# CVE-2025-66471: urllib3 Streaming API improperly handles highly compressed data > Attackers can cause excessive CPU and memory consumption in urllib3 by sending highly compressed data, leading to denial of service. - **Severity:** High 8.9 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 5 Dec 2025 - **Updated:** 5 Dec 2025 - **Also known as:** ALPINE-CVE-2025-66471, AZL-71837, AZL-71849, BELL-CVE-2025-66471, CLEANSTART-2026-JR23071, CLSA-2026-1770035896, CLSA-2026-1770046658, CLSA-2026-1787220376, DEBIAN-CVE-2025-66471, ECHO-c1ca-549f-f62b, GHSA-2xpw-w6gg-jr37, MINI-29g2-46wq-pw5x, MINI-2xv4-2mfg-wrr2, MINI-32jp-fmxp-x89h, MINI-38w9-v5vg-h9p3, MINI-5q6x-83cw-qvh4, MINI-cpr6-qwph-69pf, MINI-cw32-j3fw-5gjr, MINI-gjgm-ch9r-mgm7, MINI-h5w5-c8cv-j4vf, MINI-j76w-f2cm-vvqr, MINI-jp2h-32gh-w4cx, MINI-r38x-pgr5-94j5, MINI-x2xm-r5xf-pvr2, OESA-2026-1249, OESA-2026-1250, OESA-2026-1286, PYSEC-2026-1994, RHSA-2026:11722, RHSA-2026:1795, RHSA-2026:2060, RHSA-2026:9031, ROOT-APP-PYPI-CVE-2025-66471, SUSE-RU-2026:21397-1, SUSE-RU-2026:21430-1, SUSE-SU-2026:1067-1, SUSE-SU-2026:2854-1, SUSE-SU-2026:2854-2, UBUNTU-CVE-2025-66471, USN-7927-2, USN-7927-3, USN-8344-3, openSUSE-RU-2026:20649-1 ## What is CVE-2025-66471? CVE-2025-66471 is a high-severity denial of service vulnerability in urllib3, published on 5 Dec 2025. An attacker can exploit it over the internet without logging in, with no action from the victim. ### Official description 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. Weakness: CWE-409 (Improper Handling of Highly Compressed Data (Data Amplification)) ## Which versions are affected by CVE-2025-66471? 3 open source packages ship an affected version, including urllib3. It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `urllib3` | PyPI | From 1.0 | 2.6.0 | | `urllib3` | PyPI | From 1.0 | 2.6.0 | | `urllib3` | PyPI | From 1.0 | 2.6.0 | | `python-s3transfer` | Red Hat:enterprise_linux:7::server | All versions | 0:0.1.13-1.el7_9.5 | | `resource-agents` | Red Hat:enterprise_linux:7::server | All versions | 0:4.1.1-61.el7_9.22 | | `resource-agents-aliyun` | Red Hat:enterprise_linux:7::server | All versions | 0:4.1.1-61.el7_9.22 | | `resource-agents-debuginfo` | Red Hat:enterprise_linux:7::server | All versions | 0:4.1.1-61.el7_9.22 | | `resource-agents-gcp` | Red Hat:enterprise_linux:7::server | All versions | 0:4.1.1-61.el7_9.22 | | `python-urllib3` | Red Hat:rhel_els:6 | All versions | 0:1.10.2-3.el6_10.1 | | `python-urllib3` | Red Hat:rhel_els:7 | All versions | 0:1.10.2-7.el7_9.1 | | `resource-agents` | Red Hat:rhel_extras_sap_els:7 | All versions | 0:4.1.1-61.el7_9.22 | | `resource-agents-debuginfo` | Red Hat:rhel_extras_sap_els:7 | All versions | 0:4.1.1-61.el7_9.22 | | `resource-agents-sap` | Red Hat:rhel_extras_sap_els:7 | All versions | 0:4.1.1-61.el7_9.22 | | `sap-cluster-connector` | Red Hat:rhel_extras_sap_els:7 | All versions | 0:3.0.1-37.el7_9.22 | | `resource-agents` | Red Hat:rhel_extras_sap_hana_els:7 | All versions | 0:4.1.1-61.el7_9.22 | | `resource-agents-debuginfo` | Red Hat:rhel_extras_sap_hana_els:7 | All versions | 0:4.1.1-61.el7_9.22 | | `resource-agents-sap-hana` | Red Hat:rhel_extras_sap_hana_els:7 | All versions | 0:4.1.1-61.el7_9.22 | | `resource-agents-sap-hana-scaleout` | Red Hat:rhel_extras_sap_hana_els:7 | All versions | 0:0.164.0-6.el7_9.22 | | `python-urllib3` | SUSE:Linux Enterprise High Performance Computing 15 SP4-ESPOS | All versions | 1.25.10-150300.4.30.1 | | `python-urllib3` | SUSE:Linux Enterprise High Performance Computing 15 SP4-LTSS | All versions | 1.25.10-150300.4.30.1 | | `python-urllib3` | SUSE:Linux Enterprise High Performance Computing 15 SP5-ESPOS | All versions | 1.25.10-150300.4.30.1 | | `python-urllib3` | SUSE:Linux Enterprise High Performance Computing 15 SP5-LTSS | All versions | 1.25.10-150300.4.30.1 | | `python-urllib3` | SUSE:Linux Enterprise Micro 5.2 | All versions | 1.25.10-150300.4.24.1 | | `python-urllib3` | SUSE:Linux Enterprise Micro 5.3 | All versions | 1.25.10-150300.4.24.1 | | `python-urllib3` | SUSE:Linux Enterprise Micro 5.3 | All versions | 1.25.10-150300.4.30.1 | | `python-urllib3` | SUSE:Linux Enterprise Micro 5.4 | All versions | 1.25.10-150300.4.24.1 | | `python-urllib3` | SUSE:Linux Enterprise Micro 5.4 | All versions | 1.25.10-150300.4.30.1 | | `python-urllib3` | SUSE:Linux Enterprise Micro 5.5 | All versions | 1.25.10-150300.4.24.1 | | `python-urllib3` | SUSE:Linux Enterprise Micro 5.5 | All versions | 1.25.10-150300.4.30.1 | | `python-urllib3` | SUSE:Linux Enterprise Module for Basesystem 15 SP7 | All versions | 1.25.10-150300.4.24.1 | | `python-urllib3` | SUSE:Linux Enterprise Module for Basesystem 15 SP7 | All versions | 1.25.10-150300.4.30.1 | | `python-urllib3` | SUSE:Linux Enterprise Server 15 SP4-LTSS | All versions | 1.25.10-150300.4.30.1 | | `python-urllib3` | SUSE:Linux Enterprise Server 15 SP5-LTSS | All versions | 1.25.10-150300.4.30.1 | | `python-urllib3` | SUSE:Linux Enterprise Server 15 SP6-LTSS | All versions | 1.25.10-150300.4.30.1 | | `python-urllib3` | SUSE:Linux Enterprise Server for SAP Applications 15 SP4 | All versions | 1.25.10-150300.4.30.1 | | `python-urllib3` | SUSE:Linux Enterprise Server for SAP Applications 15 SP5 | All versions | 1.25.10-150300.4.30.1 | | `python-urllib3` | SUSE:Linux Enterprise Server for SAP Applications 15 SP6 | All versions | 1.25.10-150300.4.30.1 | Products: Python Urllib3. ## How do I fix CVE-2025-66471? Upgrade urllib3 to 2.6.0 or later. ## Is CVE-2025-66471 being exploited? There is no known exploitation of CVE-2025-66471 in the wild and no public exploit code. ## Other vulnerabilities in urllib3 - [CVE-2026-97689](https://insights.sbomify.com/CVE-2026-97689/): urllib3: HTTPResponse.stream()/read_chunked() buffers an unbounded chunk-size line into memory (high 8.9) - [CVE-2026-44432](https://insights.sbomify.com/CVE-2026-44432/): urllib3: Decompression-bomb safeguards bypassed in parts of the streaming API (high 8.9) - [CVE-2026-21441](https://insights.sbomify.com/CVE-2026-21441/): urllib3 vulnerable to decompression-bomb safeguard bypass when following HTTP redirects (streaming API) (high 8.9) - [CVE-2025-66418](https://insights.sbomify.com/CVE-2025-66418/): urllib3 allows an unbounded number of links in the decompression chain (high 8.9) - [CVE-2026-44431](https://insights.sbomify.com/CVE-2026-44431/): urllib3: Sensitive headers forwarded across origins in proxied low-level redirects (high 8.2) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2025-66471 and fix it if it is affected. What it is: CVE-2025-66471 is a high-severity denial of service vulnerability in urllib3, published on 5 Dec 2025. An attacker can exploit it over the internet without logging in, with no action from the victim. Exploitation: There is no known exploitation of CVE-2025-66471 in the wild and no public exploit code. Affected packages and fixed versions: - urllib3 (PyPI): affected from 1.0; fixed in 2.6.0 - urllib3 (PyPI): affected from 1.0; fixed in 2.6.0 - urllib3 (PyPI): affected from 1.0; fixed in 2.6.0 Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2025-66471/ (Markdown: https://insights.sbomify.com/CVE-2025-66471/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2025-66471 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-66471 - OSV: https://osv.dev/vulnerability/CVE-2025-66471 --- From sbomify Insights: https://insights.sbomify.com/CVE-2025-66471/ Summaries are generated and may contain mistakes; the official description above is the record.