12, 2026 · tarfile: Skip DIRTYPE normalization during GNU LONGNAME/LONGLINK handling
    8Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
Python Software Foundation CPython3.13.10Vendor (CNA)
Python Software Foundation CPython3.14.1Vendor (CNA)
Python Software Foundation CPython3.15.0a2Vendor (CNA)
azl3 python-py 1.11.0-1 on Azure Linux 3.0Release NotesMicrosoft (MSRC)
debian:python3.143.14.2-1 · Debian:14Package registry (OSV)
opensuse:python3113.11.14-150600.3.44.1 · openSUSE:Leap 15.6Package registry (OSV)
opensuse:python311-core3.11.14-150600.3.44.1 · openSUSE:Leap 15.6Package registry (OSV)
opensuse:python311-documentation3.11.14-150600.3.44.1 · openSUSE:Leap 15.6Package registry (OSV)
opensuse:python3123.12.12-5.1 · openSUSE:TumbleweedPackage registry (OSV)
opensuse:python3143.14.3-1.1 · openSUSE:TumbleweedPackage registry (OSV)
opensuse:python3153.15.0~a3-4.1 · openSUSE:TumbleweedPackage registry (OSV)
suse:python3113.11.14-150400.9.75.1 · SUSE:Linux Enterprise Module for Public Cloud 15 SP4Package registry (OSV)
suse:python3113.11.14-150600.3.44.1 · SUSE:Linux Enterprise Module for Python 3 15 SP7Package registry (OSV)
suse:python311-core3.11.14-150400.9.75.1 · SUSE:Linux Enterprise Module for Public Cloud 15 SP4Package registry (OSV)
suse:python311-core3.11.14-150600.3.44.1 · SUSE:Linux Enterprise Module for Basesystem 15 SP7Package registry (OSV)
suse:python311-documentation3.11.14-150400.9.75.1 · SUSE:Linux Enterprise High Performance Computing 15 SP4-ESPOSPackage registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • Special conditions such as timing or configuration are required.

If successful

Confidentiality
Integrity
Availability
Attack vector
Network
Attack complexity
High
Privileges required
None
User interaction
None
Scope
X

Weakness class (CWE)

CWE-704 · Incorrect Type Conversion or Cast

CVSS vector

CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (6 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records