GitHub MCP Server: Lockdown mode singleton in HTTP server causes cross-user GraphQL client confusion

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GitHub MCP Server is GitHub's official MCP Server. From 0.22.0 until 1.1.2, when running in HTTP mode with --lockdown-mode enabled, the RepoAccessCache is implemented as a process-global singleton initialized with the first authenticated user's GraphQL client. All subsequent requests from different users share this singleton and their lockdown-related GraphQL queries are executed using the first user's credentials. The singleton is never updated to reflect later users' tokens. This vulnerability is fixed in 1.1.2.

Source published

26 Jun 2026 16:33:32 UTC

Source modified

27 Jun 2026 02:43:34 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 6.0 CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L

GitHub MCP Server is GitHub's official MCP Server. From 0.22.0 until 1.1.2, when running in HTTP mode with --lockdown-mode enabled, the RepoAccessCache is implemented as a process-global singleton initialized with the first authenticated user's GraphQL client. All subsequent requests from different users share this singleton and their lockdown-related GraphQL queries are executed using the first user's credentials. The singleton is never updated to reflect later users' tokens. This vulnerability is fixed in 1.1.2.

Source published

Jun 26, 2026, 4:33:32 PM (timezone not stated)

Source modified

Jun 27, 2026, 2:43:34 AM (timezone not stated)

Observed here

7 Nov 2026 11:30:12 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 6.0 CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L

GitHub MCP Server is GitHub's official MCP Server. From 0.22.0 until 1.1.2, when running in HTTP mode with --lockdown-mode enabled, the RepoAccessCache is implemented as a process-global singleton initialized with the first authenticated user's GraphQL client. All subsequent requests from different users share this singleton and their lockdown-related GraphQL queries are executed using the first user's credentials. The singleton is never updated to reflect later users' tokens. This vulnerability is fixed in 1.1.2.

Source published

26 Jun 2026 17:16:33 UTC

Source modified

25 Oct 2026 17:10:00 UTC

Observed here

27 Oct 2026 13:11:53 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 6.0 CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L

### Summary When running in HTTP mode with --lockdown-mode enabled, the RepoAccessCache is implemented as a process-global singleton initialized with the first authenticated user's GraphQL client. All subsequent requests from different users share this singleton and their lockdown-related GraphQL queries are executed using the first user's credentials. The singleton is never updated to reflect later users' tokens. ### Details The singleton is defined in pkg/lockdown/lockdown.go: ```go var ( instance *RepoAccessCache instanceMu sync.Mutex ) func GetInstance(client *githubv4.Client, opts ...RepoAccessOption) *RepoAccessCache { instanceMu.Lock() defer instanceMu.Unlock() if instance == nil { instance = &RepoAccessCache{ client: client, // only stored on first call } } return instance // subsequent callers receive the same object regardless of their client } ``` In HTTP mode, pkg/github/dependencies.go calls this per request: ```go func (d *RequestDeps) GetRepoAccessCache(ctx context.Context) (*lockdown.RepoAccessCache, error) { gqlClient, err := d.GetGQLClient(ctx) // creates client with request's token ... instance := lockdown.GetInstance(gqlClient, d.RepoAccessOpts...) // gqlClient is silently dropped if singleton already exists return instance, nil } ``` The singleton's internal client field is never updated after the first initialization. All lockdown GraphQL queries that check repository access and visibility (queryRepoAccessInfo, called by IsSafeContent) run under the first authenticated user's token for the lifetime of the process. IsSafeContent is called in at least six places across pkg/github/issues.go and pkg/github/pullrequests.go to decide whether to trust or sanitize content from external contributors. ### PoC The following program demonstrates that two distinct GraphQL clients produce the same singleton pointer, confirming that the second client is discarded: ```go package main import ( "fmt" "net/http" "github.com/github/github-mcp-server/pkg/lockdown" "github.com/shurcooL/githubv4" ) func main() { httpClientA := &http.Client{} httpClientB := &http.Client{} gqlClientA := githubv4.NewEnterpriseClient("https://api.github.com/graphql", httpClientA) gqlClientB := githubv4.NewEnterpriseClient("https://api.github.com/graphql", httpClientB) fmt.Printf("gqlClientA (user A token): %p\n", gqlClientA) fmt.Printf("gqlClientB (user B token): %p\n", gqlClientB) fmt.Printf("clients are different objects: %v\n\n", gqlClientA != gqlClientB) instanceForA := lockdown.GetInstance(gqlClientA) instanceForB := lockdown.GetInstance(gqlClientB) fmt.Printf("lockdown instance returned for user A: %p\n", instanceForA) fmt.Printf("lockdown instance returned for user B: %p\n", instanceForB) fmt.Printf("same singleton returned for both users: %v\n", instanceForA == instanceForB) } ``` Output: ``` gqlClientA (user A token): 0x400044070 gqlClientB (user B token): 0x400044078 clients are different objects: true lockdown instance returned for user A: 0x400002ecc0 lockdown instance returned for user B: 0x400002ecc0 same singleton returned for both users: true ``` <img width="1642" height="450" alt="image" src="https://github.com/user-attachments/assets/bec46420-9ba7-458e-8710-62f951cb836a" /> ### Impact This affects deployments running the HTTP server with --lockdown-mode, which is the intended configuration for multi-user scenarios such as GitHub Copilot's managed MCP endpoint. Three concrete consequences: First, the ViewerLogin field in cache entries always reflects the first authenticated user's identity. The IsSafeContent check `repoInfo.ViewerLogin == strings.ToLower(username)` compares this stale value against each subsequent user's login, producing incorrect results for all users except the first. Second, repository visibility and collaborator access data stored in the cache is evaluated through the first user's token. If user A cannot see a private repository but user B can (or vice versa), the cached isPrivate and hasPushAccess values will reflect user A's view of that repository, causing IsSafeContent to return wrong decisions for user B. In lockdown mode, a wrong true result means potentially injected content from untrusted external contributors is passed to the model without sanitization. Third, if the first user's token is revoked or expires, all subsequent lockdown GraphQL queries fail with authentication errors. Since getRepoAccessInfo propagates these errors, IsSafeContent returns an error for every request, breaking lockdown protection for all users until the process is restarted.

Source published

25 Jun 2026 21:32:09 UTC

Source modified

7 Jul 2026 16:11:35 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L

Publisher rating · MODERATE

GitHub MCP Server: Lockdown mode singleton in HTTP server causes cross-user GraphQL client confusion in github.com/github/github-mcp-server

Source published

7 Jul 2026 15:26:32 UTC

Source modified

7 Jul 2026 16:00:20 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.