critical CVSS 9.1

CVE-2026-34520·Python vulnerability

AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to version 3.13.4, the C parser (the default for most installs) accepted null bytes and control characters in response headers. This issue has been patched in version 3.13.4.

Severity
critical
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 remediate this critical header parsing vulnerability. The patched version enforces stricter validation of HTTP response headers, rejecting null bytes and control characters that could be weaponized in header injection attacks. Organizations running aiohttp-dependent applications should prioritize this update to prevent potential cache poisoning, request smuggling, or downstream injection attacks.

sudo dnf update python3-aiohttp

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

HTTP response headers containing null bytes (\x00) or ASCII control characters (0x00-0x1F) in field names or values; regex pattern: ^[\x00-\x1F]+ in response header fields

WAF mitigation (if patching is not yet possible)

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

Configure WAF rules to reject any upstream HTTP responses containing null bytes or control characters in headers. Implement response header validation that strips or blocks payloads matching [\x00-\x1F] in header values before they reach the aiohttp parser.

How to check if you are affected

  1. Run 'python3 -c "import aiohttp; print(aiohttp.__version__)"' to display the installed aiohttp version and compare against 3.13.4
  2. Identify applications using aiohttp by searching the application codebase for 'import aiohttp' or checking pip freeze output with 'pip list | grep aiohttp'
  3. Monitor web server and application logs for malformed HTTP responses containing null bytes (\x00) or control characters (ASCII 0x00-0x1F) in header fields
  4. Verify the patch was applied by re-running the version check command and confirming aiohttp version is 3.13.4 or greater

FAQ

What is CVE-2026-34520?

CVE-2026-34520 is a critical vulnerability in aiohttp's C-based HTTP response parser that improperly permits null bytes and control characters in HTTP headers. Attackers can exploit this to craft malicious responses that bypass header validation, enabling header injection or cache poisoning attacks.

Is CVE-2026-34520 being actively exploited?

No, this vulnerability is not currently listed on the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits have been disclosed, though the critical CVSS score (9.1) warrants immediate attention.

What versions of Python are affected by CVE-2026-34520?

All versions of aiohttp prior to 3.13.4 are affected. The vulnerability resides in the aiohttp library itself, not Python core, though it impacts any Python application using aiohttp for HTTP client or server operations.

How do I check if my server is vulnerable to CVE-2026-34520?

Execute 'pip show aiohttp' or 'python3 -c "import aiohttp; print(aiohttp.__version__)"' and verify the version number is 3.13.4 or newer; if lower, your environment is vulnerable.

Does Defensia detect CVE-2026-34520?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Python with aiohttp is installed on a monitored server, CVE-2026-34520 will appear in your dashboard with remediation steps.

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References

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