CVE-2026-34520: aio-libs aiohttp header injection in llhttp C parser

CVE-2026-34520 · Severity: critical · CVSS 9.1 · Published 2026-04-01

AIOHTTP is a popular Python library used for building web servers and clients. A vulnerability in its default C-based message parser allows attackers to include hidden characters in web requests that can trick the system into misidentifying the source or destination of a request. This could allow an attacker to bypass security filters or gain unauthorized access to internal systems by confusing the way a reverse proxy and the application interpret the same request.

Technologies: Aiohttp, aiohttp (PyPI). Vendors: Aiohttp, PyPI.

Quick answers

Is CVE-2026-34520 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-34520 being exploited in the wild.
How severe is CVE-2026-34520?
Critical severity, with a CVSS score of 9.1, published 1 April 2026.
Which products and versions are affected?
aiohttp (PyPI) <= 3.13.3, ECOSYSTEM:introduced=0,fixed=3.13.4.
Is there a fix for CVE-2026-34520?
Yes. aiohttp (PyPI): upgrade to 3.13.4 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-34520 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-34520?
Check whether you run Aiohttp or aiohttp (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • aiohttp (PyPI): upgrade to 3.13.4 or later
    pip install "aiohttp>=3.13.4"

Technical details

A vulnerability exists in AIOHTTP's default C parser (llhttp) where it fails to properly neutralize null bytes and control characters within HTTP request headers. This is classified as an Improper Neutralization of CRLF Sequences (CWE-113). An unauthenticated remote attacker can exploit this by sending specially crafted headers that are interpreted inconsistently between AIOHTTP and other infrastructure components like reverse proxies. For example, the 'request.url.origin()' function might return a value that differs from what a proxy perceived, enabling request smuggling or security policy bypasses. The issue is addressed in version 3.13.4.

Affected products

  • aio-libs aiohttp <= 3.13.3

Identifiers

  • CVE-2026-34520
  • GHSA-63hf-3vf5-4wqf
  • PYSEC-2026-2102

Timeline

  • 2026-04-01: disclosed
  • 2026-04-01: advisory
  • 2026-04-01: patched: Fixed in version 3.13.4

References

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