CVE-2025-12781 Details
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.
View at NVD
Exploit prediction scoring system (EPSS) score for CVE-2025-12781
EPSS (Exploit Prediction Scoring System): EPSS predicts the likelihood that a vulnerability will be exploited in the wild. A higher percentage means a greater chance of an exploit occurring. The EPSS model produces a probability score between 0 and 1 (0 and 100%).
0.04 Probability of exploitation activity being observed over the next 30 days (12th percentile)
CVSS score for CVE-2025-12781
CVSS (Common Vulnerability Scoring System): An open framework owned and managed by FIRST.Org, Inc. that assigns a numerical score from 0 to 10 to software vulnerabilities to indicate their severity.
5.3 Medium
Products affected by CVE-2025-12781
Python 1.5.2
Python 2.1
Python 2.1.3
Python 2.2.1
Python 2.2.2
DVE Score by Bitsight AI
DVE scores complement traditional static scoring systems like CVSS with Bitsight AI powered real-time threat intelligence and underground cyber criminal activity so that security teams can reduce alert fatigue and focus on vulnerabilities most likely to be exploited.
Low
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CVE-2025-12781 Global Footprint

Top 10 Identified Countries

Country Observations Percentage
US 140,769 46.90%
DE 23,055 7.68%
CN 21,672 7.22%
JP 8,239 2.74%
NL 8,152 2.72%
FR 7,566 2.52%
GB 7,054 2.35%
SG 5,784 1.93%
RU 5,339 1.78%
KR 5,278 1.76%

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CVE-2025-12781 Industry Footprint

Top 10 Identified Industries

*Service provider organizations (typically Technology and Telecommunications) are disproportionally represented in the results given their upstream ownership of end-user infrastructure. See our FAQs.

Industry* Observations Percentage
Technology 185,886 83.17%
Telecommunications 24,291 10.87%
Education 8,196 3.67%
Media/Entertainment 1,881 0.84%
Government/Politics 769 0.34%
Business Services 512 0.23%
Energy/Resources 400 0.18%
Retail 330 0.15%
Real Estate 232 0.10%
Utilities 161 0.07%

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