Published: 2025-12-05T16:15:51.053
Overview
CVE-2025-66418 identifies a critical vulnerability within urllib3, a widely used HTTP client library for Python. This vulnerability, affecting versions 1.24 up to, but not including, 2.6.0, stems from an unbounded number of links permitted in the decompression chain. A malicious server could exploit this by injecting a virtually unlimited number of compression steps. This leads to excessive CPU utilization and substantial memory allocation as urllib3 attempts to decompress the data, potentially resulting in a denial-of-service (DoS) condition.
Technical Details
The vulnerability arises from the way urllib3 handles compressed responses. Specifically, the library fails to limit the depth of chained compression (e.g., gzip inside gzip inside gzip…). A malicious server can leverage this by nesting compression layers arbitrarily deeply. When urllib3 attempts to decompress such a response, it iterates through each layer, consuming CPU cycles and allocating memory for the inflated data at each step. Because there’s no limit to these decompression layers, a relatively small compressed payload can trigger an enormous amount of processing on the client side, leading to a DoS.
CVSS Analysis
Currently, the severity and CVSS score for CVE-2025-66418 are marked as N/A. However, given the potential for a denial-of-service attack, it is likely to be classified as at least a Medium severity vulnerability. A CVSS score will likely be assigned based on factors such as exploit complexity, attack vector, and scope of impact once more analysis is performed.
Possible Impact
The primary impact of this vulnerability is a denial-of-service (DoS). Successful exploitation can cause:
- High CPU Usage: Excessive decompression attempts consume significant CPU resources.
- Memory Exhaustion: Decompressing deeply nested compressed data can lead to massive memory allocation, potentially crashing the application or the entire system.
- Application Unavailability: The affected application becomes unresponsive, impacting users and services that rely on it.
This vulnerability is particularly concerning for applications that interact with untrusted servers or handle large volumes of HTTP traffic, as they are more susceptible to exploitation.
Mitigation or Patch Steps
The recommended solution is to upgrade urllib3 to version 2.6.0 or later. This version includes a fix that limits the number of allowed compression layers, preventing the unbounded decompression chain vulnerability.
- Upgrade urllib3: Use pip to upgrade to the latest version:
pip install --upgrade urllib3 - Verify the Version: After upgrading, verify the installed version:
pip show urllib3. Ensure it is 2.6.0 or greater. - Review Dependencies: Ensure that any other libraries or applications that depend on urllib3 are also updated to be compatible with the upgraded version.
