The United Nations has been grappling with so many sexual abuse allegations involving its peacekeepers that Secretary-General Ban Ki Moon recently called them “a cancer in our system.” Now, officials have learned about what appears to be a fresh scandal. Investigators discovered this month that at least four UN peacekeepers in the Central African Republic allegedly paid young girls as little as 50 cents in exchange for sex. The case is the latest to plague the UN mission in the Central African Republic, whose employees have been accused of 22 other incidents of alleged sexual abuse or sexual exploitation in the past 14 months. The most recent accusations come in the wake of Ban’s efforts to implement a “zero tolerance” policy for such offenses. As the United Nations maintains nine peacekeeping operations in Africa, employing over 100,000 people on the continent, the abuses threaten to erode the organization’s legitimacy. Other sex-crime cases have occurred in Mali, South Sudan, Liberia and Democratic Republic of Congo in recent years. Last month, the United Nations published a damning independent investigation that said poor enforcement of policies in place to deter and report abuse meant that “the credibility of the UN and peacekeeping operations are in jeopardy.” Experts and officials say systemic problems still hinder the investigation and prosecution of alleged abusers, leading to the perception of impunity within UN ranks. The abuse “undermines everything we stand for,” said Anthony Banbury, the UN assistant secretary-general for field support. The mission in the Central African Republic, where UN troops and civilians were sent in 2014 to help end a civil war and support a fledgling government, stands out for its record of sexual abuse and exploitation. “They are preying on the people they’ve come to protect,” said Parfait Onanga-Anyanga, the top UN official in the country.