eef-cve-2026-43971

{
  "affected": [
    {
      "database_specific": {
        "source": "https://cna.erlef.org/osv/EEF-CVE-2026-43971.json"
      },
      "package": {
        "ecosystem": "Hex",
        "name": "cowlib",
        "purl": "pkg:hex/cowlib"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.9.0"
            },
            {
              "fixed": "2.20.0"
            }
          ],
          "type": "SEMVER"
        }
      ],
      "versions": [
        "2.10.0",
        "2.10.1",
        "2.11.0",
        "2.12.0",
        "2.12.1",
        "2.13.0",
        "2.14.0",
        "2.15.0",
        "2.16.0",
        "2.16.1",
        "2.17.0",
        "2.17.1",
        "2.18.0",
        "2.19.0",
        "2.9.0",
        "2.9.1"
      ]
    },
    {
      "database_specific": {
        "source": "https://cna.erlef.org/osv/EEF-CVE-2026-43971.json"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "485d58dfa91b91d98135dc95e5615f421715dae5"
            },
            {
              "fixed": "89da27ee4c241f5d649ba7d9b7f2188918af6cea"
            }
          ],
          "repo": "https://github.com/ninenines/cowlib",
          "type": "GIT"
        }
      ],
      "versions": [
        "2.19.0",
        "2.18.0",
        "2.17.1",
        "2.17.0",
        "2.16.1",
        "2.16.0",
        "2.15.0",
        "2.14.0",
        "2.13.0",
        "2.12.1",
        "2.12.0",
        "2.11.0",
        "2.10.1",
        "2.10.0",
        "2.9.1",
        "2.9.0"
      ]
    }
  ],
  "aliases": [
    "CVE-2026-43971"
  ],
  "credits": [
    {
      "name": "Peter Ullrich",
      "type": "FINDER"
    },
    {
      "name": "Jonatan Männchen / EEF",
      "type": "REMEDIATION_DEVELOPER"
    },
    {
      "name": "Loïc Hoguin",
      "type": "REMEDIATION_REVIEWER"
    }
  ],
  "database_specific": {
    "capec_ids": [
      "CAPEC-137"
    ],
    "cpe_ids": [
      "cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*"
    ],
    "cwe_ids": [
      "CWE-116"
    ]
  },
  "details": "## Summary\n\nImproper Encoding or Escaping of Output vulnerability in ninenines cowlib allows Link header directive smuggling via unescaped special characters in `cow_link:link/1`.\n\n`cow_link:do_link/1` in cowlib interpolates the target URI, rel value, and attribute keys directly into the serialized `Link:` header value without escaping or token-grammar validation. A `>` byte in target prematurely closes the URI slot, allowing an attacker to append additional link entries with attacker-chosen rel directives. A `\"` or `\\` in rel escapes the quoted string and opens new parameters. Any byte — including whitespace, `=`, and `\"` — in an attribute key is emitted verbatim. Because browsers act on `Link:` directives such as `rel=\"preconnect\"`, `rel=\"preload\"`, and `rel=\"prerender\"`, an attacker who can influence these fields in an application that round-trips parsed Link headers through `cow_link:link/1` can force victim browsers to make out-of-band connections to attacker-controlled origins.\n\nThis issue affects cowlib: from 2.9.0 before 2.20.0.\n\n## Workarounds\n\nSanitize values before passing them to `cow_link:link/1`: reject or strip any target value containing `>`, any rel value containing `\"` or `\\`, and any attribute key containing characters outside the HTTP token grammar. Alternatively, ensure all Link entry fields are derived exclusively from trusted, application-controlled values rather than untrusted input.\n\n## Configurations\n\nThe application must pass values that originate from, or can be influenced by, untrusted input into `cow_link:link/1` — specifically the target URI, rel value, or attribute keys. Applications that construct Link entries exclusively from trusted, application-controlled values are not affected.",
  "id": "EEF-CVE-2026-43971",
  "modified": "2026-09-16T20:30:35.688617126Z",
  "published": "2026-08-18T09:01:53.199Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://cna.erlef.org/cves/CVE-2026-43971.html"
    },
    {
      "type": "WEB",
      "url": "https://github.com/ninenines/cowlib/commit/485d58dfa91b91d98135dc95e5615f421715dae5"
    },
    {
      "type": "FIX",
      "url": "https://github.com/ninenines/cowlib/commit/89da27ee4c241f5d649ba7d9b7f2188918af6cea"
    },
    {
      "type": "PACKAGE",
      "url": "https://hex.pm/packages/cowlib"
    }
  ],
  "schema_version": "1.9.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Link Header Directive Smuggling via Unescaped target/rel/Attribute Keys in cow_link:link/1"
}