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

### 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 cach

CVSS 3.1

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

Severity from

GitHub (reviewed advisory)

Weakness

CWE-284

Also known as

CVE-2026-48529, GO-2026-5779

• github.com/github/github-mcp-server/security/advisories/GHSA-pjp5-fpmr-3349
• github.com/github/github-mcp-server