Description

LXD: SSRF via image import from URL allows internal network probing by authenticated users ## Summary The LXD image import-from-URL endpoint (POST /1.0/images with source.type=url) makes server-side HTTP requests to an attacker-supplied URL without any private-IP or loopback blocking. An authenticated user with the can_create_images entitlement can force the LXD daemon to connect to internal network addresses (127.0.0.1, RFC1918 ranges, cloud metadata endpoints), enabling port scanning via error-based inference and interaction with internal HTTP services from the daemon's privileged network position. ## Severity - Attack Vector: Network — LXD API is network-accessible. - Attack Complexity: Low — Single crafted API request with no special conditions. - Privileges Required: Low — Requires can_create_images entitlement, a non-admin permission commonly delegated to regular users. - User Interaction: None. - Scope: Changed — The attacker leverages LXD's privileged network position to probe and interact with internal services that are outside the attacker's direct network access and outside the scope of LXD's image management authorization. - Confidentiality Impact: Low — Error messages reveal internal port states (open/closed/filtered) and service availability. Response bodies from internal services are NOT returned to the attacker (hash validation blocks content exfiltration). - Integrity Impact: None — No demonstrated ability to modify data on internal services. - Availability Impact: None — No meaningful resource exhaustion or denial of service. ## Affected Component - lxd/images.go — imgPostURLInfo (lines 668–755) — Makes HEAD request to user-supplied URL without any SSRF protections. - lxd/util/http.go — HTTPClient (lines 102–147) — Creates HTTP client with no IP blocking or URL validation. - shared/network.go — RFC3493Dialer (lines 20–49) — Dials arbitrary hosts including private/loopback ranges. - lxd/daemon_images.go — ImageDownload (lines 478–510, "direct" case) — Makes GET request to URL from attacker-controlled response header. ## Description ### Core vulnerability: no SSRF protections on server-side image fetching When a user imports an image from a URL, the imgPostURLInfo function makes an HTTP HEAD request directly to the user-supplied URL with zero validation: go // lxd/images.go:668-703 func imgPostURLInfo(ctx context.Context, s *state.State, req api.ImagesPost, ...) (*api.Image, error) { if req.Source.URL == "" { return nil, errors.New("Missing URL") } myhttp, err := util.HTTPClient("", s.Proxy) // ... // Resolve the image URL head, err := http.NewRequest("HEAD", req.Source.URL, nil) // No URL validation, no scheme check, no IP range check // ... raw, err := myhttp.Do(head) // Makes request to arbitrary destination The HTTP client is created via util.HTTPClient which uses shared.RFC3493Dialer — a standard dialer with zero private-IP filtering: go // shared/network.go:20-49 func RFC3493Dialer(context context.Context, network string, address string) (net.Conn, error) { host, port, err := net.SplitHostPort(address) // ... addrs, err := net.LookupHost(host) // ... for _, a := range addrs { c, err := net.DialTimeout(network, net.JoinHostPort(a, port), 10*time.Second) // No check: IsLoopback? IsPrivate? IsLinkLocalUnicast? // Connects to ANY resolved address including 127.0.0.1, 10.x.x.x, 169.254.169.254 The HTTP client also follows redirects unconditionally: go // lxd/util/http.go:139-144 myhttp.CheckRedirect = func(req *http.Request, via []*http.Request) error { // Replicate the headers req.Header = via[len(via)-1].Header return nil // Always follows redirects — no destination validation } ### No SSRF protections exist anywhere in the codebase An exhaustive search for private-IP checks (IsLoopback, IsPrivate, IsLinkLocalUnicast), URL allowlists, or SSRF-specific protections found zero results in any HTTP client or outbound request path. The codebase has no SSRF defense infrastructure. ### Chained SSRF via LXD-Image-URL response header After the HEAD request, the LXD-Image-URL response header is used as the target for a second HTTP GET request, creating a chained SSRF: go // lxd/images.go:705-725 hash := raw.Header.Get("LXD-Image-Hash") // Attacker controls this url := raw.Header.Get("LXD-Image-URL") // Attacker controls this info, err := ImageDownload(ctx, s, op, &ImageDownloadArgs{ Server: url, // Attacker-controlled URL from response header Protocol: "direct", // Forces unprotected direct HTTP GET Alias: hash, // Attacker-controlled hash // ... }) In ImageDownload, the "direct" case makes a GET to the attacker-controlled URL: go // lxd/daemon_images.go:478-510 case "direct": httpClient, err := util.HTTPClient(args.Certificate, s.Proxy) // ... req, err := http.NewRequest(http.MethodGet, args.Server, nil) // args.Server = attacker URL // ... raw, doneCh, err := cancel.CancelableDownload(canceler, httpClient.Do, req) This means an attacker who controls the HEAD target can redirect the second GET request to any internal service. The response body is downloaded to disk but hash validation prevents the attacker from reading it: go // lxd/daemon_images.go:541-544 result := hex.EncodeToString(sha256.Sum(nil)) if result != fp { return nil, fmt.Errorf("Hash mismatch for %q: %s != %s", args.Server, result, fp) } ### Error observability enables port scanning Errors from the outbound request propagate back to the caller via the operation result: go // lxd/operations/operations.go:612-614 if op.err != nil { retOp.Err = response.SmartError(op.err).String() } The error messages differ based on the target's state: - Port open: "Missing LXD-Image-Hash header" (HEAD succeeded, response had no expected headers) - Port closed: "connection refused" (connection rejected immediately) - Port filtered/host unreachable: "context deadline exceeded" / "no route to host" (timeout or routing error) These differences allow systematic port scanning of internal networks from the LXD daemon's network position. ### Execution chain 1. Attacker sends POST /1.0/images with {"source":{"type":"url","url":"http://127.0.0.1:6379/"}} 2. imagesPost handler verifies can_create_images entitlement (line 1163) 3. Background operation calls imgPostURLInfo (line 1361) 4. imgPostURLInfo creates HTTP client via util.HTTPClient — no SSRF protections (line 675) 5. HEAD request to http://127.0.0.1:6379/ — no IP validation (line 700) 6. RFC3493Dialer resolves 127.0.0.1 and dials it — no private-IP check (line 33) 7. Error or response propagated back via operation (line 612-614 in operations.go) 8. Attacker queries operation result via GET /1.0/operations/{id}/wait and reads the error message For chained SSRF: 1. Attacker points source.url to their own server 2. Their server responds with LXD-Image-Hash: <any> and LXD-Image-URL: http://169.254.169.254/latest/meta-data/ 3. LXD makes GET to http://169.254.169.254/... (line 493 in daemon_images.go) 4. Response body downloaded but hash validation prevents content exfiltration 5. Error messages still reveal whether the internal service responded (200 vs error) ## Proof of Concept ```bash # Error message will confirm server-side request occurred. SOCK=/var/lib/lxd/unix.socket # Create the operation OP=$(curl -s --unix-socket "$SOCK" \ -H 'Content-Type: application/json' \ -X POST http://localhost/1.0/images \ -d '{"source":{"type":"url","url":"http://127.0.0.1:1/"}}' \ | sed -n 's/."operation":"([^"])".*/\1/p') #

Mitigation

Aliases

References