high CVSS 7.5 EPSS 0.62%

CVE-2025-66471·Python vulnerability

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.0 and prior to 2.6.0, the Streaming API improperly handles highly compressed data. 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. 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.

Severity
high
Software
Python
Fixed in
2.6.0
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at 0.62% — higher than 45% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Read the two together. CVE-2025-66471 scores 7.5 on CVSS, so a successful attack would be serious — but at 0.62%, almost nobody is currently attempting it. That is a different situation from a flaw under mass exploitation, and it is usually the difference between patching on your next maintenance window and patching tonight.

Affected versions

From: 1.0

Until: 2.6.0

Fixed in: 2.6.0

How to fix this CVE

Update urllib3 to version 2.6.0 or later to resolve a decompression handling flaw in the streaming API that can cause denial of service through excessive CPU and memory consumption. Organizations using Python with urllib3 for HTTP operations should prioritize this update to prevent potential resource exhaustion attacks targeting compressed HTTP responses.

sudo dnf update python3-urllib3

Defensia detects this vulnerability

How to check if you are affected

  1. Step 1: Check urllib3 version with `python3 -c "import urllib3; print(urllib3.__version__)"`
  2. Step 2: Verify if urllib3 is imported by running `pip3 list | grep urllib3` to confirm installation
  3. Step 3: Search logs for sudden spikes in CPU usage or memory allocation correlated with HTTP streaming requests: `journalctl -u <app-service> | grep -i 'memory\|cpu\|resource'`
  4. Step 4: After updating, re-run the version check to confirm urllib3 is at 2.6.0 or later

FAQ

What is CVE-2025-66471?

CVE-2025-66471 is a vulnerability in urllib3's streaming API where improper decompression of highly compressed HTTP responses can cause the library to consume excessive CPU and memory in a single read operation, potentially leading to denial of service.

Is CVE-2025-66471 being actively exploited?

No, there are currently no known active exploits or public proof-of-concept code for CVE-2025-66471, and it is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog.

What versions of urllib3 are affected by CVE-2025-66471?

urllib3 versions 1.0 through 2.5.x are vulnerable. The vulnerability is fixed in version 2.6.0 and later.

How do I check if my server is vulnerable to CVE-2025-66471?

Run `python3 -c "import urllib3; print(urllib3.__version__)"` and check if the version is below 2.6.0. If it is, your system is vulnerable.

Does Defensia detect CVE-2025-66471?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Python with urllib3 is installed on a monitored server, CVE-2025-66471 will appear in your dashboard with remediation steps.

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References

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