CVE-2025-12781Public exploitNetwork attack vector

Data alteration via ignored alternate Base64 alphabet

Published Jan 21, 2026 · Updated Jan 21, 2026

Incorrect Base64 decoding in CPython before 3.13.10, 3.14.0, and 3.15.0a1 allows remote attackers to alter decoded data. The base64 module's b64decode(), standard_b64decode(), and urlsafe_b64decode() functions accept '+' and '/' even when altchars replaces them. Exposure requires an application to decode attacker-controlled input with an alternate alphabet that excludes those characters, producing an unintended decoded value.

CVSS severity6.3
Medium
EPSS probability0.53%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Incorrect Base64 decoding in CPython before 3.13.10, 3.14.0, and 3.15.0a1 allows remote attackers to alter decoded data. The base64 module's b64decode(), standard_b64decode(), and urlsafe_b64decode() functions accept '+' and '/' even when altchars replaces them. Exposure requires an application to decode attacker-controlled input with an alternate alphabet that excludes those characters, producing an unintended decoded value.

The record

CVE
CVE-2025-12781
Published
Jan 21, 2026
Updated
Jan 21, 2026
Vendor
Python Software Foundation
Product
CPython
Classifications
CWE-704, T1565.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 21, 2026CVE publishedPublication date reported by the CVE source.
  2. Jan 21, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <3.15.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Python Software Foundation · CPythonversion=0 <3.15.0

Published CVSS scores

5.3NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
6.3Python Software FoundationCVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-704Incorrect Type Conversion or Cast
T1565.002Transmitted Data Manipulation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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