CVE-2026-48529

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

6.0 MediumFix available

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.

Attack profile

How easy it is to exploit

Attack vector: Network
Complexity: High
Privileges needed: Low
User interaction: None

What an attacker can affect

Confidentiality: Low
Integrity: Low
Availability: Low

Exploit likelihood

0.17%Up 0.1 points since Jun 12, 2026

What each source reports

CVE List Score used 6.0

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

Record: CVE-2026-48529
CVSS 3.1: 6.0 MediumCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L
Weakness: CWE-284
Updated: 2026-11-02

EUVDEUVD-2026-39809 6.0

Record: EUVD-2026-39809
CVSS 3.1: 6.0 MediumCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L
Updated: 2026-11-02

GitHub AdvisoriesGHSA-pjp5-fpmr-3349 6.0

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:

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:

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:

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

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…

Record: GHSA-pjp5-fpmr-3349
CVSS 3.1: 6.0 MediumCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L
Weakness: CWE-284
Updated: 2026-11-02

NVD 6.0

Record: CVE-2026-48529
CVSS 3.1: 6.0 MediumCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L
Weakness: CWE-284
Updated: 2026-11-02

OSV 6.0

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

Record: CVE-2026-48529
CVSS 3.1: 6.0 MediumCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L
Weakness: CWE-284
Updated: 2026-11-02

OSVGHSA-pjp5-fpmr-3349 6.0

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

Summary

Details

The singleton is defined in pkg/lockdown/lockdown.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:

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
}

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:

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

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:

Second, repository visibility and collaborator access data stored in the…

Record: GHSA-pjp5-fpmr-3349
CVSS 3.1: 6.0 MediumCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L
Weakness: CWE-284
Updated: 2026-11-02

OSVGO-2026-5779 No score

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

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

Record: GO-2026-5779
Updated: 2026-11-02

OSVopenSUSE-SU-2026:21483-1 No score

Security update for govulncheck-vulndb

This update for govulncheck-vulndb fixes the following issues:

Update to version 0.0.20260723T184607 2026-07-23T18:46:07Z (jsc#PED-11136).

Go CVE Numbering Authority IDs added or updated with aliases:

GO-2026-4730 CVE-2026-33022 GHSA-cv4x-93xx-wgfj
GO-2026-4761 CVE-2026-33211 GHSA-j5q5-j9gm-2w5c
GO-2026-5102 CVE-2026-45021 GHSA-3vcp-chfh-f6r2
GO-2026-5272 CVE-2026-40938 GHSA-94jr-7pqp-xhcq
GO-2026-5486 CVE-2026-40924 GHSA-m2cx-gpqf-qf74
GO-2026-5542 CVE-2026-40090 GHSA-pj97-4p9w-gx3q
GO-2026-5630 CVE-2026-25542 GHSA-rmx9-2pp3-xhcr
GO-2026-5643 CVE-2026-40923 GHSA-rx35-6rhx-7858
GO-2026-5675 CVE-2026-41492 GHSA-vvf7-6rmr-m29q
GO-2026-5711 CVE-2026-40161 GHSA-wjxp-xrpv-xpff
GO-2026-5746 CVE-2026-41567 GHSA-x86f-5xw2-fm2r
GO-2026-5749 CVE-2026-41328 GHSA-x92x-px7w-4gx4
GO-2026-5837 CVE-2026-44840 GHSA-q2m9-6jp9-c6mc
GO-2026-5929 CVE-2026-44342 GHSA-26v7-h57m-gh9m
GO-2026-5930 CVE-2026-33655 GHSA-6qcr-qxgr-m7fv
GO-2026-5933 GHSA-3g4q-2f67-2gvh
GO-2026-5934 GHSA-59qp-cfj3-rp64
GO-2026-5935 GHSA-7856-g3gv-9wq8
GO-2026-5936 CVE-2026-53552 GHSA-26rh-24rg-j3vv
GO-2026-5937 CVE-2026-53508 GHSA-2jcc-mxv7-p3f9
GO-2026-5938 CVE-2026-53553 GHSA-4g5x-hcwm-82jw
GO-2026-5939 CVE-2026-10517 GHSA-698x-9w2p-7vvp
GO-2026-5940 CVE-2026-53572 GHSA-6w3m-4hhp-775q
GO-2026-5941 CVE-2026-53487 GHSA-gvhc-wv3v-7pf8
GO-2026-5942 CVE-2026-46600
GO-2026-5944 CVE-2026-50197 GHSA-659f-rgp5-w4wf
GO-2026-5945 CVE-2026-6815 GHSA-rmxx-v9rj-vpvg
GO-2026-5946 CVE-2026-52831 GHSA-v5px-423j-pf7p
GO-2026-5947 CVE-2026-53649 GHSA-xqhv-chqm-fhcc
GO-2026-5948 CVE-2026-50554 GHSA-588f-fvcv-xhvf
GO-2026-5949 CVE-2026-50553 GHSA-rqrh-8wpv-x7hh
GO-2026-5950 CVE-2026-53602 GHSA-339v-266x-79xr
GO-2026-5951 CVE-2026-10219 GHSA-6jm8-4fhr-5w64
GO-2026-5952 CVE-2026-49834 GHSA-9vcr-p3rj-q5q6
GO-2026-5953 CVE-2026-9094 GHSA-c9w5-qp6m-m395
GO-2026-5956 CVE-2026-55252 GHSA-h5g6-xmh4-hc37
GO-2026-5957 CVE-2026-54158 GHSA-5xfx-xj4h-5p7r
GO-2026-5958 CVE-2026-54068 GHSA-gcm7-57gf-953c
GO-2026-5959 CVE-2026-54072 GHSA-h29v-hj44-q8cv
GO-2026-5961 CVE-2026-54069 GHSA-hvr9-72v2-fff3
GO-2026-5962 CVE-2026-54088 GHSA-m93h-4hw7-5qcm
GO-2026-5963 CVE-2026-54067 GHSA-mvjr-vv3c-w4qv
GO-2026-5964 CVE-2026-54066 GHSA-p4m3-mgmm-c664
GO-2026-5965 CVE-2026-54070 GHSA-w7cg-whh7-xp28
GO-2026-5966 CVE-2026-54089 GHSA-xqp3-jq6g-x3qm
GO-2026-5967 CVE-2026-50551 GHSA-56mp-4f3v-fgj2
GO-2026-5968 CVE-2026-54174 GHSA-fpg8-7664-jc5q
GO-2026-5969 GHSA-g936-7jqj-mwv8
GO-2026-5970 CVE-2026-56852
GO-2026-5973 CVE-2026-54250 GHSA-jxr7-mqhw-9p98
GO-2026-5974 CVE-2026-44300 GHSA-wmj8-9953-vff5
GO-2026-5975 CVE-2026-50018 GHSA-42j2-w334-qxw7
GO-2026-5976 CVE-2026-50141 GHSA-g7mm-9vx7-jm7h
GO-2026-5977 CVE-2026-50013 GHSA-qrh4-p6v4-mrfg
GO-2026-5978 CVE-2026-50125 GHSA-qw5r-ppcg-f8rj
GO-2026-5979 CVE-2026-50006 GHSA-xrcf-6jh3-ggvx
GO-2026-5980 CVE-2026-50158 GHSA-2c7f-fxww-6w6c
GO-2026-5981 GHSA-mqxv-9rm6-w8qc
GO-2026-5982 GHSA-pqg7-v6wh-3pfp
GO-2026-5984 CVE-2026-53604 GHSA-2p2f-px33-4vv5
GO-2026-5985 GHSA-7rx3-5wx3-5v76
GO-2026-5986 CVE-2026-61699 GHSA-cm26-5974-52h8
GO-2026-5987 CVE-2026-55513 GHSA-g4x6-jcvr-9m3g
GO-2026-5988 CVE-2026-54628 GHSA-hwrq-8wxh-q4xv
GO-2026-5989 CVE-2026-55512 GHSA-m3cx-mwpg-32jg
GO-2026-5990 CVE-2026-54629 GHSA-mf78-3rpf-r784
GO-2026-5991 CVE-2026-53603 GHSA-q4vm-pq3q-8wgq
GO-2026-5992 CVE-2026-61549 GHSA-qf34-295c-26v8
GO-2026-5996 CVE-2026-54450 GHSA-pph6-vfjv-vpjw
GO-2026-5997 CVE-2026-54452 GHSA-xgch-x3mx-cm3c
GO-2026-5998 CVE-2026-54495 GHSA-398h-7f66-3h4p
GO-2026-5999 CVE-2026-10814 GHSA-jh6h-v6mp-h22v
GO-2026-6002 CVE-2026-58196 GHSA-pr64-jmmf-jp54
GO-2026-6003 CVE-2026-53714 GHSA-22xc-xg2r-9j7v
GO-2026-6004 CVE-2026-52833 GHSA-3v79-m2cg-89ww
GO-2026-6005 CVE-2026-53715 GHSA-8fv2-88gg-hm7q
GO-2026-6006 CVE-2026-53716 GHSA-cxpq-8v7q-cg56 *…

Record: openSUSE-SU-2026:21483-1
Updated: 2026-11-02

Affected products

github github-mcp-server

Affected packages

Affected packages
Package	Affected	Fixed
`github.com/github/github-mcp-server` Go	0.22.0 up to 1.1.2	1.1.2
`govulncheck-vulndb` openSUSE:Leap 16.0 From an advisory covering 1734 CVEs	before 0.0.20260723T184607-160000.1.1	0.0.20260723T184607-160000.1.1

History

History
Recorded	Changed	Sources
Nov 6, 2026	affected, facts
Nov 2, 2026	first indexed	CVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-48529 is a medium vulnerability (CVSS 6.0) in Github Github-mcp-server, published June 25, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.17%, the 5th percentile. Fixed in github.com/github/github-mcp-server 1.1.2.

References

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