CVE-2025-69228: Aiohttp Asynchronous HTTP DoS Vulnerability

CVE-2025-69228 Overview

CVE-2025-69228 is a resource exhaustion vulnerability affecting AIOHTTP, the popular asynchronous HTTP client/server framework for asyncio and Python. Versions 3.13.2 and below contain a flaw in multipart form processing that allows attackers to craft malicious requests causing uncontrolled memory consumption on the server. When an application handler utilizes the Request.post() method, an attacker can exploit this weakness to freeze the server by exhausting available memory, resulting in a denial of service condition.

Critical Impact

Attackers can completely freeze AIOHTTP-based web servers by sending specially crafted multipart form requests that bypass client_max_size restrictions, leading to memory exhaustion and service unavailability.

Affected Products

• AIOHTTP versions 3.13.2 and earlier
• Python applications using Request.post() method for multipart form handling
• Web services built on the asyncio AIOHTTP framework

Discovery Timeline

• 2026-01-06 - CVE CVE-2025-69228 published to NVD
• 2026-01-14 - Last updated in NVD database

Technical Details for CVE-2025-69228

Vulnerability Analysis

This vulnerability is classified under CWE-770 (Allocation of Resources Without Limits or Throttling). The flaw exists in how AIOHTTP handles multipart form data when processing HTTP POST requests. The core issue is that the client_max_size enforcement was implemented per-field rather than across the entire multipart form submission.

When processing multipart forms, AIOHTTP would reset the size counter for each field boundary encountered. This meant that while each individual field might respect the configured size limit, an attacker could include numerous fields in a single request—each just under the limit—to bypass the overall request size restriction. The cumulative data from all fields would then be loaded into memory, potentially exhausting server resources.

Applications are vulnerable if they use the Request.post() method in their handlers, which triggers the multipart processing code path. The network-accessible attack vector requires no authentication or user interaction, making it trivial for remote attackers to exploit.

Root Cause

The root cause stems from improper placement of the size tracking variable initialization. In vulnerable versions, the size counter was declared inside the field processing loop, causing it to reset to zero for each new multipart field. This architectural mistake allowed cumulative memory allocation to exceed the intended client_max_size limit when processing forms with multiple fields.

Attack Vector

An attacker can exploit this vulnerability by sending a crafted multipart HTTP POST request to any endpoint that processes form data using Request.post(). The attack requires no special privileges and can be executed remotely over the network. By including multiple form fields in a single request, each sized just below the configured limit, the attacker can cause the server to allocate memory far exceeding what the client_max_size setting intended to prevent. Repeated requests can exhaust server memory, causing application freezes or crashes.

             multipart = await self.multipart()
             max_size = self._client_max_size
 
+            size = 0
             field = await multipart.next()
             while field is not None:
-                size = 0
                 field_ct = field.headers.get(hdrs.CONTENT_TYPE)
 
                 if isinstance(field, BodyPartReader):

Source: GitHub Commit b7dbd35

The patch moves the size = 0 initialization outside the while loop, ensuring that the size counter tracks cumulative data across all multipart fields rather than resetting for each field.

Detection Methods for CVE-2025-69228

Indicators of Compromise

• Unusual memory consumption spikes on AIOHTTP application servers
• Multiple large multipart POST requests from single or distributed IP addresses
• Server process memory growth without corresponding legitimate traffic increases
• Application freezes or OOM (Out of Memory) killer activity in system logs

Detection Strategies

• Monitor memory utilization metrics for AIOHTTP worker processes with alerting on abnormal growth patterns
• Implement request logging to identify multipart POST requests with unusually high field counts
• Deploy web application firewall rules to detect and limit multipart forms with excessive fields
• Use application performance monitoring (APM) to track memory allocation during request processing

Monitoring Recommendations

• Configure memory usage thresholds and alerts for Python/AIOHTTP processes
• Enable detailed access logging for POST requests to endpoints using Request.post()
• Review server metrics for correlation between incoming multipart requests and memory spikes
• Implement distributed tracing to identify requests triggering high memory allocation

How to Mitigate CVE-2025-69228

Immediate Actions Required

• Upgrade AIOHTTP to version 3.13.3 or later immediately
• Audit application code to identify handlers using Request.post() method
• Review client_max_size configurations and ensure reasonable limits are set
• Consider implementing rate limiting on POST endpoints as a temporary defense layer

Patch Information

The vulnerability is fixed in AIOHTTP version 3.13.3. The fix ensures that the client_max_size limit is enforced across the entire multipart form rather than per-field. The patch modifies aiohttp/web_request.py to move the size counter initialization outside the field processing loop.

For detailed patch information, see the GitHub Security Advisory GHSA-6jhg-hg63-jvvf and the commit b7dbd35375aedbcd712cbae8ad513d56d11cce60.

Workarounds

• If immediate upgrade is not possible, implement a reverse proxy (nginx, HAProxy) with request body size limits
• Add custom middleware to count and limit multipart form fields before reaching Request.post()
• Configure operating system-level memory limits (cgroups, ulimits) for AIOHTTP processes to prevent system-wide impact
• Temporarily disable or protect endpoints that use Request.post() if they are not critical

# Example: Configure nginx reverse proxy with request size limits
# Add to nginx server block configuration
client_max_body_size 10m;
client_body_buffer_size 128k;