CVE-2025-66418

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.

Impact

urllib3 supports chained HTTP encoding algorithms for response content according to RFC 9110 (e.g., Content-Encoding: gzip, zstd).

However, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data.

Affected usages

Applications and libraries using urllib3 version 2.5.0 and earlier for HTTP requests to untrusted sources unless they disable content decoding explicitly.

Remediation

Upgrade to at least urllib3 v2.6.0 in which the library limits the number of links to 5.

If upgrading is not immediately possible, use preload_content=False and ensure that resp.headers["content-encoding"] contains a safe number of encodings before reading the response content.

HTTP library with thread-safe connection pooling, file post support, sanity friendly, and more.

Security Fix(es):

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.(CVE-2025-66418)

HTTP library with thread-safe connection pooling, file post support, sanity friendly, and more.

Security Fix(es):

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.(CVE-2025-66418)

HTTP library with thread-safe connection pooling, file post support, sanity friendly, and more.

Security Fix(es):

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.(CVE-2025-66418)

HTTP library with thread-safe connection pooling, file post support, sanity friendly, and more.

Security Fix(es):

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.(CVE-2025-66418)

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.0 and prior to 2.6.0, the Streaming API improperly handles highly compressed data. 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. When streaming a compressed response, urllib3 can perform decoding or decompression based on the HTTP Content-Encoding header (e.g., gzip, deflate, br, or zstd). The library must read compressed data from the network and decompress it until the requested chunk size is met. Any resulting decompressed data that exceeds the requested amount is held in an internal buffer for the next read operation. The decompression logic could cause urllib3 to fully decode a small amount of highly compressed data in a single operation. This can result in excessive resource consumption (high CPU usage and massive memory allocation for the decompressed data.(CVE-2025-66471)

pip is the package installer for Python. You can use pip to install packages from the Python Package Index and other indexes. %global bashcompdir %(b=$(pkg-config --variable=completionsdir bash-completion 2>/dev/null); echo ${b:-/bash_completion.d}) Name: python-pip Version: 23.3.1 Release: 6 Summary: A tool for installing and managing Python packages License: MIT and Python and ASL 2.0 and BSD and ISC and LGPLv2 and MPLv2.0 and (ASL 2.0 or BSD) URL: http://www.pip-installer.org Source0: Source1: pip.loongarch.conf BuildArch: noarch Patch1: remove-existing-dist-only-if-path-conflicts. Patch6000: dummy-certifi.patch Patch6001: backport-CVE-2023-45803-Made-body-stripped-from-HTTP-requests.patch Patch6002: backport-CVE-2024-37891-Strip-Proxy-Authorization-header-on-redirects.patch Patch6003: backport-CVE-2024-47081.patch Patch6004: backport-CVE-2025-50181.patch Patch6005: backport-CVE-2025-8869.patch

Security Fix(es):

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.(CVE-2025-66418)

pip is the package installer for Python. You can use pip to install packages from the Python Package Index and other indexes. %global bashcompdir %(b=$(pkg-config --variable=completionsdir bash-completion 2>/dev/null); echo ${b:-/bash_completion.d}) Name: python-pip Version: 23.3.1 Release: 6 Summary: A tool for installing and managing Python packages License: MIT and Python and ASL 2.0 and BSD and ISC and LGPLv2 and MPLv2.0 and (ASL 2.0 or BSD) URL: http://www.pip-installer.org Source0: Source1: pip.loongarch.conf BuildArch: noarch Patch1: remove-existing-dist-only-if-path-conflicts. Patch6000: dummy-certifi.patch Patch6001: backport-CVE-2023-45803-Made-body-stripped-from-HTTP-requests.patch Patch6002: backport-CVE-2024-37891-Strip-Proxy-Authorization-header-on-redirects.patch Patch6003: backport-CVE-2024-47081.patch Patch6004: backport-CVE-2025-50181.patch Patch6005: backport-CVE-2025-8869.patch

Security Fix(es):

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.(CVE-2025-66418)

pip is the package installer for Python. You can use pip to install packages from the Python Package Index and other indexes. %global bashcompdir %(b=$(pkg-config --variable=completionsdir bash-completion 2>/dev/null); echo ${b:-/bash_completion.d}) Name: python-pip Version: 23.3.1 Release: 6 Summary: A tool for installing and managing Python packages License: MIT and Python and ASL 2.0 and BSD and ISC and LGPLv2 and MPLv2.0 and (ASL 2.0 or BSD) URL: http://www.pip-installer.org Source0: Source1: pip.loongarch.conf BuildArch: noarch Patch1: remove-existing-dist-only-if-path-conflicts. Patch6000: dummy-certifi.patch Patch6001: backport-CVE-2023-45803-Made-body-stripped-from-HTTP-requests.patch Patch6002: backport-CVE-2024-37891-Strip-Proxy-Authorization-header-on-redirects.patch Patch6003: backport-CVE-2024-47081.patch Patch6004: backport-CVE-2025-50181.patch Patch6005: backport-CVE-2025-8869.patch

Security Fix(es):

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.(CVE-2025-66418)

pip is the package installer for Python. You can use pip to install packages from the Python Package Index and other indexes. %global bashcompdir %(b=$(pkg-config --variable=completionsdir bash-completion 2>/dev/null); echo ${b:-/bash_completion.d}) Name: python-pip Version: 23.3.1 Release: 6 Summary: A tool for installing and managing Python packages License: MIT and Python and ASL 2.0 and BSD and ISC and LGPLv2 and MPLv2.0 and (ASL 2.0 or BSD) URL: http://www.pip-installer.org Source0: Source1: pip.loongarch.conf BuildArch: noarch Patch1: remove-existing-dist-only-if-path-conflicts. Patch6000: dummy-certifi.patch Patch6001: backport-CVE-2023-45803-Made-body-stripped-from-HTTP-requests.patch Patch6002: backport-CVE-2024-37891-Strip-Proxy-Authorization-header-on-redirects.patch Patch6003: backport-CVE-2024-47081.patch Patch6004: backport-CVE-2025-50181.patch Patch6005: backport-CVE-2025-8869.patch

Security Fix(es):

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.(CVE-2025-66418)

HTTP library with thread-safe connection pooling, file post support, sanity friendly, and more.

Security Fix(es):

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.(CVE-2025-66418)

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.0 and prior to 2.6.0, the Streaming API improperly handles highly compressed data. 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. When streaming a compressed response, urllib3 can perform decoding or decompression based on the HTTP Content-Encoding header (e.g., gzip, deflate, br, or zstd). The library must read compressed data from the network and decompress it until the requested chunk size is met. Any resulting decompressed data that exceeds the requested amount is held in an internal buffer for the next read operation. The decompression logic could cause urllib3 to fully decode a small amount of highly compressed data in a single operation. This can result in excessive resource consumption (high CPU usage and massive memory allocation for the decompressed data.(CVE-2025-66471)

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.(CVE-2026-21441)

pip is the package installer for Python. You can use pip to install packages from the Python Package Index and other indexes. %global bashcompdir %(b=$(pkg-config --variable=completionsdir bash-completion 2>/dev/null); echo ${b:-/bash_completion.d}) Name: python-pip Version: 23.3.1 Release: 6 Summary: A tool for installing and managing Python packages License: MIT and Python and ASL 2.0 and BSD and ISC and LGPLv2 and MPLv2.0 and (ASL 2.0 or BSD) URL: http://www.pip-installer.org Source0: Source1: pip.loongarch.conf BuildArch: noarch Patch1: remove-existing-dist-only-if-path-conflicts. Patch6000: dummy-certifi.patch Patch6001: backport-CVE-2023-45803-Made-body-stripped-from-HTTP-requests.patch Patch6002: backport-CVE-2024-37891-Strip-Proxy-Authorization-header-on-redirects.patch Patch6003: backport-CVE-2024-47081.patch Patch6004: backport-CVE-2025-50181.patch Patch6005: backport-CVE-2025-8869.patch

Security Fix(es):

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.(CVE-2025-66418)

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. However, 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 (high CPU usage and large memory allocations for decompressed data).(CVE-2026-21441)

Impact

urllib3 supports chained HTTP encoding algorithms for response content according to RFC 9110 (e.g., Content-Encoding: gzip, zstd).

However, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data.

Affected usages

Applications and libraries using urllib3 version 2.5.0 and earlier for HTTP requests to untrusted sources unless they disable content decoding explicitly.

Remediation

Upgrade to at least urllib3 v2.6.0 in which the library limits the number of links to 5.

If upgrading is not immediately possible, use preload_content=False and ensure that resp.headers["content-encoding"] contains a safe number of encodings before reading the response content.