A teenage boy had his thumb reattached after it was blown off by a firework that was thrown at him.
Thomas McMahon, 16, was injured on Green Road in Baildon, near Bradford, on Saturday evening.
His attackers were "two males in dark clothing [with] a lit firework", West Yorkshire Police said.
Thomas's family said he had so far had 14 hours of surgery at Leeds General Infirmary, and would remain in hospital for at least two weeks for more.
Stepmother Sarah McMahon said Thomas was with friends when two older males threw a firework at him at around 18:15 BST.
"Thomas thought a rock was being thrown at him and instinctively put up his hand to protect his face," she said.
"It exploded in his hand and and his thumb was hanging off."
A neighbour administered first aid until paramedics arrived.
Ms McMahon said Thomas also had burns to his chest, arms, face, eyelashes and eyebrows.
"It's life-changing," she said. "It's lucky it wasn't his face but it's changed how he'll function.
"He already said he won't go out again and he'll become a hermit when he hears fireworks."
The family has appealed for information to help find the attackers - including CCTV footage from the area.
"This could be anybody's child, whoever is doing this clearly wants to hurt someone," Ms McMahon said.
Police have also asked for anyone with information to come forward.
The attack comes after campaigners in Bradford have been calling for tougher rules against the misuse of fireworks.
It is illegal to light fireworks after 23:00, but residents say they regularly go off in the middle of the night causing "chaos" and scaring animals, children and the elderly.