lightrag-hku: SSRF via IPv6-transition address bypass (NAT64, IPv4-compatible, 6to4) of the native-markdown image-download guard

The guard already blocks the plain and IPv4-mapped internal forms (the forms the author tested), proving the intent is to reject internal destinations; it never canonicalizes the other transition wrappers, so they pass.

Note — CPython version dependency of the table. The is_global verdicts above depend on the interpreter. CPython gh-113171 (backported to 3.10.14 / 3.11.9 / 3.12.3+, ~2024-04) added 64:ff9b:1::/48 and 2002::/16 to ipaddress._private_networks, so on any current patch release those two rows return is_global = False and are already blocked by the stdlib before the guard is reached. The 64:ff9b::/96 well-known NAT64 prefix and the IPv4-compatible ::/96 form bypass on every current CPython — and 64:ff9b::/96 is exactly the prefix real DNS64/NAT64 deployments use, so the vulnerability stands. The four-row table reflects a pre-gh-113171 interpreter. The fix classifies by the embedded IPv4 and therefore closes all forms regardless of interpreter version.

Severity

High. CWE-918 (Server-Side Request Forgery). CVSS 3.1 vector AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:L/A:N, base score 7.1. AC:H reflects the NAT64/DNS64 network precondition for end-to-end reach; PR:L because document upload requires the API key; S:C because the request pivots into an internal network segment; C:H for internal/metadata/secret disclosure.

Proof of concept (deployed, both directions, verbatim)

Environment: three Docker containers on an IPv6-enabled network - an internal victim (10.66.0.2:80, RFC1918) serving a unique secret, a DNS64/NAT64 gateway owning 64:ff9b::/96 that forwards [64:ff9b::0a42:0002]:80 to 10.66.0.2:80, and an attacker host with a client-side route 64:ff9b::/96 via <gateway>. This models a client on an IPv6-only / NAT64+DNS64 network (AWS/GCP IPv6-only subnets, many mobile and corporate networks). The attacker runs the EXACT released guard code (_validated_addresses, _host_is_public, _build_guarded_opener, the _Guarded*Connection/Handler classes, sliced verbatim from v1.5.4 parser.py, only the logger import shimmed) plus a verbatim reproduction of the released _download() scheme and host precheck. Trigger in production: an uploaded document containing ![x](http://[64:ff9b::<internal-hex>]/a.png).

RUN 1, released v1.5.4 guard (VULNERABLE):

attack URL = http://[64:ff9b::0a42:0002]:80/logo.png
===== DIRECTION A: ATTACK (NAT64-wrapped internal address) =====
guard verdict : PASSED (fetched)
SECRET LEAKED : True
body          : SSRF-LIGHTRAG-INTERNAL-METADATA-9f3a2b7c
===== control: plain internal address (must be BLOCKED) =====
verdict: BLOCKED at guard: non-public host blocked
===== DIRECTION B: PUBLIC address (must still be FETCHED) =====
guard verdict : PASSED (fetched)
public body   : PUBLIC-OK-BODY
===== SUMMARY =====
attack_leaked_internal_secret = True
plain_internal_blocked        = True
public_still_fetched          = True

RUN 2, one-line-class decode fix applied (same inputs, same network):

===== DIRECTION A: ATTACK (NAT64-wrapped internal address) =====
guard verdict : BLOCKED at guard: non-public host blocked
SECRET LEAKED : False
===== control: plain internal address (must be BLOCKED) =====
verdict: BLOCKED at guard: non-public host blocked
===== DIRECTION B: PUBLIC address (must still be FETCHED) =====
guard verdict : PASSED (fetched)
public body   : PUBLIC-OK-BODY
===== SUMMARY =====
attack_leaked_internal_secret = False
plain_internal_blocked        = True
public_still_fetched          = True

Fix-correctness matrix (isolated): genuine global IPv6 2606:4700:4700::1111 and 2001:4860:4860::8888 -> ALLOW (no false positive); NAT64/RFC8215/IPv4-compat/6to4 wrappers of an internal IPv4 -> BLOCK; NAT64 of a public IPv4 (64:ff9b::808:808 = 8.8.8.8) -> ALLOW.

Impact

A caller who can upload a markdown or textpack document (textpack uploads route to the native engine with zero config; markdown reaches it when the native engine is selected) can make the LightRAG server issue HTTP requests to internal-only addresses that the SSRF guard is meant to forbid, and the fetched body is ingested. On a deployment with NAT64/DNS64 routing this reaches cloud instance metadata (169.254.169.254, 100.100.100.200), loopback services, and RFC1918 internal hosts, exposing IAM credentials and internal service data.

Suggested fix

Decode the embedded IPv4 of any IPv6 transition wrapper and classify by the embedded IPv4 before the is_global check:

def _unwrap_embedded_ipv4(ip):
    """Return the IPv4 embedded in an IPv6 transition wrapper (IPv4-mapped,
    IPv4-compatible, NAT64 64:ff9b::/96 and RFC8215 64:ff9b:1::/48, 6to4
    2002::/16) so it is classified by IPv4 rules; else return ip unchanged."""
    if ip.version != 6:
        return ip
    if ip.ipv4_mapped is not None:
        return ip.ipv4_mapped
    if getattr(ip, "sixtofour", None) is not None:
        return ip.sixtofour
    b = ip.packed
    if b[:12] == b"\x00" * 12 and b[12:] not in (b"\x00\x00\x00\x00", b"\x00\x00\x00\x01"):
        return ip_address(b[12:])
    if b[:12] == b"\x00\x64\xff\x9b" + b"\x00" * 8:
        return ip_address(b[12:])
    if b[:6] == b"\x00\x64\xff\x9b\x00\x01":
        return ip_address(b[12:])
    return ip

Then in _validated_addresses:

judged = _unwrap_embedded_ipv4(ip)
if not (judged.is_global or any(ip in net for net in allow)):
    return []

Verified: this blocks the NAT64/IPv4-compat/6to4 internal forms while genuine public IPs and public NAT64-wrapped IPs still pass.

Resolution

Addressed by #3426 (pending merge), targeted for release in lightrag-hku 1.5.5. _validated_addresses() now decides global-routability by starting from the stdlib's is_global on the literal and only ever tightening it — the guard is never more permissive than ipaddress itself:

• Fixed-position transition wrappers are decoded and the embedded IPv4 must also be global (IPv4-mapped ::ffff:0:0/96, IPv4-compatible ::/96, and the NAT64 well-known prefix 64:ff9b::/96 — RFC 6052 §2.2, contiguous low-32 embedding). So 64:ff9b::7f00:1 (→ loopback) is blocked while 64:ff9b::808:808 (→ public 8.8.8.8) still passes.
• Blocks whose embedded-IPv4 position must not be trusted are default-denied as whole blocks, independent of the interpreter: the RFC 8215 local-use prefix 64:ff9b:1::/48 (technology-agnostic, embedded-IPv4 position not guaranteed per RFC 8215 §5 — a fixed-offset decode is itself bypassable, e.g. 64:ff9b:1:7f00:0:100:808:808 encodes 127.0.0.1 but its low 32 bits read as public 8.8.8.8) and 6to4 2002::/16 (IANA global-reachability N/A; current CPython classifies the whole block non-global — RFC 7526 deprecated only the 6to4 anycast relay path, not the 2002::/16 prefix itself). Decoding 6to4 by its embedded IPv4 would have re-permitted 2002::/16 addresses the stdlib already blocks, so it is denied outright instead.

Genuine global IPv6 and a NAT64 wrapper of a public IPv4 still pass; regression tests cover every wrapped form of loopback / RFC1918 / metadata, the RFC 6052 /48 suffix bypass, and 6to4 of a public IPv4. Boundary: a NAT64 deployment using a custom, globally-routable Network-Specific Prefix (RFC 6052 permits any /32../96), or a genuine local NAT64 / legacy 6to4 in the force-denied blocks, is not auto-permitted — such operators should pin the download egress or set NATIVE_MD_IMAGE_ALLOWED_NON_PUBLIC_CIDRS deliberately.

Credit

tonghuaroot ([email protected]).