CVE-2025-6176: Scrapy Denial of Service (DoS) Vulnerability

CVE-2025-6176 Overview

Scrapy versions up to 2.13.2 are vulnerable to a denial of service (DoS) attack due to a critical flaw in its brotli decompression implementation. The protection mechanism against decompression bombs fails to mitigate the brotli variant, allowing remote servers to crash clients with less than 80GB of available memory. This occurs because brotli can achieve extremely high compression ratios for zero-filled data, leading to excessive memory consumption during decompression.

Critical Impact

Remote attackers can exploit this vulnerability to crash Scrapy-based web crawlers and scraping applications by serving malicious brotli-compressed responses, causing complete denial of service through memory exhaustion.

Affected Products

• Scrapy versions up to and including 2.13.2
• Applications and services built on vulnerable Scrapy versions
• Web scraping and crawling infrastructure using Scrapy with brotli decompression enabled

Discovery Timeline

• 2025-10-31 - CVE-2025-6176 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2025-6176

Vulnerability Analysis

This vulnerability is classified as CWE-400 (Uncontrolled Resource Consumption), a type of resource exhaustion vulnerability that enables denial of service attacks. The flaw exists in Scrapy's response decompression handling, specifically in how it processes brotli-compressed HTTP responses.

The brotli compression algorithm can achieve exceptionally high compression ratios when processing repetitive data patterns, particularly zero-filled content. While Scrapy implements protective measures against traditional decompression bomb attacks (such as gzip bombs), these safeguards do not adequately cover the brotli compression variant.

When a malicious server returns a specially crafted brotli-compressed response, the decompression process can expand a relatively small compressed payload into an enormous amount of data, rapidly consuming all available system memory. Systems with less than 80GB of RAM are particularly susceptible to crashing under this attack.

Root Cause

The root cause lies in the incomplete implementation of decompression bomb protection within Scrapy's HTTP response handling. The existing protection mechanism was designed for other compression formats but fails to account for brotli's unique compression characteristics. Brotli's ability to achieve extremely high compression ratios for specific data patterns (particularly repetitive or zero-filled data) creates an exploitable gap in the protection logic.

The vulnerability stems from the assumption that all compression algorithms behave similarly in terms of maximum compression ratios, when in reality brotli can significantly exceed the thresholds that traditional decompression bomb checks were designed to catch.

Attack Vector

This vulnerability is exploitable over the network without requiring any authentication or user interaction. An attacker can exploit this vulnerability by operating a malicious web server that serves specially crafted brotli-compressed responses to Scrapy clients.

The attack flow involves:

1. A Scrapy-based crawler or scraping application requests a resource from a malicious server
2. The server responds with a Content-Encoding of br (brotli) and a malicious compressed payload
3. Scrapy decompresses the response without adequate bounds checking for brotli
4. The decompressed data rapidly consumes available memory
5. The client system becomes unresponsive or crashes due to memory exhaustion

A proof-of-concept demonstrating this attack is publicly available at the CVE-2025-6176 PoC repository.

Detection Methods for CVE-2025-6176

Indicators of Compromise

• Abnormally high memory consumption by Python processes running Scrapy applications
• Scrapy crawlers crashing unexpectedly with out-of-memory errors
• HTTP responses with Content-Encoding: br header and unusually small compressed sizes
• System logs showing memory allocation failures during web scraping operations

Detection Strategies

• Monitor memory usage patterns of Scrapy-based applications for sudden spikes during HTTP response processing
• Implement logging to track compression ratios of incoming brotli-compressed responses
• Deploy network monitoring to identify servers consistently sending brotli-compressed responses with suspiciously small payload sizes
• Use application-level monitoring to detect and alert on decompression operations that exceed expected memory thresholds

Monitoring Recommendations

• Configure system-level memory alerts for processes running vulnerable Scrapy versions
• Implement rate limiting and circuit breakers for external HTTP requests in scraping infrastructure
• Deploy container resource limits to prevent single-crawler memory exhaustion from affecting the entire system
• Establish baseline memory consumption metrics for normal crawling operations to facilitate anomaly detection

How to Mitigate CVE-2025-6176

Immediate Actions Required

• Upgrade Scrapy to a patched version as soon as one becomes available
• Implement memory limits for Scrapy crawler processes using system-level controls (e.g., cgroups, container limits)
• Consider temporarily disabling brotli decompression support if not critical to operations
• Add external request restrictions to prevent crawlers from accessing untrusted domains without review

Patch Information

No official patch information is currently available in vendor advisories. Organizations should monitor the Huntr Bounty Listing for updates and official remediation guidance from the Scrapy maintainers.

Workarounds

• Configure process memory limits using operating system controls to prevent complete system crashes
• Implement a custom download middleware that monitors decompression size and terminates processing when thresholds are exceeded
• Use a proxy layer that can inspect and filter brotli-compressed responses before they reach Scrapy
• Restrict crawling scope to trusted domains to reduce exposure to malicious servers

# Configuration example - Limit memory for Scrapy process using systemd
# /etc/systemd/system/scrapy-crawler.service.d/limits.conf
[Service]
MemoryMax=4G
MemoryHigh=3G