Downloaded files are corrupted. Why does it happen?

When IDM downloads a file using several connections, It requests several bytes more for every file part to match the adjacent file part data which was downloaded by another connection. When IDM resumes a download, it falls back on several bytes and compares the beginning on new data with the end of old data downloaded earlier. When the data does not match, IDM re-downloads the non-matching file part from the beginning. When the data does not match in several connections, IDM starts downloading the file from a scratch using one connection.

The corruption of data may happen because of the following reasons:

1. Such problems may occur because of conflicts between IDM and other applications. For example it can be some FireWall/Antivirus/Internet security application that controls network downloading processes and writing data to disks. Such programs interfere in opening network connections and in file creation processes. When IDM wants to write received data to a file, security programs first pass all data through their internal buffers where they scan data for possible viruses. When IDM opens several files quickly in one thread, some firewalls and antiviruses mix the data of different connections, and thus they mix file parts which leads to data corruption. In particular we confirmed such problem with some versions of Eset NOD32 or Eset Smart Security.

Users reported successful problem resolution after uninstalling a security program. Also problem disappeared if they set 1 connection in "Options→Connection" IDM dialog. Then FireWall/Antivirus did not damage files.

If you have Eset NOD32 you can also try the following recommendation from Eset NOD32:

Also note that IDM cannot in any way bypass a security program when downloading or saving files, and data corruption does not depend on IDM.

Many firewall and antivirus programs give IDM maximum access rights on downloading and writing to disks during first IDM installation. But after IDM updates, security software may see that IDM executable file has changed and starts to block access or turns on detailed download/write verification without user notification. In this case customers reported high CPU load, timeouts, and several customers reported data corruption. You can read more about it here:

Also some FireWalls, Antiviruses or Internet security may be not ready for Windows 7, Windows 8/8.1, Windows 10 or Windows 11 and work incorrectly. Or maybe some application is stable with Win 32 bit system but not fully compatible with Win 64 bit system.

Please try to TEMPORARY completely uninstall your FireWall/Antivirus/Internet security application. Note that even disabling the security program may not help because they still have their driversb in memory which do this job in background. Then look if your problem persists. If it is solved, then problem is definitely caused by that application. ou can try to contact its developers and ask them for solution or try some other FireWall/Antivirus/Internet security application.

If it does not help, maybe you have some application that functions like FireWall/Antivirus/Internet security and you do not even know about it. It may have something like "Protector", "Defender", "Spyware Remover" or "Cleaner" in its name. If you find such application on your computer, try to uninstall it as well and look if your problem persists.

2. Different "Internet accelerators" or "Video Accelerators" may also interfere in a download process. They also may cause the same problems just like security software. For example problem appears if you enable "Video Accelerator" setting in AVG 2012 antivirus.

Same problems are possible with the following applications:

  • Venturi Firewall
  • WinMount
  • PPLive
  • NVIDIA network access manager
  • NVIDIA NTune
  • IS3 Anti Spyware
  • SpeedBit Video Accelerator
  • Folder Guide
  • Proxifier 3.15
  • ESET Smart security

3. Such problems are possible with proxy servers either set in IDM or transparent proxy servers that can be used by your Internet provider. In this case try to change the proxy server, try another Internet provider, or use VPN.

4. If IDM downloads from the majority of sites without a problem, but has a problem with specific site(s), then the problem is in the site(s). There are sites which send incorrect data on resume or always send files from scratch. In this case please contact site(s) administrators.

5. This problem is possible if you used some crack for IDM (read details). After installing the genuine version of IDM and registering it with a valid serial number problem should disappear.