CCleaner (/ˈsiːˌkliːnər/; originally meaning "Crap Cleaner"),[6] developed by Piriform Software, is a utility used to clean potentially unwanted files and invalid Windows Registry entries from a computer.

CCleaner can delete potentially unwanted files left by certain programs, including Microsoft Edge, Internet Explorer, Firefox, Google Chrome, Opera, Safari, Windows Media Player, eMule, Google Toolbar, Netscape, Microsoft Office, Nero, Adobe Acrobat, McAfee, Adobe Flash Player, Sun Java, WinRAR, WinAce, WinZip and GIMP[9] along with browsing history, cookies, recycle bin, memory dumps, file fragments, log files, system caches, application data, autocomplete form history, and various other data.

The browser, built on Google's free and open-source project Chromium, includes built-in ad blocking and anti-tracking features.

[26] Upon an error in the code, the Active Monitoring component of CCleaner 5.45, which was designed to measure junk levels to trigger cleaning, switched back on again.

Piriform recognized this error and confirmed to users that the Active Monitoring feature did not report data.

[31] In July 2020, Microsoft Windows Defender began flagging the free version of CCleaner as a "potentially unwanted application", stating that "while the bundled applications themselves are legitimate, bundling of software, especially products from other providers, can result in unexpected software activity that can negatively impact user experiences.

Forty of the infected machines received a second-stage payload that appears to have targeted technology companies Samsung, Sony, Asus, Intel, VMWare, O2, Singtel, Gauselmann, Dyn, Chunghwa and Fujitsu.

[38] On 21 October 2019, Avast disclosed a second security breach during which attackers tried again to insert malware inside CCleaner releases.