AIOHTTP leaks Cookie and Proxy-Authorization headers on cross-origin redirect

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

2.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:U · score source

EPSS

• CVE-2026-34518: 0.35% estimated exploitation activity in the next 30 days; percentile 28.70 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary When following redirects to a different origin, aiohttp drops the Authorization header, but retains the Cookie and Proxy-Authorization headers. Impact The Cookie and Proxy-Authorizations headers could contain sensitive information which may be leaked to an unintended party after following a redirect. ----- Patch: https://github.com/aio-libs/aiohttp/commit/5351c980dcec7ad385730efdf4e1f4338b24fdb6

Published by the source

1 Apr 2026, 21:47 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 13:40 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-966j-vmvw-g2g9

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-966j-vmvw-g2g9

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-34518

Advisory: AIOHTTP leaks Cookie and Proxy-Authorization headers on cross-origin redirect
IDs: CVE-2026-34518, GHSA-966j-vmvw-g2g9
Record: https://patchnext.com/advisories/cve-2026-34518 (revision 3, source checked 26 Oct 2026, 13:40 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 2.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: aiohttp 3.13.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI aiohttp: affected <= 3.13.3; fixed in 3.13.4

## Suggested inspection (read-only)
- `python -m pip show aiohttp`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on aiohttp.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-966j-vmvw-g2g9 (OSV record GHSA-966j-vmvw-g2g9 lists affected ranges: PyPI aiohttp >=0 <3.13.4 (fixed))
- Verified: https://github.com/advisories/GHSA-966j-vmvw-g2g9 (GitHub-reviewed advisory GHSA-966j-vmvw-g2g9 lists affected ranges: PyPI aiohttp >=0 <=3.13.3)

Commands are suggestions: review each one before running it.