Officially, slavery was abolished back in 1962 but a judge's decision to pass on the title deed of a "slave" from one master to another has blown the lid off the hidden bondage of hundreds of Yemenis.
The judge in the town of Hajja, which is home to some 300 slaves, according to residents, said he had certified the transfer only because the new owner planned to free the slave.
But his decision has triggered a campaign by local human right activists.
A 2009 report by the human rights ministry found that males and females were still enslaved in the provinces of Hudaydah and Hajja, in northwest Yemen -- the Arab world's most impoverished country.
Mubarak, who has seven brothers and sisters, has never set foot outside the village where he was born into a family which was inherited as slaves by their local master.
Sheikh Mohammed Badawi's father had bought Mubarak's parents 50 years ago, shortly before Yemen's 1962 revolution which abolished slavery. Mubarak has known no other life except that of a slave.
"Whenever I think of freedom, I ask myself, 'Where will I go?'" he told AFP as he stood outside a hut which serves as home for him and his family.
Black-skinned Mubarak does not know his birthday but he knows he has been a slave from birth 21 years ago. He has two children with a wife who was also a slave until she was emancipated by her master, a few years before they married.
"Sometimes I wonder what the fate of my children will be, having a slave father and an emancipated mother," he said.
Mubarak and his family are just one case among many.
"We are still in the process of trying to count the numbers of slaves," the coordinator of rights group Hood, Mohammed Naji Allaw, told AFP, explaining that slaves were "owned by title deeds, or inherited within families."
The news website almasdaronline earlier spoke of "500 slaves" across Yemen.
In addition to "slaves whose owner can use them however he wants," the ministry report also refers to other groups subjected to slave-like conditions, although they are not bound by documents.
Sometimes I wonder what the fate of my children will be, having a slave father and an emancipated mother
One group includes "former slaves who have been officially set free, but remain at the service of their former masters, who continue to feed them but never pay them wages," the report said.
Allaw said such people are still referred to as "the slaves of such and such a family, or the slaves of such and such a tribe."
former slaves who have been officially set free, but remain at the service of their former masters, who continue to feed them but never pay them wages
Human Rights ministry
Descendants of an ancient empire
Enslaved groups are descendants of an empire which ruled Yemen in the 11th and 12th centuries, with their origins in ancient Ethiopia, across the Red Sea from Yemen. They were enslaved after their empire was defeated.
Under Yemeni law, slavery carries stiff penalties.
"Whoever controls another human being" can face 10 years in prison under the penal code, said Allaw, who complained of state negligence and lack of social services to such groups.
The authorities do not want to get into a conflict with the powerful tribes, who form the backbone of Yemeni society, over the slavery issue, according to Allaw.
"Local authorities in Hajja are trying to black out this phenomenon, saying it does not exist," he said.
"But these people should be compensated," said the rights activist. "They should be given houses and be rehabilitated, socially and psychologically. They should be saved from their feeling of marginalisation."
Meanwhile, Mubarak dreams of living a normal life, though he doubts being capable of coping with it.
"I dream of living like other people ... (But) I have always known myself to do nothing but work on the farm and tend the cattle," he said.
Ashram, enslaved for 50 years before being freed five years ago by his dying master, appeared to have gone through what Mubarak fears.
"When my master Sheikh Ali Hussein told me 'I have freed you, Ashram,' I was happy," he told AFP. But soon after "I started wondering how to live, where to go, and how to make a living."
Ashram decided to revert to his old life, becoming a "slave of the village," he said. "I carry water daily to the houses from a well. This gives me some assurance that I will not die of starvation."