CVE-2026-41136: free5GC AMF Missing Default Case Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-r99v-75p9-xqm5",
  "modified" : "2026-04-27T16:34:26Z",
  "published" : "2026-04-22T19:54:54Z",
  "aliases" : [ "CVE-2026-41136" ],
  "summary" : "free5GC AMF: Missing default case in Content-Type switch in HTTPUEContextTransfer",
  "details" : "## Summary\n\nThe `HTTPUEContextTransfer` handler in `internal/sbi/api_communication.go` does not include a `default` case in the `Content-Type` switch statement. When a request arrives with an unsupported `Content-Type`, the deserialization step is silently skipped, `err` remains `nil`, and the processor is invoked with a completely uninitialized `UeContextTransferRequest` object.\n\n## Details\n\nIn `internal/sbi/api_communication.go`, the `HTTPUEContextTransfer` function handles the `Content-Type` header with a switch statement that only covers `application/json` and `multipart/related`:\n\n```go\nswitch str[0] {\ncase applicationjson:\n    err = openapi.Deserialize(ueContextTransferRequest.JsonData, requestBody, contentType)\ncase multipartrelate:\n    err = openapi.Deserialize(&ueContextTransferRequest, requestBody, contentType)\n// no default case\n}\n\nif err != nil {\n    // skipped entirely when Content-Type is unsupported\n    c.JSON(http.StatusBadRequest, rsp)\n    return\n}\n\ns.Processor().HandleUEContextTransferRequest(c, ueContextTransferRequest)\n```\n\nThis is inconsistent with the two analogous handlers in the same file, `HTTPCreateUEContext` and `HTTPN1N2MessageTransfer`, which both correctly include a `default` branch:\n\n```go\ndefault:\n    err = fmt.Errorf(\"wrong content type\")\n```\n\nThe fix is simply to add the same `default` case to `HTTPUEContextTransfer`.\n\n## PoC\n\nWith a free5GC deployment running, send a POST request to the UE context transfer endpoint using any unsupported `Content-Type` (e.g. `text/plain`):\n\n```bash\ncurl -s -X POST \"http://<AMF_IP>/namf-comm/v1/ue-contexts/<ueContextId>/transfer\" \\\\\n  -H \"Content-Type: text/plain\" \\\\\n  -d '{\"test\":\"data\"}' \\\\\n  -i\n```\n\n**Expected (correct) behavior:** `400 Bad Request` from the SBI layer, rejecting the request due to unsupported Content-Type — consistent with `HTTPCreateUEContext`.\n\n**Actual (observed) behavior:** The SBI-layer error check is bypassed and the processor is reached with an empty request object, returning:\n\n```\nHTTP/1.1 400 Bad Request\n{\"status\": 400, \"cause\": \"MANDATORY_IE_MISSING\"}\n```\n\nThe `MANDATORY_IE_MISSING` cause originates from the processor's internal validation, not from the SBI handler — confirming the processor was called with an uninitialized struct.\n\n## Impact\n\nThe endpoint is an inter-NF SBI API used during AMF-to-AMF UE context handover. It is not directly reachable from external UEs and requires access to the internal 5GC SBI network. The processor's secondary mandatory field validation prevents any unintended state modification, so there is no direct exploitability. However, the SBI handler layer is the intended first line of defense — relying on the processor to compensate for a missing input check increases fragility and violates defense in depth. Any future change to the processor's validation logic could inadvertently expose the system to processing completely empty request objects.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/free5gc/amf"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "1.4.3"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/free5gc/free5gc/security/advisories/GHSA-r99v-75p9-xqm5"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-41136"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/free5gc/amf/releases/tag/v1.4.3"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/free5gc/free5gc"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-440" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-22T19:54:54Z",
    "nvd_published_at" : "2026-04-22T00:16:29Z"
  }
}