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.
CVSS Score
5.3
EPSS Score
0.001
Published
2026-01-21
When building nested elements using xml.dom.minidom methods such as appendChild() that have a dependency on _clear_id_cache() the algorithm is quadratic. Availability can be impacted when building excessively nested documents.
CVSS Score
5.3
EPSS Score
0.001
Published
2025-12-03
When reading an HTTP response from a server, if no read amount is specified, the default behavior will be to use Content-Length. This allows a malicious server to cause the client to read large amounts of data into memory, potentially causing OOM or other DoS.
CVSS Score
9.1
EPSS Score
0.001
Published
2025-12-01
When loading a plist file, the plistlib module reads data in size specified by the file itself, meaning a malicious file can cause OOM and DoS issues
CVSS Score
5.5
EPSS Score
0.0
Published
2025-12-01
A vulnerability has been found in the CPython `venv` module and CLI where path names provided when creating a virtual environment were not quoted properly, allowing the creator to inject commands into virtual environment "activation" scripts (ie "source venv/bin/activate"). This means that attacker-controlled virtual environments are able to run commands when the virtual environment is activated. Virtual environments which are not created by an attacker or which aren't activated before being used (ie "./venv/bin/python") are not affected.
CVSS Score
7.8
EPSS Score
0.001
Published
2024-10-22
There is a MEDIUM severity vulnerability affecting CPython.
Regular expressions that allowed excessive backtracking during tarfile.TarFile header parsing are vulnerable to ReDoS via specifically-crafted tar archives.
CVSS Score
7.5
EPSS Score
0.032
Published
2024-09-03
There is a LOW severity vulnerability affecting CPython, specifically the
'http.cookies' standard library module.
When parsing cookies that contained backslashes for quoted characters in
the cookie value, the parser would use an algorithm with quadratic
complexity, resulting in excess CPU resources being used while parsing the
value.
CVSS Score
7.5
EPSS Score
0.008
Published
2024-08-19
An issue was discovered in Python before 3.8.18, 3.9.x before 3.9.18, 3.10.x before 3.10.13, and 3.11.x before 3.11.5. It primarily affects servers (such as HTTP servers) that use TLS client authentication. If a TLS server-side socket is created, receives data into the socket buffer, and then is closed quickly, there is a brief window where the SSLSocket instance will detect the socket as "not connected" and won't initiate a handshake, but buffered data will still be readable from the socket buffer. This data will not be authenticated if the server-side TLS peer is expecting client certificate authentication, and is indistinguishable from valid TLS stream data. Data is limited in size to the amount that will fit in the buffer. (The TLS connection cannot directly be used for data exfiltration because the vulnerable code path requires that the connection be closed on initialization of the SSLSocket.)
CVSS Score
5.3
EPSS Score
0.006
Published
2023-08-25
An XML External Entity (XXE) issue was discovered in Python through 3.9.1. The plistlib module no longer accepts entity declarations in XML plist files to avoid XML vulnerabilities.
CVSS Score
9.8
EPSS Score
0.073
Published
2023-08-22
An issue was discovered in compare_digest in Lib/hmac.py in Python through 3.9.1. Constant-time-defeating optimisations were possible in the accumulator variable in hmac.compare_digest.
CVSS Score
5.9
EPSS Score
0.001
Published
2023-08-22
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