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CVE-2023-38691: Merge pull request from GHSA-vc7j-h8xg-fv5x · matrix-org/matrix-appservice-bridge@4c6723a
matrix-appservice-bridge provides an API for setting up bridges. Starting in version 4.0.0 and prior to versions 8.1.2 and 9.0.1, a malicious Matrix server can use a foreign user’s MXID in an OpenID exchange, allowing a bad actor to impersonate users when using the provisioning API. The library does not check that the servername part of the sub parameter (containing the user’s claimed MXID) is the the same as the servername we are talking to. A malicious actor could spin up a server on any given domain, respond with a sub parameter according to the user they want to act as and use the resulting token to perform provisioning requests. Versions 8.1.2 and 9.0.1 contain a patch. As a workaround, disable the provisioning API.
Expand Up @@ -388,6 +388,16 @@ export class ProvisioningApi { throw new ApiError("Server did not respond with the correct sub information", ErrCode.BadOpenID); } const userId = response.data.sub;
const mxidMatch = userId.match(/([^:]+):(.+)/); if (!mxidMatch) { throw new ApiError("Server did not respond with a valid MXID", ErrCode.BadOpenID); } const [, _localpart, serverName] = mxidMatch; if (serverName !== server) { throw new ApiError("Server returned a MXID belonging to another homeserver", ErrCode.BadOpenID); }
const token = this.widgetTokenPrefix + uuid().replace(/-/g, “”); const expiresTs = Date.now() + this.widgetTokenLifetimeMs; await this.store.createSession({ Expand Down
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### Impact A malicious Matrix server can use a foreign user's MXID in an OpenID exchange, allowing a bad actor to impersonate users when using the provisioning API. ### Details The library does not check that the servername part of the `sub` parameter (containing the user's *claimed* MXID) is the same as the servername we are talking to. A malicious actor could spin up a server on any given domain, respond with a `sub` parameter according to the user they want to act as and use the resulting token to perform provisioning requests. ### Workarounds Disable the provisioning API. If the bridge does not use the provisioning API, you are not vulnerable.