diff --git a/reftable/block.c b/reftable/block.c
index 4d6b11c2e7..4d285aefd7 100644
--- a/reftable/block.c
+++ b/reftable/block.c
@@ -351,6 +351,10 @@ int reftable_block_init(struct reftable_block *block,
 
 	restart_count = reftable_get_be16(block->block_data.data + block_size - 2);
 	restart_off = block_size - 2 - 3 * restart_count;
+	if (restart_off < header_size + 4 || restart_off > block_size - 2) {
+		err = REFTABLE_FORMAT_ERROR;
+		goto done;
+	}
 
 	block->block_type = block_type;
 	block->hash_size = hash_size;
diff --git a/t/unit-tests/u-reftable-block.c b/t/unit-tests/u-reftable-block.c
index 6995eb8bf1..d77048cdd0 100644
--- a/t/unit-tests/u-reftable-block.c
+++ b/t/unit-tests/u-reftable-block.c
@@ -535,3 +535,36 @@ void test_reftable_block__corrupt_block_size(void)
 	reftable_block_release(&block);
 	reftable_buf_release(&data);
 }
+
+void test_reftable_block__corrupt_restart_count(void)
+{
+	struct reftable_block_source source = { 0 };
+	struct reftable_record rec = {
+		.type = REFTABLE_BLOCK_TYPE_REF,
+		.u.ref = {
+			.value_type = REFTABLE_REF_VAL1,
+			.refname = (char *) "refs/heads/main",
+		},
+	};
+	struct reftable_block block = { 0 };
+	struct reftable_buf data = REFTABLE_BUF_INIT;
+	int block_size;
+
+	block_size = cl_reftable_write_block(&data, REFTABLE_BLOCK_TYPE_REF, &rec, 1);
+
+	/*
+	 * Corrupt the restart count to claim a bogus number of restart points.
+	 * Note that this would only cause us to perform an out-of-bounds
+	 * access when seeking into the block, but we want to refuse such a
+	 * block outright.
+	 */
+	reftable_put_be16((uint8_t *) data.buf + block_size - 2, 0xffff);
+
+	block_source_from_buf(&source, &data);
+	cl_assert_equal_i(reftable_block_init(&block, &source, 0, 0, data.len,
+					      REFTABLE_HASH_SIZE_SHA1, REFTABLE_BLOCK_TYPE_REF),
+			  REFTABLE_FORMAT_ERROR);
+
+	reftable_block_release(&block);
+	reftable_buf_release(&data);
+}