Is it safe to use cheap USB data cables?

Security issues with cables? No.

It's technically possible to have a hidden/embedded device in which case all the caveats of an untrusted USB device apply.

However the cost of a device, especially one small enough to be hidden in a cable, would be quite a bit higher than the cable itself so you probably don't need to worry about this.

Do you have reason to expect targeted attacks?

It's reasonable to assume that random cheap cables sold in large scale generally aren't modified to include offensive hardware, mostly for two reasons:

• That would raise the cost of the cable far above its price, and would be uneconomical even considering the ability to "monetize" a certain amount of random untargeted computers owned by the attack, so there are no good economic reasons for attackers to do this.
• We would have noticed such an attack. While most people wouldn't notice, if this was a mass attack, there would reasonably be some detection of that. Malware that tries to randomly hack many, many computers has obvious problems staying undetected for long.

However, if you have some reason to expect targeted, expensive attacks aimed to compromise you by people who have no qualms to perform illegal actions, then it certainly is a possibility that the hardware you receive is "special". However, that's not limited in any way to cheap USB data cables, or USB data cables - reasonably similar attacks would apply for any device you purchase in the same way, from mice/keyboards to laptops or server hardware. How do you know that your computer didn't have a hardware / firmware backdoor installed when you bought it?

If you have reason to expect such risks, you have to treat your USB data cable purchases in a similar manner as all other sensitive hardware; for example, ensure that you buy an item that cannot possibly be "adjusted" especially for you, e.g. random purchase of a generic item from a store shelf instead of a remote order that will be mailed to your address.

I really cannot imagine that the cable itself contains a malicious device as explained by @GeorgeBailey. So I would say that those cables are harmless when confidentiality or integrity is considered.

But if you consider that security also encompasses disponibility (availability), chances are that the contacts are of poor quality and that you experience occasional loss of connection when using them. Whether it is a real problem depends on actual usage...

I'd agree that the potential for malicious access is rather low, and if you are really worried about it you can always buy one more cable than you need and rip it apart to verify that nothing is in the housing that shouldn't be. I do want to note that while I wouldn't be overly concerned with potential security issues the potential for physical damage to the devices being charged is real, many of the lower-end cables available do not fully meet the USB spec and are often missing features required to correctly set the output power of a port, most likely not an issue if being used as a data cable but could be a problem if a device expects very low power input and is plugged into a USB port that is designed to be used to rapidly charge devices.

Cables are incredibly simple things. You are suggesting that the cheaper models house electronics for the purpose of compromising your security? That would only cost the supplier extra money.

What you should really be wary of is paying too much for a $5 product. Cables are not expensive. You should be paying no more than what you call "a pittance" for them.

That depends on your definition of "safe". It's unlikely that such a cable will contain a malicious payload, but it is possible for such a cable to spy on you. There's an old Powerpoint presentation floating around the Internet, supposedly used at an internal meeting by a government espionage agency, discussing a way of spying on your monitor through your cables. This presentation claimed that this device, the size of a few rice grains, was in use already, and that some cable manufacturers agreed to put this device in the cables they sold. It claimed that it beamed back your red VGA signal when a radio beam hit a small radio dish in this spying device. It seems plausible enough, RFID and NFC chips are powered in a similar way.

The good news is, if a strange truck isn't sitting outside your house, this method can't work. If it works with digital signals at all.

Full disclosure, this Powerpoint may have been a hoax. Hardware is not my strong suit, nor is electronics, especially not anything related to modulated waves.

If you are a person of interest, then anything is possible I suppose. I doubt the cable shipped from e.g. Amazon is not trustworthy, but shipments can be intercepted and what arrives in your mail box could have been tampered with. Unlikely to happen to 99.9% of us.

A bigger problem is that many USB 3 cables have been shown to be of very poor quality.

The problems stem from manufacturers not complying with the interface's specifications, specifically the use of resistors: a 56kΩ pull-up resistor should be connected to the Vbus pin to signal that one end of the cable or converter is a legacy USB device that can't handle a 3A current draw.

(http://www.theregister.co.uk/2015/11/05/google_engineer_ids_dodgy_usbs/)

The same engineer later reported frying some of his equipment due to a faulty cable:

Further analysis showed that the advertised SuperSpeed cables were missing entirely, and a 10 kΩ resistor was used instead of the 56 kΩ resistor the spec calls for. Needless to say, by the time the checks were done all of Leung's testing equipment was fried.

(https://www.engadget.com/2016/02/03/benson-leung-chromebook-pixel-usb-type-c-test/)

If the cable supplier has no way to guess at the purchaser's identity, chances for exploits are low.

However, I seem to remember that a German computer magazine had acquired and tested cheap USB3 cables that were wired wrong enough to potentially destroy devices.

There are also intentionally computer destroying USB devices around. If someone is just being an asshole, such things might get into circulation even though it is more expensive to produce than a proper cable.

And the first waves of computer viruses had no commercial payload, so assholery is not all that unheard of.