What does Windows error code 191 (ERROR_INVALID_EXE_SIGNATURE) mean?

ERROR_INVALID_EXE_SIGNATURE

What ERROR_INVALID_EXE_SIGNATURE means

ERROR_INVALID_EXE_SIGNATURE is Win32 system error code associated with the documented message: “Cannot run %1 in Win32 mode.” The significant condition is that the file does not carry a valid executable signature for the way Windows is trying to launch it.

Likely causes

• a document, script, or truncated download was renamed with an executable extension
• the DOS stub points to a missing or malformed PE/NE header
• transfer or storage corruption damaged the signature bytes

Troubleshooting steps

1. Inspect the first bytes and the header offset with a binary-aware tool
2. Compare file size and hash against the trusted distribution
3. Verify that the launching code selected the intended file rather than a similarly named data file

Code-specific investigation notes

The DOS MZ signature alone is not sufficient for a Win32 image. The loader follows the header offset to the secondary signature, which must also be present and in bounds.

Web gateways and deployment scripts sometimes replace blocked executables with HTML error pages while retaining the original filename. Inspect content and size before assuming binary corruption.

Hash comparison should use a trusted manifest or signed package. Comparing two copies from the same damaged cache does not establish integrity.

Difference from related errors

ERROR_EXE_MARKED_INVALID means the executable is explicitly marked unusable; it means the expected image signature cannot be validated.

References

• Microsoft: System error codes 0–499