diff --git a/.gitignore b/.gitignore
index 44ce48c6..fc0b0813 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,6 @@ node_modules/
 /action_kit_api.go
 **/coverage.out
 **/*.o
+
+# macOS Finder metadata
+.DS_Store
diff --git a/go/action_kit_commons/network/proxyfault/lifecycle_test.go b/go/action_kit_commons/network/proxyfault/lifecycle_test.go
index 6a8010aa..61e494d7 100644
--- a/go/action_kit_commons/network/proxyfault/lifecycle_test.go
+++ b/go/action_kit_commons/network/proxyfault/lifecycle_test.go
@@ -54,7 +54,7 @@ func Test_waitExited_returns_when_never_started(t *testing.T) {
 // value, so a second Stop blocked and Exited() went back to reporting running.
 func Test_exited_stable_across_repeated_stop(t *testing.T) {
 	b := newProcessBase()
-	require.NoError(t, b.startAndMonitor(exec.Command("sh", "-c", "exit 0"), "test"))
+	require.NoError(t, b.startAndMonitor(exec.Command("sh", "-c", "exit 0"), "test", nil))
 	waitForExit(t, &b)
 
 	ex, err := b.Exited()
@@ -74,10 +74,24 @@ func Test_exited_stable_across_repeated_stop(t *testing.T) {
 // A non-zero exit is surfaced (and remains readable after Stop).
 func Test_exited_reports_error(t *testing.T) {
 	b := newProcessBase()
-	require.NoError(t, b.startAndMonitor(exec.Command("sh", "-c", "exit 7"), "test"))
+	require.NoError(t, b.startAndMonitor(exec.Command("sh", "-c", "exit 7"), "test", nil))
 	waitForExit(t, &b)
 	b.waitExited()
 	ex, err := b.Exited()
 	require.True(t, ex)
 	require.Error(t, err)
 }
+
+// The interception CA is handed over on stdin, so it never appears on the
+// command line and is never written to a filesystem the target can reach.
+func Test_startAndMonitor_writesStdin(t *testing.T) {
+	b := newProcessBase()
+	// cat echoes stdin to stdout, which the metrics scanner drains; the point is
+	// that the process sees the payload and reaches EOF so it can exit.
+	require.NoError(t, b.startAndMonitor(exec.Command("cat"), "test", []byte("PEM-PAYLOAD\n")))
+	waitForExit(t, &b)
+
+	ex, err := b.Exited()
+	require.True(t, ex, "cat must reach EOF once stdin is closed")
+	require.NoError(t, err)
+}
diff --git a/go/action_kit_commons/network/proxyfault/proxyfault.go b/go/action_kit_commons/network/proxyfault/proxyfault.go
index d8c9ad6e..8913da6a 100644
--- a/go/action_kit_commons/network/proxyfault/proxyfault.go
+++ b/go/action_kit_commons/network/proxyfault/proxyfault.go
@@ -62,6 +62,41 @@ type Fault struct {
 	Hosts       []string
 }
 
+// TLSInterceptCA carries the certificate authority the proxy uses to mint
+// per-SNI certificates, which is what lets an HTTPStatus fault reach an HTTPS
+// dependency instead of only a cleartext one.
+//
+// The CA belongs to the customer: they generate it, choose how long it lives,
+// and install it in the truststores of the workloads they want to fault. This
+// package only relays it.
+//
+// It is carried as PEM rather than as file paths on purpose. The runc backend
+// runs the proxy in a bundle whose rootfs is an overlay of the extension's "/",
+// and an overlay does not carry the extension's submounts — so a CA mounted
+// from a Kubernetes Secret is simply not visible by path inside the sidecar
+// (verified: the mount point appears as an empty directory). Passing the PEM
+// over the process's stdin works identically for both backends, keeps the key
+// off the command line, and never writes it to a disk the target could reach.
+type TLSInterceptCA struct {
+	CertPEM []byte
+	KeyPEM  []byte
+	// LeafValidity, when >0, overrides how long the per-SNI certificates the
+	// proxy mints stay valid. Always clamped to the CA's own expiry by the
+	// proxy. Zero keeps the proxy's built-in default.
+	LeafValidity time.Duration
+}
+
+// pemStream is what the proxy reads from stdin: one PEM stream carrying both
+// halves. Order is irrelevant to the parser on the other side.
+func (c *TLSInterceptCA) pemStream() []byte {
+	out := make([]byte, 0, len(c.CertPEM)+len(c.KeyPEM)+1)
+	out = append(out, c.CertPEM...)
+	if len(c.CertPEM) > 0 && c.CertPEM[len(c.CertPEM)-1] != '\n' {
+		out = append(out, '\n')
+	}
+	return append(out, c.KeyPEM...)
+}
+
 // Opts configures interception and the injected fault.
 type Opts struct {
 	ExecutionId string
@@ -83,6 +118,10 @@ type Opts struct {
 	ExcludeCIDRs []net.IPNet
 	Ports        []uint16
 	Fault        Fault
+	// TLSInterceptCA, when set, lets an HTTPStatus fault also apply to HTTPS
+	// connections. Nil (the default) means TLS is never decrypted and HTTPS is
+	// spliced through untouched.
+	TLSInterceptCA *TLSInterceptCA
 }
 
 // Proxy is a running transparent-proxy instance.
@@ -104,18 +143,23 @@ type HostStat struct {
 
 // Snapshot mirrors the transparent-proxy metrics JSON emitted on stdout.
 type Snapshot struct {
-	ConnectionsMatched    int64               `json:"connections_matched"`
-	ConnectionsActive     int64               `json:"connections_active"`
-	ConnectionsProxied    int64               `json:"connections_proxied"`
-	ConnectionsAborted    int64               `json:"connections_aborted"`
-	ConnectionsDropped    int64               `json:"connections_dropped"`
-	ConnectionsFaulted    int64               `json:"connections_faulted"`
-	LatencyApplied        int64               `json:"latency_applied"`
-	HTTPResponsesInjected int64               `json:"http_responses_injected"`
-	UpstreamErrors        int64               `json:"upstream_errors"`
-	BytesToUpstream       int64               `json:"bytes_to_upstream"`
-	BytesToClient         int64               `json:"bytes_to_client"`
-	PerHost               map[string]HostStat `json:"per_host,omitempty"`
+	ConnectionsMatched    int64 `json:"connections_matched"`
+	ConnectionsActive     int64 `json:"connections_active"`
+	ConnectionsProxied    int64 `json:"connections_proxied"`
+	ConnectionsAborted    int64 `json:"connections_aborted"`
+	ConnectionsDropped    int64 `json:"connections_dropped"`
+	ConnectionsFaulted    int64 `json:"connections_faulted"`
+	LatencyApplied        int64 `json:"latency_applied"`
+	HTTPResponsesInjected int64 `json:"http_responses_injected"`
+	// TLSInterceptRejected counts HTTPS connections on which the client refused
+	// the minted certificate. A non-zero value is the canonical "the CA is not in
+	// the target's truststore (or the client pins certificates)" signal — the
+	// fault never applied, so these are deliberately not counted as faulted.
+	TLSInterceptRejected int64               `json:"tls_intercept_rejected"`
+	UpstreamErrors       int64               `json:"upstream_errors"`
+	BytesToUpstream      int64               `json:"bytes_to_upstream"`
+	BytesToClient        int64               `json:"bytes_to_client"`
+	PerHost              map[string]HostStat `json:"per_host,omitempty"`
 }
 
 // SortedHosts returns the per-host keys in a stable order.
@@ -226,9 +270,40 @@ func (o Opts) startArgs() []string {
 	if len(o.Fault.Hosts) > 0 {
 		args = append(args, "--fault-hosts", strings.Join(o.Fault.Hosts, ","))
 	}
+	// Start-only: --revert reconstructs the interception rules, which do not
+	// depend on the CA. The PEM itself goes over stdin, never argv.
+	//
+	// Gated on the same condition as the payload: telling the proxy to read its
+	// CA from stdin while writing nothing there would leave it reading an empty
+	// stream, which is a far more confusing failure than not enabling it.
+	if _, ok := o.interceptCAPayload(); ok {
+		args = append(args, "--tls-ca-stdin")
+		if o.TLSInterceptCA.LeafValidity > 0 {
+			args = append(args, "--tls-leaf-validity", o.TLSInterceptCA.LeafValidity.String())
+		}
+	}
 	return args
 }
 
+// interceptCAPayload returns the PEM to hand the proxy on stdin, and whether a
+// usable CA was configured at all. A half-populated CA counts as unusable: the
+// proxy needs both halves, and silently sending one produces a startup failure
+// that reads like a certificate problem.
+func (o Opts) interceptCAPayload() ([]byte, bool) {
+	ca := o.TLSInterceptCA
+	if ca == nil || len(ca.CertPEM) == 0 || len(ca.KeyPEM) == 0 {
+		return nil, false
+	}
+	return ca.pemStream(), true
+}
+
+// stdinPayload is what gets written to the proxy's stdin at start: the
+// interception CA, or nothing when HTTPS is not being decrypted.
+func (o Opts) stdinPayload() []byte {
+	payload, _ := o.interceptCAPayload()
+	return payload
+}
+
 // sortedKeys returns a map's keys in a deterministic order so the built argv is
 // stable (which keeps revert-arg matching and tests predictable).
 func sortedKeys(m map[string]string) []string {
@@ -326,12 +401,29 @@ func (b *processBase) waitExited() {
 	<-b.done
 }
 
-func (b *processBase) startAndMonitor(cmd *exec.Cmd, logId string) error {
+// startAndMonitor starts cmd and watches it. stdin, when non-empty, is written
+// to the process and the pipe then closed — this is how the interception CA is
+// handed over without touching argv or the filesystem.
+func (b *processBase) startAndMonitor(cmd *exec.Cmd, logId string, stdin []byte) error {
 	logger := log.With().Str("id", logId).Logger()
+	var stdinPipe io.WriteCloser
+	if len(stdin) > 0 {
+		w, err := cmd.StdinPipe()
+		if err != nil {
+			return fmt.Errorf("failed to pipe transparent-proxy stdin: %w", err)
+		}
+		stdinPipe = w
+	}
 	// stdout carries the JSON metrics stream (scraped for statistics); stderr
 	// carries the proxy's structured logs.
 	stdout, err := cmd.StdoutPipe()
 	if err != nil {
+		// Unreachable in practice (StdoutPipe only fails when cmd.Stdout is
+		// already set, and cmd is always freshly built), but returning here
+		// without closing the stdin pipe would strand its write end.
+		if stdinPipe != nil {
+			_ = stdinPipe.Close()
+		}
 		return fmt.Errorf("failed to pipe transparent-proxy stdout: %w", err)
 	}
 	cmd.Stderr = &logWriter{logger: logger}
@@ -339,6 +431,17 @@ func (b *processBase) startAndMonitor(cmd *exec.Cmd, logId string) error {
 		return fmt.Errorf("failed to start transparent-proxy: %w", err)
 	}
 	b.started.Store(true)
+	// Only after a successful Start. On the error paths above os/exec never
+	// closes the child end of the pipe, so writing there would leak a descriptor
+	// and leave a copy of the CA key resident for a process that never ran.
+	if stdinPipe != nil {
+		go func() {
+			defer func() { _ = stdinPipe.Close() }()
+			if _, werr := stdinPipe.Write(stdin); werr != nil {
+				logger.Warn().Err(werr).Msg("failed to write CA to transparent-proxy stdin")
+			}
+		}()
+	}
 	scanDone := make(chan struct{})
 	go func() {
 		defer close(scanDone)
@@ -387,7 +490,7 @@ func newNetnsProcess(targetProcess ociruntime.LinuxProcessInfo, opts Opts) (Prox
 
 func (p *netnsProxy) Start() error {
 	log.Trace().Str("cmd", p.opts.String()).Msg("starting transparent-proxy via ip netns exec")
-	return p.startAndMonitor(p.cmd, "transparent-proxy")
+	return p.startAndMonitor(p.cmd, "transparent-proxy", p.opts.stdinPayload())
 }
 
 func (p *netnsProxy) Stop() error {
@@ -463,7 +566,7 @@ func (d *runcProxy) Start() error {
 	if err != nil {
 		return fmt.Errorf("failed to create run command: %w", err)
 	}
-	return d.startAndMonitor(cmd, d.bundle.ContainerId())
+	return d.startAndMonitor(cmd, d.bundle.ContainerId(), d.opts.stdinPayload())
 }
 
 func (d *runcProxy) Stop() error {
diff --git a/go/action_kit_commons/network/proxyfault/proxyfault_test.go b/go/action_kit_commons/network/proxyfault/proxyfault_test.go
index fcb85590..d7bcd496 100644
--- a/go/action_kit_commons/network/proxyfault/proxyfault_test.go
+++ b/go/action_kit_commons/network/proxyfault/proxyfault_test.go
@@ -14,6 +14,7 @@ import (
 	"github.com/rs/zerolog"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func mustCIDR(t *testing.T, s string) net.IPNet {
@@ -86,6 +87,44 @@ func TestStartArgs_probability(t *testing.T) {
 		"nil must omit the flag so the proxy default applies")
 }
 
+func TestStartArgs_tlsInterceptCA(t *testing.T) {
+	o := sampleOpts(t)
+
+	// Unset: TLS is never decrypted, so the flags must be absent entirely.
+	got := strings.Join(o.startArgs(), " ")
+	assert.NotContains(t, got, "--tls-ca")
+
+	assert.Nil(t, o.stdinPayload())
+
+	o.TLSInterceptCA = &TLSInterceptCA{CertPEM: []byte("CERT-PEM"), KeyPEM: []byte("KEY-PEM")}
+	got = strings.Join(o.startArgs(), " ")
+	assert.Contains(t, got, "--tls-ca-stdin")
+	// The key must never reach the command line.
+	assert.NotContains(t, got, "CERT-PEM")
+	assert.NotContains(t, got, "KEY-PEM")
+
+	// Both halves are handed over on stdin as one PEM stream.
+	payload := string(o.stdinPayload())
+	assert.Contains(t, payload, "CERT-PEM")
+	assert.Contains(t, payload, "KEY-PEM")
+
+	// Revert only reconstructs interception rules, which do not depend on the CA.
+	assert.NotContains(t, strings.Join(o.revertArgs(), " "), "--tls-ca")
+}
+
+func TestSnapshot_tlsInterceptRejected(t *testing.T) {
+	// The rejected counter must survive the stdout round-trip, since it is the
+	// signal that the CA is missing from the target's truststore.
+	var c metricsCollector
+	c.collectFromReader(strings.NewReader(
+		`{"connections_matched":2,"connections_faulted":0,"tls_intercept_rejected":2}`+"\n"), zerolog.Nop())
+
+	snap, ok := c.snapshot()
+	require.True(t, ok)
+	assert.Equal(t, int64(2), snap.TLSInterceptRejected)
+	assert.Equal(t, int64(0), snap.ConnectionsFaulted)
+}
+
 func TestRevertArgs(t *testing.T) {
 	got := strings.Join(sampleOpts(t).revertArgs(), " ")
 	// Revert must reproduce the same chain identity (exec-id) and filter so the
@@ -155,3 +194,31 @@ func TestStartArgs_MetricsStdoutAndNoFlush(t *testing.T) {
 		t.Errorf("missing no-flush flag: %s", args)
 	}
 }
+
+// A half-populated CA is unusable: the proxy needs both halves. Telling it to
+// read the CA from stdin while writing nothing there would leave it reading an
+// empty stream — a failure that reads like a certificate problem rather than a
+// configuration one.
+func TestStartArgs_tlsInterceptCA_halfPopulated(t *testing.T) {
+	for _, ca := range []*TLSInterceptCA{
+		{CertPEM: []byte("CERT-ONLY")},
+		{KeyPEM: []byte("KEY-ONLY")},
+		{},
+	} {
+		o := sampleOpts(t)
+		o.TLSInterceptCA = ca
+		assert.NotContains(t, strings.Join(o.startArgs(), " "), "--tls-ca-stdin",
+			"the flag must not be set without a complete CA")
+		assert.Nil(t, o.stdinPayload())
+	}
+}
+
+func TestStartArgs_tlsLeafValidity(t *testing.T) {
+	o := sampleOpts(t)
+	o.TLSInterceptCA = &TLSInterceptCA{CertPEM: []byte("C"), KeyPEM: []byte("K")}
+	// Unset: the proxy's own default applies, so the flag is omitted.
+	assert.NotContains(t, strings.Join(o.startArgs(), " "), "--tls-leaf-validity")
+
+	o.TLSInterceptCA.LeafValidity = 2 * time.Hour
+	assert.Contains(t, strings.Join(o.startArgs(), " "), "--tls-leaf-validity 2h0m0s")
+}