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CVE-2026-115559

imaplib STARTTLS serves plaintext bytes buffered before the handshake as if they came over TLS

IMAP4.starttls() wraps the socket in TLS but leaves self._readbuf, imaplib's own read-ahead buffer, untouched. Any plaintext bytes that arrived in the same recv() as the tagged OK to STARTTLS are returned by readline() after the handshake, so the client treats them as the first responses on the protected channel. A network attacker who cannot break TLS can append forged lines after the server's OK, let the handshake complete with the real server and a valid certificate, and have the client accept the forged CAPABILITY and the forged tagged completion of the next command. This is the STARTTLS command/response injection class of CVE-2011-0411.

CVSS 4.0 · 6.3 Medium EPSS < 1% KEV · Not listed
Vendors
Python Python Software Foundation
Products
Cpython Cpython
CVE-2026-16052

Cookie Attribute Injection via Unescaped Semicolons in http.cookies.Morsel

The http.cookies.Morsel class allows separator semicolons in attribute values and includes them verbatim in all output formats. An attacker who can control these values could use this to relax the cookie's Domain or SameSite restrictions.

CVSS 4.0 · 2.3 Low EPSS n/a KEV · Not listed
Vendors
Python Python Software Foundation
Products
Cpython Cpython
CVE-2026-19552

ssl.get_server_certificate(addr, ca_certs=...) verifies the chain but never the identity

When ca_certs parameter was supplied to ssl.get_server_certificate() function, the identity of the certificate is not checked.

CVSS 4.0 · 0 Low EPSS < 1% KEV · Not listed
Vendors
Python Python Software Foundation
Products
Cpython Cpython
CVE-2026-112899

Pillow: EPS load(scale) lets Ghostscript render past the decompression bomb limit

Pillow is a Python imaging library. In versions 2.3.0 through 12.3.0, EPS rendering does not validate the scaled image dimensions before invoking Ghostscript. Applications that process untrusted EPS or PostScript files using an application-selected scale greater than one can incur excessive Ghostscript CPU and temporary disk consumption. In versions with decompression-bomb protection, the scaled output is checked only after Ghostscript has completed rendering, allowing resource consumption beyond the configured pixel limit. The amplification depends on the scale selected by the application, not an attacker-controlled scale value. This issue is fixed in version 13.0.0.

CVSS 4.0 · 6.3 Medium EPSS < 1% KEV · Not listed
Vendors
Python Python-pillow
Products
Pillow Pillow
CVE-2026-112902

Pillow: Memory disclosure and segfault in libtiff encoder via crafted TIFF tags

Pillow is a Python imaging library. In versions through 12.3.0, the libtiff TIFF encoder can pass arguments to  TIFFSetField()  that do not match libtiff’s definition of the corresponding tag. When an application saves attacker-controlled image metadata through this encoder, crafted tag values can cause invalid memory reads, copying process memory into the encoded output or crashing the process. Applications that return the encoded output to an attacker can disclose that memory. The issue is reachable when re-saving compressed TIFF images and through certain conversions that preserve EXIF metadata into compressed TIFF output. This vulnerability is fixed in version 13.0.0.

CVSS 4.0 · 8.3 High EPSS < 1% KEV · Not listed
Vendors
Python Python-pillow
Products
Pillow Pillow
CVE-2026-21441

urllib3 vulnerable to decompression-bomb safeguard bypass when following HTTP redirects (streaming API)

urllib3 is an HTTP client library for Python. urllib3's streaming API is designed for the efficient handling of large HTTP responses by reading the content in chunks, rather than loading the entire response body into memory at once. urllib3 can perform decoding or decompression based on the HTTP `Content-Encoding` header (e.g., `gzip`, `deflate`, `br`, or `zstd`). When using the streaming API, the library decompresses only the necessary bytes, enabling partial content consumption. Starting in version 1.22 and prior to version 2.6.3, for HTTP redirect responses, the library would read the entire response body to drain the connection and decompress the content unnecessarily. This decompression occurred even before any read methods were called, and configured read limits did not restrict the amount of decompressed data. As a result, there was no safeguard against decompression bombs. A malicious server could exploit this to trigger excessive resource consumption on the client. Applications and libraries are affected when they stream content from untrusted sources by setting `preload_content=False` when they do not disable redirects. Users should upgrade to at least urllib3 v2.6.3, in which the library does not decode content of redirect responses when `preload_content=False`. If upgrading is not immediately possible, disable redirects by setting `redirect=False` for requests to untrusted source.

CVSS 4.0 · 8.9 High EPSS 2.9% KEV · Not listed
Vendors
Python Urllib3
Products
Urllib3 Urllib3
CVE-2026-9669

bz2.BZ2Decompressor reuse after error can cause a stack buffer overflow

bz2.BZ2Decompressor objects could be reused after a decompression error. If an application caught the resulting OSError and retried with the same decompressor, crafted input could cause the decompressor to resume from an invalid internal state and perform out-of-bounds writes to a stack buffer. This could crash the process when processing untrusted data.

CVSS 4.0 · 8.2 High EPSS < 1% KEV · Not listed
Vendors
Python
Products
Cpython
CVE-2026-8328

FTP PASV SSRF, ftpcp() does not use actual peer address, trusts server-supplied PASV host address

The ftpcp() function in Lib/ftplib.py was not updated when CVE-2021-4189 was fixed. While makepasv() was patched to replace server-supplied PASV host addresses with the actual peer address (getpeername()[0]), ftpcp() still calls parse227() directly and passes the raw attacker-controllable IP address and port to target.sendport(). This patch is related to CVE-2021-4189.

CVSS 4.0 · 5.9 Medium EPSS < 1% KEV · Not listed
Vendors
Python
Products
Cpython
CVE-2026-6019

BaseCookie.js_output() does not neutralize embedded characters

http.cookies.Morsel.js_output() returns an inline <script> snippet and only escapes " for JavaScript string context. It does not neutralize the HTML parser-sensitive sequence </script> inside the generated script element. Mitigation base64-encodes the cookie value to disallow escaping using cookie value.

CVSS 4.0 · 2.1 Low EPSS < 1% KEV · Not listed
Vendors
Python
Products
Cpython Python
CVE-2026-3276

Potential DoS via quadratic complexity in unicodedata.normalize()

unicodedata.normalize() can take excessive CPU time when processing specially crafted Unicode input containing long runs of combining characters with alternating Canonical Combining Class values. This affects all normalization forms.

CVSS 4.0 · 6.3 Medium EPSS < 1% KEV · Not listed
Vendors
Python
Products
Cpython
CVE-2026-1502

HTTP client proxy tunnel headers not validated for CR/LF

CR/LF bytes were not rejected by HTTP client proxy tunnel headers or host.

CVSS 4.0 · 5.7 Medium EPSS < 1% KEV · Not listed
Vendors
Python
Products
Cpython
CVE-2026-19553

SSLContext.wrap_bio() missing validation of server_hostname parameter

ssl.SSLContext.wrap_bio() didn't require the server_hostname argument to not be None if ssl.SSLContext.check_hostname was set. Due to a missing parameter check in SSLObject, if the server_hostname argument isn't supplied then hostname verification would be silently skipped. This defect could lead to programs where certificate hostname verification *appeared* to be succeeding with SSLContext.check_hostname = True and no ValueError being raised due to misconfiguration. If the program passes a server_hostname value that isn't an empty string or None to any of these APIs then certificate hostname verification proceeds as expected and the program is not affected by this vulnerability. Mitigating this vulnerability doesn't require updating Python or applying the patch. To mitigate, pass a valid non-None and non-empty server_hostname value to SSLContext.wrap_bio(), asyncio.create_connection(), or asyncio.loop.start_tls() and certificate hostname verification will proceed as expected. Upgrading to the latest version of Python or applying the patch only changes the behavior from silently skipping hostname verification to raising a ValueError, similar to SSLContext.wrap_socket(), when server_hostname isn't supplied.

CVSS 4.0 · 7.6 High EPSS < 1% KEV · Not listed
Vendors
Python
Products
Cpython
CVE-2026-19445

Use-after-free of a server-side SSLContext when sni_callback switches contexts

A remote, unauthenticated TLS client can make a server crash or call through a freed pointer if its sni_callback assigns a different context to SSLSocket.context (the documented way to select a certificate per server name) and nothing else keeps the original ssl.SSLContext alive. Typical cases are servers that create an SSLContext per connection or replace it while connections are open; servers that wrap their listening socket with it are not affected. Mitigation: keep a reference to every SSLContext that sets sni_callback for the lifetime of the server. TLS clients are not affected.

CVSS 4.0 · 9.2 Critical EPSS < 1% KEV · Not listed
Vendors
Python Redhat
Products
Cpython Hummingbird
CVE-2026-18503

Super-linear CPU usage for unbounded input to csv.Sniffer.sniff()

Attacker-controlled CSV samples can trigger super-linear regular-expression work during dialect sniffing and consume significant CPU when applications pass unbounded input to csv.Sniffer.sniff().

CVSS 4.0 · 2.4 Low EPSS < 1% KEV · Not listed
Vendors
Python
Products
Cpython
CVE-2026-17084

stringprep.map_table_b2() deviates from RFC 3454 Table B.2

The "stringprep" module didn't process characters from RFC 3454 tables B.2 or B.3 correctly: the latest Unicode codepoint attributes were used instead of the specified Unicode 3.2.0. This behavior would cause mismatches when processing domain names using IDNA 2003 (the "idna" codec) and the in_table_b2() function of the "stringprep" module. This only affects domain names containing characters that were not previously registered or had their Unicode attributes such as case-folding behavior updated since Unicode 3.2.0.

CVSS 4.0 · 6 Medium EPSS < 1% KEV · Not listed
Vendors
Python
Products
Cpython
CVE-2026-15806

`HTTPPasswordMgr` can send saved HTTPS credentials via HTTP because of incorrect scheme matching

The HTTPPasswordMgr class in the urllib.request module, along with its subclasses HTTPPasswordMgrWithDefaultRealm and HTTPPasswordMgrWithPriorAuth, did not take the URL scheme into account when matching stored credentials against a requested URL. Credentials added for an https:// URL were also used for requests to the same host over http://, so an attacker able to redirect or downgrade a client to plain HTTP (for example, via an HTTPS-to-HTTP redirect or an on-path position) could capture credentials in cleartext. Credentials added for http:// URLs could likewise be sent over https://. Credential matching is now scoped by URL scheme. Credentials registered with a URL that includes a scheme are only used for requests with the same scheme. Credentials registered with a bare authority (such as example.com or example.com:8080) continue to match any scheme, preserving compatibility with existing code, including proxy authentication. Users who cannot upgrade immediately can mitigate by ensuring that applications never make plain http:// requests to hosts for which credentials are registered, for example by not following redirects to http:// URLs.

CVSS 4.0 · 6 Medium EPSS < 1% KEV · Not listed
Vendors
Python
Products
Cpython
CVE-2026-15310

zipfile: bzip2/LZMA/Zstandard members decompress without a max_length bound, defeating chunked-read memory limits

When decompressing crafted zip files using the bzip/LZMA/Zstandard compressions, Python could use an attacker-controlled size to pre-allocate memory, possibly resulting in memory exhaustion.

CVSS 4.0 · 2.1 Low EPSS < 1% KEV · Not listed
Vendors
Python
Products
Cpython
CVE-2026-15308

Incremental HTMLParser feed() allows CPU-exhaustion DoS via repeated unterminated markup declarations

The incremental HTML parser (html.parser.HTMLParser) allows for CPU denial-of-service through repeated unterminated markup declarations when processing uncontrolled data.

CVSS 4.0 · 8.7 High EPSS < 1% KEV · Not listed
Vendors
Python
Products
Cpython Python
CVE-2021-4189

python: ftplib should not use the host from the PASV response

A flaw was found in Python, specifically in the FTP (File Transfer Protocol) client library in PASV (passive) mode. The issue is how the FTP client trusts the host from the PASV response by default. This flaw allows an attacker to set up a malicious FTP server that can trick FTP clients into connecting back to a given IP address and port. This vulnerability could lead to FTP client scanning ports, which otherwise would not have been possible.

CVSS 3.1 · 5.3 Medium EPSS 3.6% KEV · Not listed
Vendors
Debian Netapp Python and 1 more
Products
Debian Linux Ontap Select Deploy Administration Utility Python and 3 more
CVE-2026-124249

`http.cookiejar` applies expired `Set-Cookie` to cookie store before policy validation

http.cookiejar.CookieJar deleted a stored cookie when a response carried an already expired Set-Cookie header for it, using the Domain and Path attributes from the header without consulting the cookie policy. A server could therefore delete cookies belonging to other domains from a shared cookie jar, such as the one used by urllib's HTTPCookieProcessor. The fix consults the cookie policy before an expired cookie may delete an existing one, as required by RFC 6265 section 5.3.

CVSS 4.0 · 6.3 Medium EPSS < 1% KEV · Not listed
Vendors
Python Python Software Foundation
Products
Cpython Cpython