ERROR_ILLEGAL_DLL_RELOCATION - 623 (0x26F)
{Illegal System DLL Relocation} The system DLL %hs was relocated in memory. The application will not run properly. The relocation occurred because the DLL %hs occupied an address range reserved for Windows system DLLs. The vendor supplying the DLL should be contacted for a new DLL.
Updated: Feb 21, 2026
Technical Background
The ERROR_ILLEGAL_DLL_RELOCATION error code (623, 0x26F) indicates that a system Dynamic Link Library (DLL) has been relocated in memory, which is not allowed. This error typically occurs when an application attempts to load or use a DLL that conflicts with the reserved address space of Windows system DLLs.
Error Details
Cause
The relocation occurred because the DLL occupied an address range reserved for Windows system DLLs. This can happen if the vendor supplying the DLL has incorrectly implemented the loading mechanism, leading to an illegal memory access or conflict.
Impact
When this error occurs, the application will not run properly due to the invalid memory layout of the DLL. The system may terminate the application or display a message indicating that the DLL is incompatible with the current environment.
Common Causes
- Incorrect implementation by the vendor supplying the DLL
- Attempting to load a user-defined DLL into an address space reserved for system use
- Incompatible version of the DLL being used
Real-World Context
This error can occur in various scenarios, such as when integrating third-party software or updating system components. It is critical to ensure that all DLLs are compatible with the operating environment and do not conflict with system-reserved memory regions.
Is This Error Critical?
Yes, this error can lead to application failure and instability if not addressed properly. It is essential to handle such errors gracefully and provide appropriate feedback to users or developers.
How to Diagnose
Reviewing Operation Context
- Verify the version of the DLL being used
- Check for any recent updates or installations that may have caused this issue
- Ensure that no other applications are using conflicting memory regions
Validating Parameters
- Confirm that all parameters passed to the application are valid and do not contain invalid pointers or addresses
- Validate the integrity of the DLL by checking its checksum against known good values
How to Resolve
Correct Parameter Usage
- Ensure that all parameters used in loading the DLL are correct and do not conflict with system-reserved memory regions
- Use appropriate APIs for loading DLLs, such as
LoadLibraryorGetProcAddress, which handle memory management correctly
Adjust Operation Context
- If possible, update the application to use a different version of the DLL that does not conflict with system memory
- Contact the vendor supplying the DLL and request an updated version that is compatible with the current environment
Developer Notes
Developers should be cautious when integrating third-party libraries or modifying existing code. Ensure that all components are thoroughly tested in the target environment to avoid such issues.
Related Errors
- ERROR_DLL_NOT_FOUND (193)
- ERROR_INVALID_PARAMETER (1208)
- ERROR_BAD_EXE_FORMAT (193)
FAQ
Q: What does the ERROR_ILLEGAL_DLL_RELOCATION error mean?
A: This error indicates that a system DLL has been relocated in memory, causing an application to fail due to a conflict with reserved address space.
Q: How can I prevent this error from occurring?
A: Ensure that all third-party libraries and custom DLLs are compatible with the operating environment. Use appropriate APIs for loading DLLs and verify their integrity before use.
Summary
The ERROR_ILLEGAL_DLL_RELOCATION error (623) is a specific issue related to memory management in Windows applications. It occurs when a user-defined DLL conflicts with reserved system memory regions, leading to application failure. Proper handling of this error requires careful validation and coordination between the application and its dependencies.