CVE-2026-21441
Python Urllib3 1.22 – 2.6.3
Raw 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:XSummary
CVE-2026-21441 is a high-severity Data Amplification (CWE-409) vulnerability in Python Urllib3. Its CVSS base score is 8.9 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Endpoint Denial of Service (T1499); ranked in the top 15% of CVEs by exploit likelihood; it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to SI-10 (Information Input Validation) and SC-5 (Denial-of-service Protection) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
CVE-2026-21441 affects urllib3, an HTTP client library for Python, in versions starting from 1.22 and prior to 2.6.3. The vulnerability arises in the library's streaming API, which is used for efficient handling of large HTTP responses by reading content in chunks with `preload_content=False`. For HTTP redirect responses, urllib3 reads and decompresses the entire response body to drain the connection, even before any read methods are called and regardless of configured read limits. This behavior enables decompression bombs, as there are no safeguards against maliciously crafted compressed content (e.g., via `gzip`, `deflate`, `br`, or `zstd` encodings indicated by the `Content-Encoding` header), leading to excessive resource consumption on the client.
Any remote attacker controlling an untrusted HTTP server can exploit this vulnerability by serving a redirect response with a highly compressible payload that expands dramatically during decompression. Client applications or libraries using urllib3's streaming mode without disabling redirects are vulnerable when fetching content from such servers. Successful exploitation results in denial-of-service via resource exhaustion, with a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H), stemming from CWE-409 (Improper Handling of Highly Compressed Data).
Advisories recommend upgrading to urllib3 version 2.6.3 or later, where the library no longer decodes content in redirect responses when `preload_content=False`. As an interim mitigation, disable redirects by setting `redirect=False` for requests to untrusted sources. Relevant resources include the urllib3 security advisory (GHSA-38jv-5279-wg99), the fixing commit (8864ac407bba8607950025e0979c4c69bc7abc7b), and Debian LTS announcements.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-1188
Vulnerability Data
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…
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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.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Input validation can reject or limit decompression of data whose expansion ratio exceeds safe thresholds.
DoS protection limits the resource-exhaustion impact when a decompression bomb is processed.
Resource allocation controls bound memory/CPU consumption that a data-amplification attack would otherwise exhaust.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Secure-development practices include input-validation and resource-limit checks that prevent improper handling of compressed data.
Runtime monitoring of compute resources can detect exhaustion caused by decompression bombs.
Capacity planning and monitoring directly limits the availability impact of data-amplification attacks.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing can uncover decompression-bomb vulnerabilities before release.
Redundancy helps availability but does not address the root cause of the weakness.
Monitoring can detect anomalous resource usage but does not prevent the weakness.
Secure development lifecycle includes input validation and resource-limit checks that mitigate data-amplification attacks.
Application security requirements can mandate limits on decompression size and ratio.
Secure architecture principles encourage defensive design against resource-exhaustion threats.