fix(security): fail closed when the snapshot checksum is unavailable

Whether a snapshot was verified at all was decided by pre.HasMD5Sidecar,
a flag set from a plaintext HEAD of the .md5sum URL. Anyone on the path
could answer that HEAD with a 404 and the tarball was then extracted with
expectedMD5 empty, no error, exit 0 - the attacker-controlled response was
effectively a 'skip integrity checking' switch, and the CLI reported
'(md5 sidecar absent - not verified)' as a success.

The sidecar is now fetched unconditionally whenever the operator has not
opted out, and a 404, non-200 or transport failure returns a typed
VerificationUnavailableError before any tarball byte is requested, so
nothing is written. Driven A/B against a binary built from develop: base
extracts attacker state with exit 0 on a 404, a 500 and a transport reset;
this refuses all three with an empty destination.

The MCP snapshot_download tool had no opt-out at all, so it always
extracted unverified on a 404; it gains a no_verify arg alongside the
CLI's existing --no-verify, which survives the --detach re-exec.

All eight mirrors were re-probed and every one returns 200 with a
coreutils-format body, so no in-repo caller needs the opt-out. The docs
still say sidecars can be absent - corrected to say the download now
refuses rather than extracting.

Adds one additive optional field to snapshot-download.schema.json, so
SchemaVersion goes 1.12.2 -> 1.12.3 with a regenerated baseline.

Author: barbatos2011

AGENTS.md

@@ -310,6 +310,13 @@ A pin is only worth the channel you got it over — a digest read off the
 same cleartext mirror buys nothing. Surface `plaintext_transport` to the
 user rather than deciding for them; if they have an out-of-band digest,
 pass it through `--sha256` (or the MCP `snapshot_download` `sha256` arg).
+If a download exits non-zero with `VERIFICATION_UNAVAILABLE`, the mirror's
+`.md5sum` sidecar could not be fetched, so trond refused to extract a chain
+database it cannot check — nothing was written to the destination. Retry,
+or pick another backup/mirror. Only pass `--no-verify` (MCP:
+`no_verify: true`) when the user has explicitly accepted an unverified
+chain database; a completed download reports
+`"verification_skipped": true` in that case.
 
 When the download finishes, its manifest + log stay under
 `~/.trond/snapshots/<id>.{json,log}` so you can audit later. Stale

CHANGELOG.md

@@ -246,6 +246,14 @@ The agent-ergonomics arc lands across four sequenced PRs:
   download. Existing MD5 behaviour, verification ordering, and disk
   headroom are unchanged. Schema `1.12.2` → `1.12.3` (additive optional
   fields on one schema).
+- `snapshot download` fails closed on integrity: the `.md5sum` sidecar is
+  always fetched (a preflight HEAD no longer decides whether to verify),
+  and a 404 / transport failure aborts with `VERIFICATION_UNAVAILABLE`
+  (exit 1) instead of silently extracting an unverified chain database.
+  `--no-verify` (MCP `snapshot_download` gains `no_verify`, previously
+  absent) is the only way to skip the check; results carry
+  `verification_skipped` so `md5_verified: false` can no longer be read
+  as "the mirror had no sidecar". Schema 1.12.2 → 1.12.3
 
 ## [0.1.0-alpha] — 2026-XX-XX
 

cmd/snapshot/detach_test.go

@@ -61,6 +61,14 @@ func TestStripDetach(t *testing.T) {
 			in:   []string{"trond", "--state-dir", "/tmp", "snapshot", "download", "--detach", "--to", "/data"},
 			want: []string{"trond", "--state-dir", "/tmp", "snapshot", "download", "--to", "/data"},
 		},
+		{
+			// The detached child is the process that actually fetches the
+			// sidecar and refuses to extract without it, so the operator's
+			// deliberate opt-out has to survive the re-exec.
+			name: "preserves --no-verify so the opt-out reaches the detached child",
+			in:   []string{"trond", "snapshot", "download", "--detach", "--no-verify", "--network", "nile"},
+			want: []string{"trond", "snapshot", "download", "--no-verify", "--network", "nile"},
+		},
 	}
 
 	for _, c := range cases {

cmd/snapshot/download.go

@@ -36,8 +36,10 @@ var downloadCmd = &cobra.Command{
 	Short: "Stream a snapshot tarball into a local directory",
 	Long: `Download a chain database snapshot, streaming the tarball through
 gunzip + tar so the .tgz is never persisted to disk. Verifies the
-upstream MD5 sidecar (when published), pre-checks free disk space, and
-refuses to overwrite an existing database without --force.
+upstream MD5 sidecar — if the sidecar cannot be fetched the download
+aborts rather than extracting unchecked data, unless you pass
+--no-verify. Also pre-checks free disk space and refuses to overwrite an
+existing database without --force.
 
 The default destination is ./output-directory under the current working
 directory — same convention as the official tron-docker tooling. Pass
@@ -77,7 +79,7 @@ func init() {
 	downloadCmd.Flags().StringVar(&dlDest, "to", "", "Destination directory (default ./output-directory)")
 	downloadCmd.Flags().StringVar(&dlNode, "node", "", "Managed node name; resolves --to from state")
 	downloadCmd.Flags().BoolVar(&dlForce, "force", false, "Overwrite existing database in destination")
-	downloadCmd.Flags().BoolVar(&dlNoVerify, "no-verify", false, "Skip MD5 verification (not recommended)")
+	downloadCmd.Flags().BoolVar(&dlNoVerify, "no-verify", false, "Extract without checking the MD5 sidecar (UNSAFE; otherwise a missing sidecar aborts the download)")
 	downloadCmd.Flags().StringVar(&dlSHA256, "sha256", "",
 		"Expected SHA-256 of the tarball, obtained out of band (64 hex chars). "+
 			"Unlike the upstream .md5sum — which travels the same channel as the "+
@@ -181,6 +183,15 @@ func runDownload(cmd *cobra.Command, _ []string) error {
 		if errors.As(err, &ow) {
 			return output.NewError("HUMAN_REQUIRED", output.ExitHumanRequired, ow.Error())
 		}
+		var vu *snapshot.VerificationUnavailableError
+		if errors.As(err, &vu) {
+			return output.NewError("VERIFICATION_UNAVAILABLE", output.ExitGeneralError, vu.Error()).
+				WithSuggestions(
+					"Retry — the sidecar may be published a few minutes after the tarball, or the mirror may be briefly unhealthy",
+					fmt.Sprintf("Pick another backup: trond snapshot list --network %s", src.Network),
+					"Only if you accept an unauthenticated chain database: re-run with --no-verify",
+				)
+		}
 		return output.NewError("DOWNLOAD_ERROR", output.ExitGeneralError, err.Error())
 	}
 
@@ -193,8 +204,11 @@ func runDownload(cmd *cobra.Command, _ []string) error {
 		humanGB(uint64(res.BytesDownloaded)), res.Duration.Round(time.Second), res.FilesExtracted, dest)
 	if res.MD5Verified {
 		fmt.Fprint(cmd.OutOrStdout(), " (md5 ✓)")
-	} else if !dlNoVerify {
-		fmt.Fprint(cmd.OutOrStdout(), " (md5 sidecar absent — not verified)")
+	} else {
+		// Only reachable with --no-verify: without it, a sidecar that is
+		// missing or unfetchable aborts the download rather than landing
+		// an unchecked chain database here.
+		fmt.Fprint(cmd.OutOrStdout(), " (NOT VERIFIED — --no-verify was passed; this chain database is unauthenticated)")
 	}
 	if res.SHA256Verified {
 		fmt.Fprint(cmd.OutOrStdout(), " (sha256 pin ✓)")
@@ -225,11 +239,14 @@ func downloadPayload(src *snapshot.Source, backup, dest string, res *snapshot.Do
 		"bytes_downloaded": res.BytesDownloaded,
 		"duration_ms":      res.DurationMs,
 		"md5_verified":     res.MD5Verified,
-		"actual_md5":       res.ActualMD5,
-		"files_extracted":  res.FilesExtracted,
-		"userdata_present": pre.UserdataPresent,
-		"sha256":           res.SHA256,
-		"sha256_verified":  res.SHA256Verified,
+		// Distinguishes "checked and good" from "deliberately unchecked":
+		// a missing sidecar is now an error, never a silent skip.
+		"verification_skipped": res.VerificationSkipped,
+		"actual_md5":           res.ActualMD5,
+		"files_extracted":      res.FilesExtracted,
+		"userdata_present":     pre.UserdataPresent,
+		"sha256":               res.SHA256,
+		"sha256_verified":      res.SHA256Verified,
 		// The cleartext-transport fact has to reach agents that read
 		// stdout only and never see the stderr warning.
 		"plaintext_transport": res.PlaintextTransport,
@@ -271,6 +288,10 @@ func emitPlan(outputFmt string, src *snapshot.Source, backup, dest string, pre *
 	if pre.PlaintextTransport {
 		fmt.Print(snapshot.PlaintextWarning(pre.URL, dlSHA256 != ""))
 	}
+	if !pre.HasMD5Sidecar {
+		fmt.Println("  WARNING: the sidecar did not answer this probe — the download will")
+		fmt.Println("           refuse to extract unverified data unless you pass --no-verify.")
+	}
 	if pre.WouldOverwrite {
 		fmt.Println("  WARNING: existing database would be overwritten (use --force).")
 	}

internal/knowledge/files/snapshots.md

@@ -70,8 +70,9 @@ Before any tarball bytes hit the wire, trond:
 
 1. Sends an HTTP HEAD to the tarball URL → reads `Content-Length`. The
    body is never opened on this probe.
-2. Issues a separate HEAD to the `.md5sum` sidecar → records whether
-   inline verification will be possible.
+2. Issues a separate HEAD to the `.md5sum` sidecar → records whether the
+   mirror advertises one. Informational only: it never decides whether
+   verification happens (see "MD5 verification").
 3. `Statfs(destination)` → reads available bytes (Bavail × Bsize, the
    same number `df` shows).
 4. Refuses to start the GET if free space < `Content-Length × 2`.
@@ -118,10 +119,12 @@ If a mirror ever gains HTTPS, switch its `BaseURL` in
 
 ## What the MD5 sidecar does and does not buy
 
-Mainnet mirrors publish `<tarball>.tgz.md5sum` sidecars. trond:
+Mainnet and Nile mirrors both publish `<tarball>.tgz.md5sum` sidecars.
+trond:
 
-1. HEADs the sidecar in preflight (records "has md5 sidecar: true/false").
-2. Downloads the sidecar (a few hundred bytes).
+1. HEADs the sidecar in preflight (records "has md5 sidecar: true/false"
+   for `--dry-run`; this answer decides nothing).
+2. Downloads the sidecar (a few hundred bytes) before the tarball GET.
 3. Hashes the tarball stream while extracting.
 4. Compares — mismatch fails the operation with the database in whatever
    partial state extraction reached.
@@ -134,10 +137,21 @@ substitute the sidecar, and the two will agree. MD5 is also collision-prone
 on its own merits. So the sidecar check proves the transfer was not
 *corrupted*; it proves nothing about where the bytes came from.
 
-Nile and the occasional outage may leave the sidecar absent. trond will
-still extract; the result message reads `(md5 sidecar absent — not
-verified)`. Pass `--no-verify` to suppress that note when you've made
-the choice deliberately.
+A mirror outage — or a mirror that stops publishing sidecars — can still
+leave the sidecar absent. **trond then refuses to download**: step 2
+fails with `VERIFICATION_UNAVAILABLE` (exit 1) and nothing is written to
+the destination. The same applies to any transport failure fetching the
+sidecar. The preflight HEAD from step 1 is not consulted: it arrives over
+the same unauthenticated HTTP channel as the tarball, so a 404 there is
+never taken as permission to skip the check.
+
+If you accept an unverified chain database, say so explicitly with
+`--no-verify` (MCP: `no_verify: true`); the flag carries through
+`--detach` to the background child. The result line then reads
+`(NOT VERIFIED — --no-verify was passed; this chain database is
+unauthenticated)` and JSON output carries `"verification_skipped": true`.
+Treat such a database as untrusted input — it is the state your node will
+serve to every dApp, explorer and exchange that queries it.
 
 ## `--sha256`: the check that can detect substitution
 
@@ -301,6 +315,14 @@ version change.
   - Malformed pin, rejected before the transfer starts. SHA-256 digests are
     64 hex characters; you may have pasted an MD5 (32).
 
+`VERIFICATION_UNAVAILABLE: cannot verify snapshot integrity: <url> is
+unavailable ...`
+  - The `.md5sum` sidecar 404'd or the fetch failed, so trond has nothing
+    to check the tarball against and refuses to extract. Retry (sidecars
+    are sometimes published a few minutes after the tarball), pick another
+    backup with `trond snapshot list`, or switch `--region` / `--domain`.
+    `--no-verify` bypasses the check and should be a deliberate choice.
+
 ## Local database backup with `db_cp`
 
 `scripts/db_cp.sh` makes a fast, space-efficient local copy of a

internal/mcp/tools_snapshot.go

@@ -32,6 +32,12 @@ type snapshotDownloadArgs struct {
 	Force   bool   `json:"force,omitempty" jsonschema:"overwrite an existing chain DB (DESTRUCTIVE)"`
 	DryRun  bool   `json:"dry_run,omitempty" jsonschema:"print the plan and exit without downloading"`
 	SHA256  string `json:"sha256,omitempty" jsonschema:"expected SHA-256 of the tarball (64 hex chars) obtained out of band; a mismatch fails the download. The mainnet mirrors are cleartext HTTP and their .md5sum sidecar rides the same channel, so this pin is the only check that can detect a substituted archive"`
+	// NoVerify is the deliberate opt-out from integrity checking. Without
+	// it, a sidecar that cannot be fetched aborts the download instead of
+	// silently extracting unverified chain data — mirroring `--no-verify`
+	// on the CLI so an agent isn't stuck when a mirror genuinely stops
+	// publishing sidecars.
+	NoVerify bool `json:"no_verify,omitempty" jsonschema:"UNSAFE: extract without checking the MD5 sidecar; otherwise a missing/unfetchable sidecar aborts the download"`
 }
 
 func registerSnapshotTools(s *mcp.Server) {
@@ -59,7 +65,7 @@ func registerSnapshotTools(s *mcp.Server) {
 	mcp.AddTool(s, &mcp.Tool{
 		Name:  "snapshot_download",
 		Title: "Download a chain DB snapshot",
-		Description: `Stream a snapshot tarball into a destination directory, gunzip + tar in one pipeline (no .tgz on disk). Pre-checks free disk space (HEAD probe + Statfs, requires 2× headroom). Refuses overwrite of an existing chain DB unless force=true. Preserves any pre-existing userdata/. MD5-verifies inline against the published sidecar when present.
+		Description: `Stream a snapshot tarball into a destination directory, gunzip + tar in one pipeline (no .tgz on disk). Pre-checks free disk space (HEAD probe + Statfs, requires 2× headroom). Refuses overwrite of an existing chain DB unless force=true. Preserves any pre-existing userdata/. MD5-verifies inline against the published sidecar; if the sidecar cannot be fetched the download fails instead of extracting unverified chain data — pass no_verify=true only to accept that risk deliberately.
 
 Use dry_run=true to inspect the plan first. The tool emits MCP progress notifications during the actual download so the client can render a live progress bar. NOTE: this MCP tool runs the download in-process and blocks until completion or context cancellation; for fire-and-forget mainnet-full sized downloads (multi-hour) prefer the CLI with --detach.
 
@@ -147,6 +153,7 @@ func snapshotDownloadTool(ctx context.Context, req *mcp.CallToolRequest, args sn
 		DestDir:        args.Dest,
 		Force:          args.Force,
 		ExpectedSHA256: args.SHA256,
+		NoVerify:       args.NoVerify,
 	}
 
 	pre, err := snapshot.Preflight(ctx, opts)
@@ -188,11 +195,14 @@ func snapshotDownloadTool(ctx context.Context, req *mcp.CallToolRequest, args sn
 		"bytes_downloaded": res.BytesDownloaded,
 		"duration_ms":      res.DurationMs,
 		"md5_verified":     res.MD5Verified,
-		"actual_md5":       res.ActualMD5,
-		"files_extracted":  res.FilesExtracted,
-		"userdata_present": pre.UserdataPresent,
-		"sha256":           res.SHA256,
-		"sha256_verified":  res.SHA256Verified,
+		// A missing sidecar is an error now, so md5_verified=false means
+		// exactly one thing: the caller passed no_verify.
+		"verification_skipped": res.VerificationSkipped,
+		"actual_md5":           res.ActualMD5,
+		"files_extracted":      res.FilesExtracted,
+		"userdata_present":     pre.UserdataPresent,
+		"sha256":               res.SHA256,
+		"sha256_verified":      res.SHA256Verified,
 		// An MCP caller has no stderr to read the warning from, so the
 		// cleartext-transport fact has to travel in the payload.
 		"plaintext_transport": res.PlaintextTransport,

internal/schema/embed.go

@@ -113,7 +113,13 @@ import (
 //	        inferring it, and pin an out-of-band digest via `--sha256` /
 //	        the MCP `sha256` arg. One existing schema, additive optional
 //	        fields only — PATCH.
-const SchemaVersion = "1.12.4"
+//	1.12.5 — snapshot-download output gains `verification_skipped`. A
+//	        missing or unfetchable `.md5sum` sidecar is now a hard error
+//	        (`VERIFICATION_UNAVAILABLE`) rather than a silent skip, so
+//	        `md5_verified: false` on a successful download means one
+//	        thing only: the operator passed `--no-verify` / `no_verify`.
+//	        One existing schema, one additive optional field — PATCH.
+const SchemaVersion = "1.12.5"
 
 // JSONSchemaURLBase is the canonical URL prefix for individual output
 // schema files. Embedded $id values inside each schema mirror this so

internal/schema/files/snapshot-download.schema.json

@@ -24,6 +24,10 @@
         "actual_md5":       { "type": "string", "pattern": "^[0-9a-f]{32}$" },
         "expected_md5":     { "type": "string", "pattern": "^[0-9a-f]{32}$" },
         "md5_verified":     { "type": "boolean", "description": "The upstream .md5sum sidecar matched. On a plaintext_transport mirror the sidecar arrives over the same unauthenticated channel as the tarball, so this attests transfer integrity only — not provenance." },
+        "verification_skipped": {
+          "type": "boolean",
+          "description": "true only when --no-verify / no_verify was passed. A missing or unfetchable .md5sum sidecar is an error (VERIFICATION_UNAVAILABLE), never a silent skip, so md5_verified=false always means the operator opted out."
+        },
         "dest":             { "type": "string" },
         "userdata_present": { "type": "boolean" },
         "source":           { "type": "object" },

internal/schema/version_baseline.json

@@ -1,5 +1,5 @@
 {
-  "schema_version": "1.12.4",
+  "schema_version": "1.12.5",
   "entries": [
     {
       "name": "apply",
@@ -103,7 +103,7 @@
     },
     {
       "name": "snapshot-download",
-      "hash": "24767d4d39d7201e46c9f0d23e575283a768bc5867cab81cb9255ce0f5eb3d66"
+      "hash": "14517cc14171dcf3f615fa2cf69be553bceab3b83fcea2ac7f913260e64e5c2c"
     },
     {
       "name": "snapshot-jobs",