# CVE-2025-66418: urllib3 allows an unbounded number of links in the decompression chain > A malicious server can cause high CPU and memory usage in urllib3 by sending an unbounded decompression chain, 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-2025-66418, AZL-71834, AZL-71846, BELL-CVE-2025-66418, CLEANSTART-2026-GI63491, DEBIAN-CVE-2025-66418, ECHO-2d1b-2193-003f, GHSA-gm62-xv2j-4w53, MINI-2cfp-mr5p-v9xh, MINI-2j5j-mx6v-5v36, MINI-48c8-v3h8-88pj, MINI-7fh3-8hwm-xq38, MINI-86rf-527g-vj73, MINI-8p4j-jxf7-qm55, MINI-cc7r-3x5v-c76p, MINI-cw83-g2m8-hqfv, MINI-fcqc-vwm2-6257, MINI-m2f3-vrqw-wh79, MINI-qqqr-7wqv-2qf5, MINI-v6wh-6pjc-26fj, MINI-vq9w-8629-wf7g, OESA-2026-1233, OESA-2026-1234, OESA-2026-1235, OESA-2026-1252, OESA-2026-1253, OESA-2026-1332, OESA-2026-1333, PYSEC-2026-1998, RHSA-2026:1329, RHSA-2026:1330, RHSA-2026:1331, RHSA-2026:1332, RHSA-2026:1336, RHSA-2026:1337, RHSA-2026:1338, RHSA-2026:1339, RHSA-2026:1340, RHSA-2026:1701, RHSA-2026:1702, RHSA-2026:2279, ROOT-APP-PYPI-CVE-2025-66418, ROOT-OS-UBUNTU-2204-CVE-2025-66418, UBUNTU-CVE-2025-66418, USN-8010-1, openSUSE-SU-2026:10431-1 ## What is CVE-2025-66418? CVE-2025-66418 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 1.24 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. Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling) ## Which versions are affected by CVE-2025-66418? 3 open source packages ship an affected version, including urllib3. It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `urllib3` | PyPI | From 1.24 | 2.6.0 | | `urllib3` | PyPI | From 1.24 | 2.6.0 | | `urllib3` | PyPI | From 1.24 | 2.6.0 | | `python-s3transfer` | Red Hat:enterprise_linux:7::server | All versions | 0:0.1.13-1.el7_9.3 | | `resource-agents` | Red Hat:enterprise_linux:7::server | All versions | 0:4.1.1-61.el7_9.21 | | `resource-agents-aliyun` | Red Hat:enterprise_linux:7::server | All versions | 0:4.1.1-61.el7_9.21 | | `resource-agents-debuginfo` | Red Hat:enterprise_linux:7::server | All versions | 0:4.1.1-61.el7_9.21 | | `resource-agents-gcp` | Red Hat:enterprise_linux:7::server | All versions | 0:4.1.1-61.el7_9.21 | | `fence-agents` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-aliyun-debuginfo` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-all` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-amt-ws` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-apc` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-apc-snmp` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-bladecenter` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-brocade` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-cisco-mds` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-cisco-ucs` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-common` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-compute` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-debuginfo` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-debugsource` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-drac5` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-eaton-snmp` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-emerson` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-eps` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-heuristics-ping` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-hpblade` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-ibm-powervs` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-ibm-vpc` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-ibmblade` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-ifmib` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-ilo-moonshot` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-ilo-mp` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-ilo-ssh` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-ilo2` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-intelmodular` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-ipdu` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-ipmilan` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | | `fence-agents-kdump` | Red Hat:rhel_aus:8.4::appstream | All versions | 0:4.2.1-65.el8_4.24 | Products: Python Urllib3. ## How do I fix CVE-2025-66418? Upgrade urllib3 to 2.6.0 or later. ## Is CVE-2025-66418 being exploited? There is no known exploitation of CVE-2025-66418 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-2025-66471](https://insights.sbomify.com/CVE-2025-66471/): urllib3 Streaming API improperly handles highly compressed data (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-2025-66418 and fix it if it is affected. What it is: CVE-2025-66418 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-2025-66418 in the wild and no public exploit code. Affected packages and fixed versions: - urllib3 (PyPI): affected from 1.24; fixed in 2.6.0 - urllib3 (PyPI): affected from 1.24; fixed in 2.6.0 - urllib3 (PyPI): affected from 1.24; 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-2025-66418/ (Markdown: https://insights.sbomify.com/CVE-2025-66418/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2025-66418 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-66418 - OSV: https://osv.dev/vulnerability/CVE-2025-66418 --- From sbomify Insights: https://insights.sbomify.com/CVE-2025-66418/ Summaries are generated and may contain mistakes; the official description above is the record.