fetch: SSRF guard misses RFC 6052 Well-Known NAT64 prefix (64:ff9b::/96) and ::/96
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评估
调研方向
从src/fetch/src/mcp_server_fetch/server.py开始，阅读_NON_PUBLIC_NETWORKS和_is_public_address()。检查现有的IPv4-mapped和Local-Use NAT64条目是如何测试的，然后为报告的两个前缀及其嵌入的私有IPv4地址补充测试。完成标准是这些地址会被拒绝，同时公共目标仍保持预期行为。
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描述
Which server?
fetch (mcp-server-fetch)
Server version
latest (post PR #5033 / v2/main)
How did you run the server?
uvx or pip (PyPI package)
Transport
stdio
Protocol era
Modern (2026-07-28)
MCP client and version
Claude Desktop / MCP Inspector
Operating system and runtime
Linux / macOS / Windows, Python 3.10+
Steps to reproduce
Following the merge of PR #5033 ("fix(fetch): refuse private, loopback and metadata addresses by default (#4838)"), _is_public_address() in src/fetch/src/mcp_server_fetch/server.py checks IP addresses against _NON_PUBLIC_NETWORKS.
- Start
mcp-server-fetch. - Request fetching a target whose resolved IPv6 address maps to an internal IPv4 address using RFC 6052 Well-Known Prefix NAT64 (
64:ff9b::/96) or deprecated IPv4-compatible IPv6 (::/96, RFC 4291).
Examples:64:ff9b::127.0.0.1(hex64:ff9b::7f00:1)64:ff9b::169.254.169.254(hex64:ff9b::a9fe:a9fe)::127.0.0.1
_is_public_address(ipaddress.ip_address("64:ff9b::127.0.0.1"))is evaluated.
Expected behavior
Under defense-in-depth egress filtering, addresses that translate/embed loopback, RFC 1918, link-local, or cloud metadata IPv4 addresses should be rejected by default, in the same manner as IPv4-mapped IPv6 (::ffff:0:0/96) and Local-Use NAT64 (64:ff9b:1::/48).
Actual behavior
In src/fetch/src/mcp_server_fetch/server.py, _NON_PUBLIC_NETWORKS includes:
ipaddress.ip_network("64:ff9b:1::/48")(RFC 8215 Local-Use IPv4/IPv6 Translation)ipaddress.ip_network("::ffff:0:0/96")(RFC 4291 IPv4-mapped IPv6)
However, it omits:
64:ff9b::/96(RFC 6052 Well-Known Prefix for IPv4/IPv6 translation)::/96(RFC 4291 IPv4-compatible IPv6)
Because standard Python ipaddress.IPv6Address("64:ff9b::127.0.0.1").is_global evaluates to True, _is_public_address() returns True, allowing the request to bypass the private destination refusal guard in dual-stack / NAT64 environments.
Logs or error output
>>> import ipaddress
>>> from mcp_server_fetch.server import _is_public_address
>>>
>>> # Currently blocked:
>>> _is_public_address(ipaddress.ip_address("127.0.0.1"))
False
>>> _is_public_address(ipaddress.ip_address("::ffff:127.0.0.1"))
False
>>>
>>> # Currently bypasses filter:
>>> _is_public_address(ipaddress.ip_address("64:ff9b::127.0.0.1"))
True
>>> _is_public_address(ipaddress.ip_address("64:ff9b::169.254.169.254"))
True
>>> _is_public_address(ipaddress.ip_address("::127.0.0.1"))
True
Already prototyped a fix?
Add both prefixes to _NON_PUBLIC_NETWORKS in src/fetch/src/mcp_server_fetch/server.py:
_NON_PUBLIC_NETWORKS: tuple[ipaddress.IPv4Network | ipaddress.IPv6Network, ...] = (
# ... existing networks ...
ipaddress.ip_network("64:ff9b::/96"), # Well-Known NAT64 Prefix (RFC 6052)
ipaddress.ip_network("64:ff9b:1::/48"), # Local-Use IPv4/IPv6 Translation (RFC 8215)
ipaddress.ip_network("::ffff:0:0/96"), # IPv4-mapped IPv6 (RFC 4291)
ipaddress.ip_network("::/96"), # IPv4-compatible IPv6 (RFC 4291)
)
(Or inspect the embedded 32-bit IPv4 address for any address falling into /96 translation scopes to re-verify against IPv4 private definitions).
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