VulnerabilityPublished 21 Jan 2026

CVE-2025-12781: Python Software Foundation CPython · CWE-704

Affects Python Software Foundation CPythonSource CVE Database V5

Not known to be exploited · CVSS 6.3 Medium · fixed in 3.15.0a2 : update Python Software Foundation CPython when convenient.

Data as of 26 Oct 2026, 08:30 UTC

CVSS 4.0 base score

6.3Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.8% chance of exploitation in 30 days – higher than 58% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 3.15.0a2, 3.14.1, 3.13.10

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

Attack Vector: Network
Attack Complexity: High
Attack Requirements: Present
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: Low
Vulnerable system availability: None
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: The successful attack depends on the evasion or circumvention of security-enhancing techniques in place that would otherwise hinder the attack. These include: Evasion of exploit mitigation techniques, for example, circumvention of address space randomization (ASLR) or data execution prevention (DEP) must be performed for the attack to be successful; Obtaining target-specific secrets. The attacker must gather some target-specific secret before the attack can be successful. A secret is any piece of information that cannot be obtained through any amount of reconnaissance. To obtain the secret the attacker must perform additional attacks or break otherwise secure measures (e.g. knowledge of a secret key may be needed to break a crypto channel). This operation must be performed for each attacked target.
Attack Requirements: Present: The successful attack depends on the presence of specific deployment and execution conditions of the vulnerable system that enable the attack. These include: a race condition must be won to successfully exploit the vulnerability (the successfulness of the attack is conditioned on execution conditions that are not under full control of the attacker, or the attack may need to be launched multiple times against a single target before being successful); the attacker must inject themselves into the logical network path between the target and the resource requested by the victim (e.g. vulnerabilities requiring an on-path attacker).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact to the Vulnerable System.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 21 Jan 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade Python Software Foundation CPython to 3.15.0a2, 3.14.1 or 3.13.10.

Affected software

Product	Affected	Fixed in
Python Software FoundationCPython	Affected <3.13.10 >=3.14.0 <3.14.1 >=3.15.0a1 <3.15.0a2	Fixed in 3.15.0a2 3.14.1 3.13.10
PyPIcpython`pkg:pypi/cpython`Other threats	Affected =0 =3.14.0 =3.15.0	Fixed in–
GitHubCPython`pkg:github/CPython`Other threats	Affected <3.13.10 >=3.14.0 <3.14.1 >=3.15.0a1 <3.15.0a2	Fixed in 3.13.10 3.14.1 3.15.0a2

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

European organizations should implement the following specific mitigations: 1) Audit all Python applications and libraries to identify usage of the base64 module's b64decode(), standard_b64decode(), and urlsafe_b64decode() functions, especially where the altchars parameter is used to specify alternate alphabets. 2) Implement strict input validation to ensure that any base64-encoded data strictly conforms to the expected alphabet, rejecting or sanitizing inputs containing '+' or '/' when these are not part of the intended encoding scheme. 3) Where possible, refactor applications to avoid reliance on alternate base64 alphabets or to handle decoding explicitly and safely. 5) Incorporate static and dynamic code analysis tools to detect improper base64 decoding usage patterns. 6) Educate developers and security teams about the implications of this vulnerability and the importance of encoding validation. 7) For critical systems, consider implementing additional data integrity checks post-decoding to detect corrupted or malformed data early. These steps go beyond generic advice by focusing on code auditing, input validation, and proactive patch management tailored to the specifics of this vulnerability.

Description

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.

Impact

For European organizations, the impact of CVE-2025-12781 depends largely on the use of Python applications that employ alternate base64 alphabets for encoding and decoding data. Sectors such as finance, telecommunications, and government agencies that rely on Python for secure data transmission and storage may face risks of data integrity issues if the vulnerability is exploited or if corrupted data is processed unknowingly. This could lead to subtle data corruption, misinterpretation of encoded data, or failures in cryptographic or authentication processes that depend on strict base64 encoding. While the vulnerability does not directly compromise confidentiality or availability, the integrity issues could undermine trust in data processing pipelines and cause operational disruptions. Given the widespread use of Python in European IT environments, especially in critical infrastructure and software development, organizations must assess their exposure. The lack of known exploits reduces immediate risk, but the medium severity rating and planned deprecation indicate that this is a vulnerability that should be proactively addressed to avoid future exploitation or data integrity failures.

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 29 Jan 2026, 08:42 UTC

WeaknessesCWE-704

Technical details

References

Community reviews

Related threats