Exploit proof-of-concepts (PoCs) for known vulnerabilities are widely shared in the security community. They help security analysts to learn from each other and they facilitate security assessments and red teaming tasks. In the recent years, PoCs have been widely distributed, e.g., via dedicated websites and platforms, and public code repositories such as GitHub. However, there is no guarantee that PoCs in public code repositories come from trustworthy sources or even that they do what they are supposed to do. In this work we investigate GitHub-hosted PoCs for known vulnerabilities discovered in 2017--2021. We discovered that not all PoCs are trustworthy. Some proof-of-concepts are malicious, e.g., they attempt to exfiltrate data from the system they are being run on, or they try to install malware on this system, and in some cases they have hard-coded reverse shell listener. To measure the prevalence of this threat, we propose an approach to detecting malicious PoCs. Our approach relies on the maliciousness symptoms we have observed in our PoC dataset: calls to malicious IP addresses, encoded malicious code, and included Trojanized binaries. With this approach, we have discovered 899 malicious repositories out of 47,285 repositories that have been downloaded and checked (i.e., 1.9% of the studied repositories have indicators of malicious intent). This figure shows a worrying prevalence of dangerous malicious PoCs among the exploit code distributed on GitHub.