urllib3: Improper Handling of Highly Compressed Data (Data Amplification)
CVE-2026-21441 is a high-severity improper handling of highly compressed data (data amplification) vulnerability in urllib3, with a CVSS 3.1 base score of 7.5 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected urllib3 versions: >= 1.22, < 2.6.3. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 2.92% chance of exploitation within 30 days, higher than 87% of scored CVEs.
High severity with no evidence of exploitation so far.
What to do
- Check which version of urllib3 you run against the affected versions below, and install the vendor's latest release.
- Read the advisory on github.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
Version data comes from the organisation that assigned the CVE; 35 further products are omitted here. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- NoneNo data is disclosed.
- Integrity
- NoneNothing can be modified.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 4.0 | 8.9 | High | CNA (github.com) |
| CVSS 3.1 | 7.5 | High | NVD |
| CVSS 3.1 | 7.5 | High | CNA |
Official 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.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Improper Handling of Highly Compressed Data (Data Amplification)
The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output.
More Improper Handling of Highly Compressed Data (Data Amplification) vulnerabilities (CWE-409)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Patches and release notes
Other references
- lists.debian.org/debian-lts-announce/2026/01/msg00017.html
- access.redhat.com/errata/RHSA-2026:0981
- access.redhat.com/errata/RHSA-2026:0990
- access.redhat.com/errata/RHSA-2026:10184
- access.redhat.com/errata/RHSA-2026:1038
- access.redhat.com/errata/RHSA-2026:1041
- access.redhat.com/errata/RHSA-2026:1042
- access.redhat.com/errata/RHSA-2026:1086
- access.redhat.com/errata/RHSA-2026:1087
- access.redhat.com/errata/RHSA-2026:1088
- access.redhat.com/errata/RHSA-2026:1089
- access.redhat.com/errata/RHSA-2026:1166
- access.redhat.com/errata/RHSA-2026:1168
- access.redhat.com/errata/RHSA-2026:1176
- access.redhat.com/errata/RHSA-2026:1224
Other vulnerabilities in urllib3
- 7.5 High severity
- 8.1 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 8.1 High severity
Recent Improper Handling of Highly Compressed Data (Data Amplification) vulnerabilities
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
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