From d045b0f3e8d519ae6016536f069b26ba65064f1b Mon Sep 17 00:00:00 2001
From: iho <iho@users.noreply.github.com>
Date: Thu, 9 Jul 2026 11:28:30 +0300
Subject: [PATCH] fix: use FutureTimeLimit instead of CorruptedData for skew
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Headers slightly beyond future_time_limit are not corrupted data —
clocks differ. Return ser::Error::FutureTimeLimit so the condition is
explicit and not lumped with real corruption/POW failures.

Addresses #3360.
---
 core/src/core/block.rs | 14 ++++++++------
 core/src/ser.rs        |  6 ++++++
 core/tests/block.rs    | 26 +++++++++++++++++++++++++-
 3 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/core/src/core/block.rs b/core/src/core/block.rs
index 7198e1e049..8331d04376 100644
--- a/core/src/core/block.rs
+++ b/core/src/core/block.rs
@@ -448,14 +448,16 @@ impl Readable for UntrustedBlockHeader {
 		let header = read_block_header(reader)?;
 		let ftl = global::get_future_time_limit();
 		if header.timestamp > Utc::now() + Duration::seconds(ftl as i64) {
-			// refuse blocks whose timestamp is too far in the future
-			// this future_time_limit (FTL) is specified in grin-server.toml
-			// TODO add warning in p2p code if local time is too different from peers
-			error!(
-				"block header {} validation error: block time is more than {} seconds in the future",
+			// Refuse blocks whose timestamp is too far in the future.
+			// FTL is configured in grin-server.toml. This is *not* corrupted data —
+			// local clocks differ; use a dedicated error so peers are not treated
+			// as sending garbage (#3360).
+			// TODO: warn in p2p if local time is systematically different from peers
+			warn!(
+				"block header {} rejected: block time is more than {} seconds in the future (clock skew?)",
 				header.hash(), ftl
 			);
-			return Err(ser::Error::CorruptedData);
+			return Err(ser::Error::FutureTimeLimit);
 		}
 
 		// Check the block version before proceeding any further.
diff --git a/core/src/ser.rs b/core/src/ser.rs
index 38cd9aa1b7..9e3ee8bf18 100644
--- a/core/src/ser.rs
+++ b/core/src/ser.rs
@@ -72,6 +72,10 @@ pub enum Error {
 	InvalidBlockVersion,
 	/// Unsupported protocol version
 	UnsupportedProtocolVersion,
+	/// Block/header timestamp is beyond the future time limit.
+	/// Not corrupted data — peer clocks may simply disagree; treat as non-fatal
+	/// for peer reputation (see #3360).
+	FutureTimeLimit,
 }
 
 impl From<io::Error> for Error {
@@ -102,6 +106,7 @@ impl fmt::Display for Error {
 			Error::HexError(ref e) => write!(f, "hex error {:?}", e),
 			Error::InvalidBlockVersion => f.write_str("invalid block version"),
 			Error::UnsupportedProtocolVersion => f.write_str("unsupported protocol version"),
+			Error::FutureTimeLimit => f.write_str("future time limit exceeded"),
 		}
 	}
 }
@@ -126,6 +131,7 @@ impl error::Error for Error {
 			Error::HexError(_) => "hex error",
 			Error::InvalidBlockVersion => "invalid block version",
 			Error::UnsupportedProtocolVersion => "unsupported protocol version",
+			Error::FutureTimeLimit => "future time limit exceeded",
 		}
 	}
 }
diff --git a/core/tests/block.rs b/core/tests/block.rs
index b2451ffefb..33b68163f4 100644
--- a/core/tests/block.rs
+++ b/core/tests/block.rs
@@ -26,7 +26,7 @@ use crate::core::core::{Committed, CompactBlock};
 use crate::core::libtx::build::{self, input, output};
 use crate::core::libtx::ProofBuilder;
 use crate::core::{global, pow, ser};
-use chrono::Duration;
+use chrono::{Duration, Utc};
 use grin_core as core;
 use keychain::{BlindingFactor, ExtKeychain, Keychain};
 use util::{secp, ToHex};
@@ -466,6 +466,30 @@ fn deserialize_untrusted_header_weight() {
 	assert!(res.is_ok());
 }
 
+/// Future timestamps must not be reported as CorruptedData (#3360).
+#[test]
+fn deserialize_untrusted_header_future_time() {
+	test_setup();
+	let keychain = ExtKeychain::from_random_seed(false).unwrap();
+	let builder = ProofBuilder::new(&keychain);
+	let prev = BlockHeader::default();
+	let key_id = ExtKeychain::derive_key_id(1, 1, 0, 0, 0);
+	let mut b = new_block(&[], &keychain, &builder, &prev, &key_id);
+
+	// Far beyond future_time_limit (default 300s for AutomatedTesting? check global)
+	b.header.timestamp = Utc::now() + Duration::seconds(3600);
+	set_pow(&mut b.header);
+
+	let mut vec = Vec::new();
+	ser::serialize_default(&mut vec, &b.header).expect("serialization failed");
+	let res: Result<UntrustedBlockHeader, _> = ser::deserialize_default(&mut &vec[..]);
+	assert_eq!(
+		res.err(),
+		Some(ser::Error::FutureTimeLimit),
+		"future time must use FutureTimeLimit, not CorruptedData"
+	);
+}
+
 #[test]
 fn serialize_deserialize_block() {
 	test_setup();