Answer in brief
CVE-2025-66471 records a High severity (CVSS 7.5) vulnerability in urllib3 streaming API improperly handles highly compressed data. The current sources do not mark it as known exploited. The current feed maps urllib3 (pip), urllib3 (pypi). Check affected ranges and fixed versions before updating.
Analysis pending evidence review
HOL Guard separates source facts from reviewed analysis. See the methodology.
CVSS is 7.5. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.
Analysis status
Analysis pending evidence review
Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.
The current feed maps urllib3 (pip), urllib3 (pypi). Check affected ranges and fixed versions before updating.
| Product | Affected versions | Fixed versions |
|---|---|---|
| cpe:2.3:a:python:urllib3:*:*:*:*:*:*:*:* | Not reported | Not reported |
| Package | Affected range | Fixed version |
|---|---|---|
| urllib3pip | >=1.0,<2.6.0 | 2.6.0 |
| urllib3pypi | >=1.0 <2.6.0 | 2.6.0 |
Published upstream
Dec 5, 2025
Evidence: source:ghsa:source_dates:source-dates:recordSource modified
Sep 25, 2026
Evidence: source:ghsa:source_dates:source-dates:recordFirst seen by HOL
Sep 25, 2026
### Impact urllib3's [streaming API](https://urllib3.readthedocs.io/en/2.5.0/advanced-usage.html#streaming-and-i-o) 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; CWE-409) on the client side, even if the application only requested a small chunk of data. ### Affected usages Applications and libraries using urllib3 version 2.5.0 and earlier to stream large compressed responses or content from untrusted sources. `stream()`, `read(amt=256)`, `read1(amt=256)`, `read_chunked(amt=256)`, `readinto(b)` are examples of `urllib3.HTTPResponse` method calls using the affected logic unless decoding is disabled explicitly. ### Remediation Upgrade to at least urllib3 v2.6.0 in which the library avoids decompressing data that exceeds the requested amount. If your environment contains a package facilitating the Brotli encoding, upgrade to at least Brotli 1.2.0 or brotlicffi 1.2.0.0 too. These versions are enforced by the `urllib3[brotli]` extra in the patched versions of urllib3. ### Credits The issue was reported by @Cycloctane. Supplemental information was provided by @stamparm during a security audit performed by [7ASecurity](https://7asecurity.com/) and facilitated by [OSTIF](https://ostif.org/).
Quoted source text, attributed separately from HOL analysis.