GHSA-c279-989m-238f: Sliver Nil Pointer Dereference Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-c279-989m-238f",
  "modified" : "2026-03-29T15:25:42Z",
  "published" : "2026-03-29T15:25:42Z",
  "aliases" : [ ],
  "summary" : "Sliver: Nil Pointer Dereference in tunnelCloseHandler causes panic when a reverse tunnel (rportfwd) close is attempted",
  "details" : "### Summary\nA nil pointer dereference in `tunnelCloseHandler` causes the handler goroutine to panic whenever a reverse tunnel (rportfwd) close is attempted. Both the legitimate close path AND the unauthorized close path dereference `tunnel.SessionID` where `tunnel` is guaranteed nil. This means rportfwd tunnels can never be cleanly closed, and any authenticated implant can trigger repeated goroutine panics.\n\n### Details\nFile: `server/handlers/sessions.go` lines 172 and 175\n\nThe function enters an `else` block precisely because `core.Tunnels.Get(tunnelData.TunnelID)` returned `nil`. Both conditions inside that else block then dereference `tunnel.SessionID` instead of `rtunnel.SessionID`:\n```go\n} else {\n    rtunnel := rtunnels.GetRTunnel(tunnelData.TunnelID)\n\n    if rtunnel != nil && session.ID == tunnel.SessionID {      // LINE 172 — nil deref\n        rtunnel.Close()\n        rtunnels.RemoveRTunnel(rtunnel.ID)\n    } else if rtunnel != nil && session.ID != tunnel.SessionID { // LINE 175 — nil deref\n        sessionHandlerLog.Warnf(\"...\")\n    }\n}\n```\n\nNote: The identical bug was already fixed in `tunnelDataHandler` at lines 124/126 (correctly uses `rtunnel.SessionID`), but the fix was \nnot applied to `tunnelCloseHandler`.\n\n### PoC\n```go\ntunnel := GetTunnel(999)   // returns nil — no normal tunnel with this ID\n// tunnel is nil here\n\nrtunnel := GetRTunnel(999) // returns valid rtunnel owned by session-AAAA\n\n// Both lines below panic with:\n// runtime error: invalid memory address or nil pointer dereference\nif rtunnel != nil && sessionID == tunnel.SessionID { ... }      // line 172\n} else if rtunnel != nil && sessionID != tunnel.SessionID { ... } // line 175\n```\n\nConfirmed on master commit `7ac4db3fa` with standalone reproducer.\nOutput:\n```\nPANIC on line 172 (legitimate close): runtime error: invalid memory address or nil pointer dereference\nPANIC on line 175 (unauthorized close): runtime error: invalid memory address or nil pointer dereference\n```\n\n![1](https://github.com/user-attachments/assets/93b24286-3282-454f-80a4-b01abe4f1d63)\n![2](https://github.com/user-attachments/assets/d4219aea-eb18-474c-b69a-a5e20e97161f)\n![3](https://github.com/user-attachments/assets/5a76b0d7-ae5b-4d91-bfe9-730d3e5c322c)\n\n### Impact\n- rportfwd tunnels **cannot be closed** — functional regression\n- Any authenticated implant can trigger repeated handler goroutine panics\n- rtunnel map entries leak (never cleaned up on close failure)\n- `recoverAndLogPanic()` prevents full server crash but silently drops the close operation\n\n### Fix\nReplace `tunnel.SessionID` with `rtunnel.SessionID` on both lines:\n```diff\n-  if rtunnel != nil && session.ID == tunnel.SessionID {\n+  if rtunnel != nil && session.ID == rtunnel.SessionID {\n       rtunnel.Close()\n       rtunnels.RemoveRTunnel(rtunnel.ID)\n-  } else if rtunnel != nil && session.ID != tunnel.SessionID {\n+  } else if rtunnel != nil && session.ID != rtunnel.SessionID {\n```",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/bishopfox/sliver"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "1.7.3"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/BishopFox/sliver/security/advisories/GHSA-c279-989m-238f"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/BishopFox/sliver"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-476" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-29T15:25:42Z",
    "nvd_published_at" : null
  }
}