# AIOHTTP accepts duplicate Host headers

Advisory record · `GHSA-c427-h43c-vf67`

- **Identifiers:** `GHSA-c427-h43c-vf67` · `CVE-2026-34525` · `PYSEC-2026-2103`
- **State:** Confirmed
- **Severity:** CVSS 6.3 Medium
- **Critical:** No
- **Weakness (CWE):** [CWE-20](https://cwe.mitre.org/data/definitions/20.html) · [CWE-444](https://cwe.mitre.org/data/definitions/444.html)
- **Tags:** [HTTP request smuggling](https://agentprime.io/advisories/tag/request-smuggling.md) · [CWE-20](https://agentprime.io/advisories/tag/cwe-20.md)
- **Published:** 1 Apr 2026
- **Updated:** 10 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 1 Apr 2026, 21:49 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `aiohttp` (PyPI): affected `<3.13.4`; fixed in `3.13.4`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> AIOHTTP (aiohttp) versions before 3.13.4 accept requests with multiple Host headers. The source says this mainly matters when a reverse proxy applies security rules based on the Host header, since the proxy and aiohttp could then interpret different host names and a request might reach a privileged sub app set up with Application.add_domain(). Upgrade to aiohttp 3.13.4 or later.

Written 1 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 6.3 Medium, CVSS 4.0: `CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:L/SI:L/SA:N`, from CNA (GitHub_M)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0030, the 22nd percentile, on 6 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:11 UTC: First seen by agentprime
- 31 Oct 2026, 14:11 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-c427-h43c-vf67)

## Sources

- [GitHub Advisory Database: GHSA-c427-h43c-vf67](https://github.com/advisories/GHSA-c427-h43c-vf67)
- [OSV: GHSA-c427-h43c-vf67](https://osv.dev/vulnerability/GHSA-c427-h43c-vf67)
- <https://github.com/aio-libs/aiohttp/security/advisories/GHSA-c427-h43c-vf67>
- <https://nvd.nist.gov/vuln/detail/CVE-2026-34525>
- <https://github.com/aio-libs/aiohttp/commit/53e2e6fc58b89c6185be7820bd2c9f40216b3000>
- <https://github.com/aio-libs/aiohttp/commit/e00ca3cca92c465c7913c4beb763a72da9ed8349>
- <https://github.com/aio-libs/aiohttp>
- <https://github.com/aio-libs/aiohttp/releases/tag/v3.13.4>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-c427-h43c-vf67>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-c427-h43c-vf67/>. Everything for agents: <https://agentprime.io/llms.txt>.