CVE-2025-62727: Starlette ASGI Framework DoS Vulnerability

CVE-2025-62727 Overview

Starlette is a lightweight ASGI framework/toolkit widely used for building high-performance Python web applications. A denial-of-service vulnerability exists in Starlette versions 0.39.0 through 0.49.0 that allows an unauthenticated attacker to send a crafted HTTP Range header that triggers quadratic-time processing in the FileResponse Range parsing and merging logic. This enables CPU exhaustion per request, causing denial-of-service for endpoints serving files such as StaticFiles or any use of FileResponse.

Critical Impact
Unauthenticated attackers can exhaust server CPU resources through malicious HTTP Range headers, causing denial-of-service for file-serving endpoints without requiring authentication.

Affected Products

Starlette versions >= 0.39.0 and < 0.49.1
Applications using Starlette's FileResponse class
Applications using Starlette's StaticFiles middleware

Discovery Timeline

2025-10-28 - CVE-2025-62727 published to NVD
2025-11-04 - Last updated in NVD database

Technical Details for CVE-2025-62727

Vulnerability Analysis

This vulnerability is classified as CWE-407 (Inefficient Algorithmic Complexity), representing an algorithmic complexity attack that leads to CPU exhaustion. The flaw resides in how Starlette's FileResponse class processes HTTP Range headers when serving files.

When a client requests partial content from a file-serving endpoint using the Range header, Starlette parses and merges the specified byte ranges. The vulnerable implementation uses an algorithm with quadratic time complexity (O(n²)), meaning processing time increases exponentially with the number of ranges specified. An attacker can craft a Range header with numerous overlapping or complex range specifications that cause the server to spend excessive CPU cycles on parsing and merging operations.

This attack requires no authentication and can be launched remotely over the network. The impact is limited to availability—there is no data confidentiality or integrity breach—but the ability to exhaust CPU resources makes this a significant denial-of-service vector for any Starlette application serving static files.

Root Cause

The root cause lies in the inefficient algorithmic implementation of the Range header parsing and merging logic in Starlette's responses.py module. The original implementation used regular expression-based parsing that exhibited quadratic time complexity when processing headers containing many range specifications. The patch removes the inefficient regex-based approach in favor of a more efficient parsing mechanism.

Attack Vector

The attack is network-based and requires no authentication, privileges, or user interaction. An attacker simply sends HTTP requests with specially crafted Range headers to any endpoint that serves files using Starlette's FileResponse or StaticFiles. By including a large number of range specifications in a single header, the attacker forces the server to perform excessive computation, consuming CPU resources and potentially rendering the service unavailable for legitimate users.

python

# Patch removes inefficient regex-based Range parsing (from responses.py)
# Before: Used regex for parsing
 import http.cookies
 import json
 import os
-import re
 import stat
 import sys
 import warnings

Source: GitHub Commit

Detection Methods for CVE-2025-62727

Indicators of Compromise

Unusually high CPU utilization on servers running Starlette applications
HTTP requests containing Range headers with excessive number of byte range specifications
Elevated response times for file-serving endpoints without corresponding increase in legitimate traffic
Server logs showing repeated requests to static file endpoints from single sources

Detection Strategies

Monitor for HTTP requests with abnormally long or complex Range headers targeting file-serving endpoints
Implement rate limiting on static file endpoints to identify potential abuse patterns
Configure web application firewalls (WAF) to detect and block Range headers exceeding normal complexity thresholds
Analyze server performance metrics for CPU spikes correlated with requests to static file endpoints

Monitoring Recommendations

Set up alerting for CPU utilization anomalies on Starlette application servers
Log and analyze Range header contents for requests to FileResponse and StaticFiles endpoints
Monitor request latency metrics for file-serving endpoints to detect degradation patterns
Implement connection tracking to identify sources sending excessive requests with complex Range headers

How to Mitigate CVE-2025-62727

Immediate Actions Required

Upgrade Starlette to version 0.49.1 or later immediately
Audit applications to identify all uses of FileResponse and StaticFiles components
Implement network-level rate limiting for static file endpoints as a temporary protection measure
Review web server and reverse proxy configurations to add Range header complexity limits

Patch Information

The vulnerability has been fixed in Starlette version 0.49.1. The patch addresses the inefficient algorithmic complexity in the Range header parsing logic by removing the regex-based approach that caused quadratic-time processing.

Relevant resources:

Workarounds

Deploy a reverse proxy (nginx, Apache, Caddy) in front of Starlette applications configured to reject or limit complex Range headers
Implement middleware to validate and sanitize Range headers before they reach FileResponse processing
Temporarily disable Range header support for static file endpoints if partial content delivery is not required
Use CDN or separate static file serving infrastructure that is not affected by this vulnerability

bash

# Example nginx configuration to limit Range header abuse
# Add to your server block configuration
location /static/ {
    # Limit the size of Range header
    if ($http_range ~* "bytes=.{500,}") {
        return 416;
    }
    proxy_pass http://starlette_backend;
}