What is the MSP file format?
An .msp file is a Windows Installer Patch Package - a binary delta encoding the difference between an original MSI application installation and its updated version. Introduced with Windows Installer 1.5 in Windows 2000, .msp has been the standard distribution format for application patches across Microsoft products including Office, SQL Server, and Visual Studio for over two decades.
Structurally an .msp file is an OLE2 Compound File Binary, the same container used by legacy Office documents and MSI packages. Its root storage contains a SummaryInformation stream with patch metadata - description, target product codes, revision number - and one sub-storage per patched product GUID. Each sub-storage holds MST (transform) streams describing database table changes, plus embedded CAB (Cabinet) archives carrying binary-delta-patched or entirely new files using the MSPATCH encoding algorithm.
Patches are applied by Windows Installer: msiexec.exe /p patch.msp. An .msp file cannot be run directly - it must reference an already-installed product matching its embedded target GUID. Applied patches are tracked in the Windows Installer database and can be uninstalled if flagged as removable. Microsoft digitally signs its patches with Authenticode; the signature lives in the DigitalSignature and MsiDigitalSignatureEx streams.
For OS component updates, .msp has been supplemented by the .msu (Windows Update Standalone Package) format since Windows Vista, but .msp remains the norm for enterprise application patches.
Security & safety
RISK: MEDIUMAn MSP applies executable changes to installed software. Only apply MSPs from trusted sources (Microsoft, verified software vendors). Malicious MSPs could modify system software or introduce backdoors. Always verify Authenticode digital signatures before applying. MSPs from untrusted download sites may be tampered.
Format details
in a nutshellPrograms that open MSP files
Windows5 apps
7-Zip
Open-source Right-click → Open archive - can browse OLE/CAB contents of MSP for inspection (read-only). 
Windows Installer (msiexec.exe)
Built-in Apply patch: msiexec /p patch.msp [/qb for quiet UI]. Double-click also works if target product is installed.
Advanced Installer
Freemium Can create and inspect MSP patch packages; GUI-based alternative to WiX/Orca. Technical details
deep spec| Container format | OLE2 Compound File Binary (CFBF / Structured Storage) |
| File signature | D0 CF 11 E0 A1 B1 1A E1 (OLE2 signature, offset 0) |
| Byte order | Little-endian |
| Patch encoding | MSPATCH binary delta algorithm for file-level differences |
| Sub-file container | CAB (Cabinet) archives embedded as OLE streams |
| Database changes | MST (transform) streams, one per patched product GUID |
| Digital signature | Authenticode in DigitalSignature / MsiDigitalSignatureEx streams (optional but standard for Microsoft patches) |
| Application command | msiexec.exe /p patch.msp |
| Patch metadata stream | SummaryInformation (product codes, description, revision number) |
| Typical size | 100 KB - 500 MB |
| Associated OS | Windows (Windows 2000 and later) |
| MIME type | application/octet-stream |
| Patch tracking | Applied patches recorded in Windows Installer database; removable if flagged as uninstallable |
| Released | 2000 (Windows Installer 1.5, Windows 2000) |
| Latest version | Windows Installer 5.0 (Windows 7 / Server 2008 R2, 2009) |
| Specification | learn.microsoft.com |
MSP conversions
Community Q&A
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