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

LightRAG's native markdown parser downloads external images referenced by an uploaded markdown or textpack document. The only SSRF guard, _validated_addresses() in lightrag/parser/markdown/parser.py, resolves the image host and rejects it when the resolved IP is not is_global. That check is evaluated on the raw resolved address and never decodes IPv6 transition wrappers that embed an internal IPv4. Python's ipaddress classifies a NAT64 (64:ff9b::/96 and the RFC 8215 64:ff9b:1::/48 prefix), IPv4-compatible (::a.b.c.d), or 6to4 (2002::/16) literal that wraps an internal IPv4 as globally routable, so the guard passes it. On a host with NAT64/DNS64 routing the request is then delivered to the embedded internal target (loopback, RFC1918, or a cloud metadata endpoint), and the fetched body is ingested. The plain and IPv4-mapped (::ffff:) forms of the same internal address are correctly blocked, so this is an encoding that defeats the existing filter.

• Package: lightrag-hku (LightRAG), the native markdown image-download path.
• Component: lightrag/parser/markdown/parser.py, guard _validated_addresses() (the if not (ip.is_global or ...) check), reached from _download() -> _build_guarded_opener().open(req).
• Enabled by default: download_enabled = _env_bool("NATIVE_MD_IMAGE_DOWNLOAD_ENABLED", True).
• Affected: LightRAG <= 1.5.4 (latest release at time of report, commit 9a45b64).
• Precondition: the caller can upload a document (API key via Depends(combined_auth)); the host has NAT64/DNS64 routing for the encoded address to reach the internal endpoint.

Evaluated on the exact resolved addresses (CPython ipaddress):

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.

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.

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):

 1attack URL = http://[64:ff9b::0a42:0002]:80/logo.png
 2===== DIRECTION A: ATTACK (NAT64-wrapped internal address) =====
 3guard verdict : PASSED (fetched)
 4SECRET LEAKED : True
 5body          : SSRF-LIGHTRAG-INTERNAL-METADATA-9f3a2b7c
 6===== control: plain internal address (must be BLOCKED) =====
 7verdict: BLOCKED at guard: non-public host blocked
 8===== DIRECTION B: PUBLIC address (must still be FETCHED) =====
 9guard verdict : PASSED (fetched)
10public body   : PUBLIC-OK-BODY
11===== SUMMARY =====
12attack_leaked_internal_secret = True
13plain_internal_blocked        = True
14public_still_fetched          = True

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

 1===== DIRECTION A: ATTACK (NAT64-wrapped internal address) =====
 2guard verdict : BLOCKED at guard: non-public host blocked
 3SECRET LEAKED : False
 4===== control: plain internal address (must be BLOCKED) =====
 5verdict: BLOCKED at guard: non-public host blocked
 6===== DIRECTION B: PUBLIC address (must still be FETCHED) =====
 7guard verdict : PASSED (fetched)
 8public body   : PUBLIC-OK-BODY
 9===== SUMMARY =====
10attack_leaked_internal_secret = False
11plain_internal_blocked        = True
12public_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.

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.

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

 1def _unwrap_embedded_ipv4(ip):
 2    """Return the IPv4 embedded in an IPv6 transition wrapper (IPv4-mapped,
 3    IPv4-compatible, NAT64 64:ff9b::/96 and RFC8215 64:ff9b:1::/48, 6to4
 4    2002::/16) so it is classified by IPv4 rules; else return ip unchanged."""
 5    if ip.version != 6:
 6        return ip
 7    if ip.ipv4_mapped is not None:
 8        return ip.ipv4_mapped
 9    if getattr(ip, "sixtofour", None) is not None:
10        return ip.sixtofour
11    b = ip.packed
12    if b[:12] == b"\x00" * 12 and b[12:] not in (b"\x00\x00\x00\x00", b"\x00\x00\x00\x01"):
13        return ip_address(b[12:])
14    if b[:12] == b"\x00\x64\xff\x9b" + b"\x00" * 8:
15        return ip_address(b[12:])
16    if b[:6] == b"\x00\x64\xff\x9b\x00\x01":
17        return ip_address(b[12:])
18    return ip

Then in _validated_addresses:

 1judged = _unwrap_embedded_ipv4(ip)
 2if not (judged.is_global or any(ip in net for net in allow)):
 3    return []

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

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.

tonghuaroot (tonghuaroot@gmail.com).