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Add an egress CIDR allowlist to the SSRF guard (closes #204)
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The SSRF blocklist had no escape hatch, so the thing webhooker is
mostly for — taking a public webhook and forwarding it to something
on your own network — could not be configured at all. Every private
address, Docker sibling and loopback service was permanently
unreachable as a delivery destination.

ALLOWED_EGRESS_CIDRS (default empty) names blocks that delivery
targets may reach despite the default blocklist. It is an allowlist
and only ever adds destinations: there is no boolean, and no value
disables SSRF protection wholesale. Empty, the guard behaves exactly
as before.

A fixed set of addresses is refused before the allowlist is
consulted, so no supplied CIDR opens one — not the exact address,
not a supernet, not 0.0.0.0/0 or ::/0. It is the two link-local
blocks (169.254.0.0/16, fe80::/10) plus host routes for the cloud
metadata endpoints that sit outside them: AWS's IPv6 IMDS at
fd00:ec2::254, which lives in ordinary ULA space, and Alibaba's
100.100.100.200, which lives in CGNAT. Allowlisting fd00::/8 or
100.64.0.0/10 (Tailscale's range) is an ordinary thing for an
operator to do and must not reopen instance-credential theft. The
IPv4-compatible (::a9fe:a9fe) and NAT64 (64:ff9b::a9fe:a9fe)
spellings of 169.254.169.254 are listed too, because To4() does not
normalise them into the link-local block the way it does the
IPv4-mapped form. Reaching any of these is credential theft rather
than delivery to an internal service.

The policy now lives in one function, Guard.checkIP, which both
target-creation validation and the delivery dialer call. The two
paths previously decided separately, which is how they came to
disagree about a destination. The guard is built once from config
and injected via fx into both the handlers and the delivery engine,
so there is a single instance and a single answer.

A set-but-unparseable value aborts startup naming the variable,
reusing the existing envPrefixList parser. A non-empty list is
logged at startup with the blocks spelled out, not counted, so the
hole is visible in the log of any deployment that has one.

Tests: an allowlisted loopback CIDR both validates and delivers to a
live server (and the same URL still fails without the allowlist); a
private address outside the listed block stays refused on both
paths; every unconditionally blocked address stays refused on both
paths under an allowlist that covers it, and the set itself is
pinned entry by entry; public addresses are unaffected either way;
and config coverage for parsing, startup abort, and the warning's
contents.
This commit is contained in:
Jeffrey Paul
2026-08-20 04:13:06 +00:00
parent aba02bc509
commit f9362101e6
16 changed files with 1218 additions and 61 deletions
Download Patch File Download Diff File Expand all files Collapse all files

261
internal/delivery/ssrf.go
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@@ -6,8 +6,11 @@ import (
"fmt"
"net"
"net/http"
"net/netip"
"net/url"
"time"
"sneak.berlin/go/webhooker/internal/config"
)
const (
@@ -25,20 +28,49 @@ var (
errBlockedIP = errors.New(
"blocked private/reserved IP range",
)
errBlockedMetadata = errors.New(
"blocked link-local or cloud instance metadata " +
"address: ALLOWED_EGRESS_CIDRS cannot open it",
)
errInvalidScheme = errors.New(
"only http and https are allowed",
)
)
// blockedNetworks contains all private/reserved IP ranges
// that should be blocked to prevent SSRF attacks.
// that should be blocked to prevent SSRF attacks. An operator
// can permit specific blocks out of this set with
// ALLOWED_EGRESS_CIDRS; see Guard.
//
//nolint:gochecknoglobals // package-level network list is appropriate here
var blockedNetworks []*net.IPNet
// alwaysBlockedNetworks are the ranges no configuration can
// open: the link-local blocks and the cloud instance metadata
// endpoints that live outside them. Reaching one is credential
// theft rather than delivery to an internal service, so a
// supplied CIDR that covers such an address still leaves it
// blocked.
//
// Some of these are also in blockedNetworks and this list is
// what makes them unconditional; two are public unicast and are
// reachable by default without it. Every metadata endpoint
// outside the link-local range is a /32 or /128 host route, so
// blocking it costs an operator nothing else on the surrounding
// network.
//
// Derive membership from the address, never from the vendor's
// prose. Several providers call these endpoints "link-local" or
// even "localhost" in their own documentation while the address
// is a ULA outside fe80::/10, so a set derived from the docs
// comes out wrong.
//
//nolint:gochecknoglobals // package-level network list is appropriate here
var alwaysBlockedNetworks []*net.IPNet
//nolint:gochecknoinits // init is the idiomatic way to parse CIDRs once at startup
func init() {
cidrs := []string{
blockedNetworks = mustParseCIDRs([]string{
"127.0.0.0/8",
"10.0.0.0/8",
"172.16.0.0/12",
@@ -56,7 +88,77 @@ func init() {
"::1/128",
"fc00::/7",
"fe80::/10",
}
})
// Every entry is named. The set must not grow or shrink
// without a matching change to
// TestAlwaysBlockedNetworks_PinnedSet.
//
// The IPv4-mapped form ::ffff:169.254.169.254 needs no
// entry: net.IPNet.Contains normalises it via To4() before
// comparing, so 169.254.0.0/16 already matches it. To4()
// does not normalise the IPv4-compatible or NAT64 forms,
// which is why those are listed separately.
alwaysBlockedNetworks = mustParseCIDRs([]string{
// IPv4 link-local, carrying the 169.254.169.254
// metadata service used by AWS, Azure, Alibaba,
// DigitalOcean, Hetzner, OpenStack and others.
"169.254.0.0/16",
// IPv6 link-local, its IPv6 counterpart.
"fe80::/10",
// IPv6 metadata endpoints in ULA space. Each is a host
// route, and fd00::/8 is an ordinary block for an
// operator to allowlist, so without these entries that
// one allowlist line hands out cloud credentials on
// every provider below.
//
// AWS IPv6 IMDS.
"fd00:ec2::254/128",
// AWS EKS Pod Identity Agent, which issues pod identity
// credentials. A second AWS endpoint, distinct from
// IMDS above. AWS's own docs call it "localhost".
"fd00:ec2::23/128",
// GCP metadata server for IPv6-only instances.
"fd20:ce::254/128",
// Oracle OCI IMDS, serving /opc/v2 instance principals.
"fd00:c1::a9fe:a9fe/128",
// Scaleway metadata, serving /user_data and /conf.
"fd00:42::42/128",
// Linode/Akamai metadata. Akamai's docs call it
// "link-local"; it is not.
"fd00:a9fe:a9fe::1/128",
// IPv4 metadata endpoints outside link-local.
//
// Alibaba Cloud metadata. It sits in CGNAT
// 100.64.0.0/10, which Tailscale also uses, so an
// operator allowlisting a Tailscale peer's range would
// otherwise reopen it.
"100.100.100.200/32",
// Oracle Cloud Classic metadata. Inside the blocked
// 192.0.0.0/24, so this entry is what stops an
// allowlist from opening it.
"192.0.0.192/32",
// Azure WireServer, carrying goalstate and extension
// settings on ports 80 and 32526. Public unicast, so
// this entry is what blocks it at all.
"168.63.129.16/32",
// Equinix Metal metadata. Public unicast, likewise.
"147.75.207.243/32",
// 169.254.169.254 as an IPv4-compatible IPv6 address.
"::a9fe:a9fe/128",
// 169.254.169.254 behind the NAT64 well-known prefix.
"64:ff9b::a9fe:a9fe/128",
})
}
// mustParseCIDRs parses a list of CIDR literals, panicking on a
// bad one. The inputs are compile-time constants, so a failure
// is a programming error rather than a runtime condition.
func mustParseCIDRs(cidrs []string) []*net.IPNet {
networks := make([]*net.IPNet, 0, len(cidrs))
for _, cidr := range cidrs {
_, network, err := net.ParseCIDR(cidr)
@@ -67,16 +169,15 @@ func init() {
))
}
blockedNetworks = append(
blockedNetworks, network,
)
networks = append(networks, network)
}
return networks
}
// isBlockedIP checks whether an IP address falls within
// any blocked private/reserved network range.
func isBlockedIP(ip net.IP) bool {
for _, network := range blockedNetworks {
// matchesAny reports whether ip falls inside any of networks.
func matchesAny(networks []*net.IPNet, ip net.IP) bool {
for _, network := range networks {
if network.Contains(ip) {
return true
}
@@ -85,9 +186,40 @@ func isBlockedIP(ip net.IP) bool {
return false
}
// isBlockedIP checks whether an IP address falls within
// any blocked private/reserved network range, before any
// operator allowlist is considered.
func isBlockedIP(ip net.IP) bool {
return matchesAny(blockedNetworks, ip)
}
// Guard makes every SSRF decision in the process.
//
// It holds the operator's ALLOWED_EGRESS_CIDRS allowlist and
// applies it in exactly one place, checkIP, which both the
// target-creation validator (ValidateTargetURL) and the delivery
// dialer call. Routing both through the same function is the
// point: when the two paths decided separately they drifted and
// disagreed, which is what made a target creatable but
// undeliverable.
//
// The guard is always on. The allowlist only ever adds specific
// networks to what the default blocklist refuses, and no
// configuration turns the guard off wholesale.
type Guard struct {
// allowed is the operator's ALLOWED_EGRESS_CIDRS. Empty
// (the default) means the default blocklist stands as-is.
allowed []netip.Prefix
}
// NewGuard builds the process-wide SSRF guard from configuration.
func NewGuard(cfg *config.Config) *Guard {
return &Guard{allowed: cfg.AllowedEgressCIDRs}
}
// ValidateTargetURL checks that an HTTP delivery target
// URL is safe from SSRF attacks.
func ValidateTargetURL(
func (g *Guard) ValidateTargetURL(
ctx context.Context, targetURL string,
) error {
parsed, err := url.Parse(targetURL)
@@ -111,36 +243,79 @@ func ValidateTargetURL(
}
if ip := net.ParseIP(host); ip != nil {
return checkBlockedIP(ip)
return g.checkIP(ip)
}
return validateHostname(ctx, host)
return g.validateHostname(ctx, host)
}
func validateScheme(scheme string) error {
if scheme != "http" && scheme != "https" {
// NewSSRFSafeTransport creates an http.Transport with a
// custom DialContext that refuses connections to any address
// this guard blocks. It resolves and checks at dial time, so a
// name that passed validation but now answers with a blocked
// address (DNS rebinding) is still refused.
func (g *Guard) NewSSRFSafeTransport() *http.Transport {
return &http.Transport{
DialContext: g.ssrfDialContext,
}
}
// allows reports whether ip falls inside the operator's
// configured egress allowlist.
func (g *Guard) allows(ip net.IP) bool {
if len(g.allowed) == 0 {
return false
}
addr, ok := netip.AddrFromSlice(ip)
if !ok {
return false
}
// Config unmaps every parsed prefix, so an IPv4-mapped
// address has to be unmapped too or it would never match.
addr = addr.Unmap()
for _, prefix := range g.allowed {
if prefix.Contains(addr) {
return true
}
}
return false
}
// checkIP is the single point at which SSRF policy is decided.
//
// The order is the policy:
//
// 1. alwaysBlockedNetworks is refused before the allowlist is
// consulted, so no configured CIDR reaches link-local or a
// cloud instance metadata endpoint.
// 2. The allowlist is consulted next, so a listed private
// network becomes reachable.
// 3. Everything else keeps the default blocklist's answer.
func (g *Guard) checkIP(ip net.IP) error {
if matchesAny(alwaysBlockedNetworks, ip) {
return fmt.Errorf(
"unsupported URL scheme %q: %w",
scheme, errInvalidScheme,
"target IP %s: %w", ip, errBlockedMetadata,
)
}
return nil
}
if g.allows(ip) {
return nil
}
func checkBlockedIP(ip net.IP) error {
if isBlockedIP(ip) {
return fmt.Errorf(
"target IP %s is in a blocked "+
"private/reserved range: %w",
ip, errBlockedIP,
"target IP %s: %w", ip, errBlockedIP,
)
}
return nil
}
func validateHostname(
func (g *Guard) validateHostname(
ctx context.Context, host string,
) error {
dnsCtx, cancel := context.WithTimeout(
@@ -165,11 +340,11 @@ func validateHostname(
}
for _, ipAddr := range ips {
if isBlockedIP(ipAddr.IP) {
err = g.checkIP(ipAddr.IP)
if err != nil {
return fmt.Errorf(
"hostname %q resolves to blocked "+
"IP %s: %w",
host, ipAddr.IP, errBlockedIP,
"hostname %q resolves to a blocked address: %w",
host, err,
)
}
}
@@ -177,16 +352,7 @@ func validateHostname(
return nil
}
// NewSSRFSafeTransport creates an http.Transport with a
// custom DialContext that blocks connections to
// private/reserved IP addresses.
func NewSSRFSafeTransport() *http.Transport {
return &http.Transport{
DialContext: ssrfDialContext,
}
}
func ssrfDialContext(
func (g *Guard) ssrfDialContext(
ctx context.Context,
network, addr string,
) (net.Conn, error) {
@@ -209,11 +375,11 @@ func ssrfDialContext(
}
for _, ipAddr := range ips {
if isBlockedIP(ipAddr.IP) {
err = g.checkIP(ipAddr.IP)
if err != nil {
return nil, fmt.Errorf(
"ssrf: connection to %s (%s) "+
"blocked: %w",
host, ipAddr.IP, errBlockedIP,
"ssrf: connection to %s blocked: %w",
host, err,
)
}
}
@@ -225,3 +391,14 @@ func ssrfDialContext(
net.JoinHostPort(ips[0].IP.String(), port),
)
}
func validateScheme(scheme string) error {
if scheme != "http" && scheme != "https" {
return fmt.Errorf(
"unsupported URL scheme %q: %w",
scheme, errInvalidScheme,
)
}
return nil
}