Multiple Host headers were allowed in aiohttp.
Mostly this doesn't affect aiohttp security itself, but if a reverse proxy is applying security rules depending on the target Host, it is theoretically possible that the proxy and aiohttp could process different host names, possibly resulting in bypassing a security check on the proxy and getting a request processed by aiohttp in a privileged sub app when using Application.add_domain().
Patch: https://github.com/aio-libs/aiohttp/commit/e00ca3cca92c465c7913c4beb763a72da9ed8349 Patch: https://github.com/aio-libs/aiohttp/commit/53e2e6fc58b89c6185be7820bd2c9f40216b3000
{
"github_reviewed": true,
"github_reviewed_at": "2026-04-01T21:49:45Z",
"severity": "MODERATE",
"cwe_ids": [
"CWE-20",
"CWE-444"
],
"nvd_published_at": "2026-04-01T21:17:00Z"
}