high CVSS 7.5

CVE-2026-34516·Python vulnerability

AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to version 3.13.4, a response with an excessive number of multipart headers may be allowed to use more memory than intended, potentially allowing a DoS vulnerability. This issue has been patched in version 3.13.4.

Severity
high
Software
Python
Fixed in
3.13.4
Published
2026-04-01

Affected versions

Until: 3.13.4

Fixed in: 3.13.4

How to fix this CVE

Update Python's aiohttp library to version 3.13.4 or later to mitigate a denial-of-service vulnerability triggered by malformed multipart HTTP responses. The vulnerability allows attackers to exhaust server memory by crafting responses with an abnormally high number of multipart headers. Ensure all systems running aiohttp-dependent applications apply this patch immediately.

sudo dnf update python3-aiohttp

Defensia detects this vulnerability

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement request size limits and multipart header count restrictions at the load balancer or reverse proxy level (e.g., LimitRequestFields, LimitRequestFieldSize in Apache; client_max_body_size, large_client_header_buffers in Nginx) to prevent oversized multipart payloads from reaching the vulnerable aiohttp application.

How to check if you are affected

  1. Check installed aiohttp version: python3 -c 'import aiohttp; print(aiohttp.__version__)'
  2. Verify if aiohttp is used in running applications: pip3 list | grep aiohttp or dpkg -l | grep aiohttp
  3. Search application logs for memory exhaustion errors or OOM killer events: grep -i 'out of memory\|oom\|memory exhausted' /var/log/syslog /var/log/kern.log
  4. After patching, re-run the version check to confirm aiohttp is at 3.13.4 or later

FAQ

What is CVE-2026-34516?

CVE-2026-34516 is a denial-of-service vulnerability in aiohttp's multipart response parsing that allows remote attackers to consume excessive memory by sending HTTP responses with an unusually large number of multipart headers, potentially crashing the application.

Is CVE-2026-34516 being actively exploited?

No, this vulnerability is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits are available.

What versions of Python/aiohttp are affected by CVE-2026-34516?

All versions of aiohttp prior to 3.13.4 are vulnerable; the fix is included in version 3.13.4 and later.

How do I check if my server is vulnerable to CVE-2026-34516?

Run 'python3 -c "import aiohttp; print(aiohttp.__version__)"' and verify the version is 3.13.4 or higher. If it is lower, your system is vulnerable.

Does Defensia detect CVE-2026-34516?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If aiohttp is installed on a monitored server, CVE-2026-34516 will appear in your dashboard with remediation steps.

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References

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