This work studies time security of ten popular package managers. These package managers use different security mechanisms that provide varying levels of usability and resilience to attack. We find that, despite their existing security mnechanisms, all of these package managers have vulnerabilities that can be exploited by a man-in-the-middle or a malicious mirror. While all current package managers suffer frons vulnerabilities. their security is also positively or negatively impacted by the distribution's security practices. Weaknesses in package managers are more easily exploited when distributions use third-party nurrors as official mirrors. We were successful in using false credentials to obtain an official nurror on all five of the distributions we attempted. We also found that some security mechanisms that control where a client obtains metadata and packages frotn may actually decrease security. We analyze current package managers to show that by exploiting vulnerabilities, an attacker with a mirror can compromise or crash hundreds to thousands of clients weekly. The problemns we disclose are now being corrected by many different package manager maintainers.