How severe is CVE-2025-66418?

CVE-2025-66418 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2025-66418 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (46th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2025-66418?

CVE-2025-66418 affects Python Urllib3. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2025-66418?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2025-66418 have an EU (EUVD) identifier?

Yes. CVE-2025-66418 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-201421.

When was CVE-2025-66418 published?

CVE-2025-66418 was published on 2025-12-05 and last updated on 2026-06-17.

CVE-2025-66418

CVE-2025-66418 is a high-severity vulnerability in Python Urllib3 with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-770.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v4: 8.9
EPSS exploit prediction: 1% (46th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2025-201421
Weakness: CWE-770
Affected product: Python Urllib3
Published: 2025-12-05
Last modified: 2026-06-17

Description

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.

Frequently asked questions

What is CVE-2025-66418?: urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.
How severe is CVE-2025-66418?: CVE-2025-66418 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2025-66418 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (46th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2025-66418?: CVE-2025-66418 affects Python Urllib3. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2025-66418?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2025-66418 have an EU (EUVD) identifier?: Yes. CVE-2025-66418 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-201421.
When was CVE-2025-66418 published?: CVE-2025-66418 was published on 2025-12-05 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:python:urllib3:*:*:*:*:*:*:*:*

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