high CVSS 7.5

CVE-2025-53643·Python vulnerability

AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to version 3.12.14, the Python parser is vulnerable to a request smuggling vulnerability due to not parsing trailer sections of an HTTP request. If a pure Python version of aiohttp is installed (i.e. without the usual C extensions) or AIOHTTP_NO_EXTENSIONS is enabled, then an attacker may be able to execute a request smuggling attack to bypass certain firewalls or proxy protections. Version 3.12.14 contains a patch for this issue.

Severity
high
Software
Python
Fixed in
3.12.14
Published
2025-07-14

Affected versions

Until: 3.12.14

Fixed in: 3.12.14

How to fix this CVE

Upgrade aiohttp to version 3.12.14 or later to patch a request smuggling vulnerability in the Python parser. This vulnerability only affects systems running the pure Python implementation of aiohttp (without C extensions) or those with AIOHTTP_NO_EXTENSIONS enabled. Ensure all Python environments and virtual machines using aiohttp are updated immediately to prevent potential bypass of firewall and proxy security controls.

sudo dnf update python3-aiohttp

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST|GET|PUT|DELETE|PATCH .* HTTP/1\.1.*Transfer-Encoding: chunked.*Trailer: .*

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 or sanitize HTTP requests containing Trailer headers in the request line, as legitimate requests rarely use this feature. Specifically, block requests with malformed or suspicious trailer sections that do not conform to RFC 7230 specifications.

How to check if you are affected

  1. Run `python3 -c "import aiohttp; print(aiohttp.__version__)"` to check the installed aiohttp version and verify if it is below 3.12.14
  2. Check if AIOHTTP_NO_EXTENSIONS environment variable is set by running `echo $AIOHTTP_NO_EXTENSIONS` — if set to 1, the pure Python parser is active and vulnerable
  3. Review application logs and proxy/firewall logs for unusual HTTP requests containing trailer headers or chunked transfer encoding patterns that appear malformed
  4. After upgrading, run `python3 -c "import aiohttp; print(aiohttp.__version__)"` again to confirm version is 3.12.14 or higher

FAQ

What is CVE-2025-53643?

CVE-2025-53643 is a request smuggling vulnerability in aiohttp's pure Python HTTP parser that fails to properly parse HTTP trailer headers, allowing attackers to craft malicious requests that bypass security controls on firewalls and proxies.

Is CVE-2025-53643 being actively exploited?

No, this vulnerability is not currently listed on the CISA Known Exploited Vulnerabilities catalog and no public exploits are available, though the attack is theoretically feasible.

What versions of aiohttp are affected by CVE-2025-53643?

All versions of aiohttp prior to 3.12.14 are vulnerable when using the pure Python parser implementation or when AIOHTTP_NO_EXTENSIONS is explicitly enabled.

How do I check if my server is vulnerable to CVE-2025-53643?

Run `python3 -c "import aiohttp; print(aiohttp.__version__)"` to see the installed version. If it is below 3.12.14, verify whether the C extensions are available by checking if the version string contains 'speedups' or whether AIOHTTP_NO_EXTENSIONS is set.

Does Defensia detect CVE-2025-53643?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If aiohttp is installed on a monitored server, CVE-2025-53643 will appear in your dashboard with remediation steps.

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References

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