{"cve":"CVE-2026-21441","nvd":{"published":"2026-01-07T22:15:44.040","modified":"2026-09-15T12:16:58.133","status":"Modified","description":"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_v4":{"vector":"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","score":8.9},"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","score":7.5},"cwes":["CWE-409"],"references":["https://github.com/urllib3/urllib3/commit/8864ac407bba8607950025e0979c4c69bc7abc7b","https://github.com/urllib3/urllib3/security/advisories/GHSA-38jv-5279-wg99","https://lists.debian.org/debian-lts-announce/2026/01/msg00017.html","https://access.redhat.com/errata/RHSA-2026:0981","https://access.redhat.com/errata/RHSA-2026:0990"],"products":["python:urllib3"],"source":"nvd","synced_at":"2026-10-19T22:19:41.481987139+00:00"},"epss":{"cve":"CVE-2026-21441","score":0.02922,"percentile":0.87044,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-38jv-5279-wg99","url":"https://osv.dev/vulnerability/GHSA-38jv-5279-wg99","summary":"Decompression-bomb safeguards bypassed when following HTTP redirects (streaming API)","modified":"2026-09-10T03:50:32.562010895Z","packages":[{"ecosystem":"PyPI","name":"urllib3","purl":"pkg:pypi/urllib3","ranges":[">=1.22 <2.6.3"],"fixed":["2.6.3"]}]},{"id":"PYSEC-2026-1996","url":"https://osv.dev/vulnerability/PYSEC-2026-1996","summary":"Decompression-bomb safeguards bypassed when following HTTP redirects (streaming API)","modified":"2026-07-07T17:48:15.382053236Z","packages":[{"ecosystem":"PyPI","name":"urllib3","purl":"pkg:pypi/urllib3","ranges":[">=1.22 <2.6.3"],"fixed":["2.6.3"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}