Side note: most of the examples and sources I am going to be citing are from the Holy Roman Empire in the Late Middle Ages, as this is my area of expertise, but the general principles are true for most of the European Middle Ages. I will also talk about the symbolic implications of plundering, as the specifics of how plundering worked are usually glossed over by chroniclers

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Yes, they absolutely would be attacked. Their specific fate depended on a variety of circumstances, including how the raiders were feeling that day, but peasants
were seen as valid targets in warfare. After all, peasants were producing the food that kept the enemy army fed, therefore they were considered legitimate targets, if not outright enemy combatants. People living near castles were kinda lucky, as they could flee behind the walls. Most peasants would be raided without warning, however. Raiding the countryside was a very common part of medieval warfare – most historians now think it was actually more common than actual battles or sieges. However, medieval chroniclers would often rather write about those battles, as they were seen as more impactful and relevant than the day-to-day raiding business, even though this “small war” was a very integral part of medieval warfare.

As a nobleman going to war, you didn’t just raid the countryside to get loot or damage your enemy’s economy though. Put yourself in the shoes of some local lord. You’ve just received a letter announcing your neighbour’s intention to start a feud over that one piece of land you two have been squabbling about. So, you call your friends, your vassals, levies etc., gather your supplies and hole up in your castle. Now your neighbour lord will have to invest considerable amounts of time and manpower to get you out - time he may not have. So, all’s well for you, right? Well, now your countryside is largely unprotected – but that’s fine, right? A few villages getting burned is sure bad and is going to hurt your economy, but your army is safe and intact, your granaries are full and safe behind your castle’s wall. And then your enemy denounces you as a coward for not defending your country. And then two of your men-at-arms ask you what happened to the oath you took as the liege of this country – the oath in which you swore to protect the lands from harm.

Chroniclers often use descriptions of looting and plundering to indicate that a foe is defeated.
The logic being if you can plunder without anyone hindering you then you must have won. So, just the fact that your neighbour has looted your lands can already be used by him to claim victory over you. And now your vassals start getting demoralized since, well, the enemy can just burn down our lands with impunity – have we
been defeated? Plundering and raiding are thus not only methods of economic warfare but were also seen as a symbolic signifier for victory.

We do not know a lot about what actually went down during those lootings, as most chroniclers usually just write “Lord X came to the lands of Lord Y and burned down villages, looted the countryside and took the cattle.” Looting and plundering was considered such a normal part of war, most chroniclers wouldn’t even bother mentioning it. One chronicler, Ludwig von Eyb the Younger, who chronicled the life of German knight Wilwolt von Schaumberg at one point straight up says he doesn’t want to describe the minutia of looting and the “small war” as that would just bore his readers:

“Nichtsdesterweniger ward zw bayden tayllen vill reütterey getribenn, stett eingenommen, fleckenn vnnd dorffer gebucht, geplundert, gebrennt, das vich genomen, reyssig vnd zwfus, wie den das kriegs siett, nyder geworffenn, gefanngen, gegen einander wider ledig gelassenn, das ich, als ümb kürtz vnnd
verdrus der lessenden, zwschreibenn vnnterlas.”/ “Nonetheless there was much riding war waged by both sides, cities taken, hamlets and villages razed, plundered, burned, the cattle taken, mounted troops and foot soldiers, as is the custom of war, laid low, captured, released for one another, of which I, for the sake of brevity and my readers’ frustration, will refrain from writing about.” (von Eyb, p. 132,
translation mine)

There is one very striking exception that I find very
interesting. The chronicle of one Martin von Bolkenhain, writing about the Hussite Wars and their aftermath. He describes how his city of Bolkenhain is sacked
during a feud between two local nobles and how the enemy invades the city. He then describes his personal experience of being on the receiving end of a
plundering, which is an incredibly uncommon perspective. He describes how the plunderers go out of their way to seek through rich looking houses and
churches, how they enter his house, break open his shop and loot everything they can get their hands on, holding him at sword point. They only stop at his
wife, who according to him had given birth a few weeks ago and was still lying in bed. According to him, some of the looters knew his wife and spared the two,
even allowing them to take their most valuable possessions with them. They then advised
them to head to the cellar, as they intended to burn down the city, once they are done with looting and move on. So, whether you survived or not often times
depended on whether or not you got lucky.

u/ocolor gives a really good overview from his area of expertise. I can't expect to differ a whole lot from his response except but offer some insights from my own area: the Albigensian Crusade (1209-1229).

In short, yes, they were attacked and attacking them was considered logistically and military expedient.

One aspect that ought to be touched upon was one of the guiding principles of Medieval Warfare was the importance placed upon raiding. Vegetius's military work De Rei Militari was well read during the High Middle Ages, in fact, it's the most extent Medieval manuscript we have copies for with over 200 known copies. Richard the Lionheart was known to read it. One of Vegetius's insights was the importance of avoiding open battle and seeking to defeat one's enemies through terror and logistical deprivation. Vegetius's regarded a pitched battle as incredibly risky because battles have an incalculable chance element in them. Case in point, Simon de Montfort and a small force of a few hundred crusader knights were able to rout around 10,000 troops from the combined Occitan-Aragonese coalition at the Battle of Muret when they killed Peter II. An open battle could often times change the course of the war. In the 13th century we see such instances at the Battle of Bouvines and the Battle of Las Navas de Tolosas. Raiding was much safer as it required less troops, could be carried out with high intensity, and served to weaken one's foe by depriving them of food, manpower, and generally demoralizing the enemy. During the crusade, the crusader forces took to burning down and uprooting the vineyards surrounding the city of Toulouse during the Second and Third sieges. This particularly hurt the people of Toulouse as the vineyards took years to grow and they watched with despair and hopelessness as generations of work were undone.

Vegetius's point was also relevant when the discussion regarding the quality and number of troops came into play. The bulk of Medieval forces were not well trained knights, but rather a collection of progressively bigger contingents of professional, semi-professional, volunteer and conscript footmen of vastly different quality and martial ability. Many of these troops did not have the discipline for a pitched battle nor the training to reasonably do well on one. A small force of knights, who trained since childhood for warfare, would easily demolish a large group of farmers who have never seen a day of battle. Raiding was a far more efficient and safer use of the lesser trained troops where combat against a superior foe could be avoided. Moreover, gaining loot was often times an incentive for volunteers or a form of payment meant to pay for their upkeep. At the Siege of Beziers in 1209, it was a peasant force that stormed the city and immediately took to looting, regarding it as part of the reward for going on crusade. When they didn't get their way with the loot from the city they took to burning it down so no one could get anything. Loot was that important, even in a crusade.

Raiding was a sound military strategy and a way to pad one's pocket or pay for the motley array of soldiers who had limited combat utility. Peasants were absolutely targeted as they were considered prime candidates for raiding.

Primary:

The History of the Albigensian Crusade by Peter Les Vaux de Cernay

The Song of the Cathar Wars by William of Tudela & the Anonymous

The Chronicle of William of Puylaurens

Secondary

The Occitan War: A Military and Political History of the Albigensian Crusade, 1209-1218 by Laurence Marvin

French Medieval Armies: 1000-1300 (Men-at-Arms) by David Nicolle

Medieval Warfare Source Book: Warfare In Western Christendom by David Nicolle

Muret 1213 by Martin Alvara Cabrer