CVE-2026-49838: Github.com/osrg/gobgp/v4 GoBGP Confederation Validation Panics On Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-frrj-87jh-2772",
  "modified" : "2026-07-09T23:21:05Z",
  "published" : "2026-07-09T23:21:05Z",
  "aliases" : [ "CVE-2026-49838" ],
  "summary" : "GoBGP confederation validation panics on empty AS_PATH attribute",
  "details" : "Found through variant analysis based on `CVE-2026-41643`\n\n## Summary\nGoBGP accepts a zero-length AS_PATH during UPDATE decoding and later panics while validating that attribute for a confederation eBGP peer. The vulnerable path is in the BGP UPDATE validator: a malformed UPDATE that should be rejected as a malformed AS_PATH instead reaches an unchecked `p.Value[0]` access, allowing a configured confederation eBGP peer to trigger a denial of service.\n\n## Affected\n- Project: gobgp\n- Repo: https://github.com/osrg/gobgp\n- Pinned ref: c24629411ba49f160d9dc09126f418218127e016\n\n## Root cause\nAn established peer's receive path reads BGP bytes from the network connection in `pkg/server/fsm.go:1267`, parses UPDATE bodies through the BGP message decoder, and validates decoded UPDATEs with peer state at `pkg/server/fsm.go:1849`. The UPDATE decoder walks the path-attribute list in `pkg/packet/bgp/bgp.go:15773` and selects the concrete attribute parser from the attacker-controlled attribute type at `pkg/packet/bgp/bgp.go:15855`. For AS_PATH, `PathAttributeAsPath.DecodeFromBytes` returns nil when the decoded attribute length is zero (`pkg/packet/bgp/bgp.go:11533`, `pkg/packet/bgp/bgp.go:11538`), leaving `p.Value` empty rather than reporting a malformed attribute. Validation then dispatches each decoded attribute through `ValidateAttribute` (`pkg/packet/bgp/validate.go:34`); in the confederation eBGP branch, `pkg/packet/bgp/validate.go:162` indexes `p.Value[0]` before checking that any AS_PATH segment was decoded. The eBGP and confederation guards are normal peer-state gates: `pkg/config/oc/util.go:127` defines eBGP as peer AS differing from local AS, `pkg/config/oc/util.go:116` checks confederation membership, and `pkg/server/fsm.go:740` and `pkg/server/fsm.go:741` copy those results into the FSM state used by the validator.\n\n## Reproduction\n\n[INT-bgp-gobgp-confed-empty-aspath-panic.zip](https://github.com/user-attachments/files/28203698/INT-bgp-gobgp-confed-empty-aspath-panic.zip)\n\n```bash\nbash ./poc/run.sh\n```\n\n```text\nTRIGGERED: confed empty AS_PATH validation panic: runtime error: index out of range\n```\n\nThe `TRIGGERED` line is the recovered panic fingerprint from the confederation eBGP validation path after a zero-length AS_PATH has decoded successfully. A build failure or any output without that fingerprint would not demonstrate this bug, because the signal is tied to the unchecked AS_PATH segment access.\n\n## Impact\nA remote unauthenticated peer that is configured as a confederation eBGP neighbor can establish a BGP session and send a single malformed UPDATE containing a syntactically valid AS_PATH attribute header with zero value length. Because the decode path does not turn that empty AS_PATH into a `MessageError`, normal malformed-attribute handling is bypassed and validation panics before GoBGP can return a BGP NOTIFICATION. The demonstrated effect is denial of service for the receive goroutine and peer session, with potential process termination if the panic is not recovered by the runtime path; no memory corruption, data disclosure, authentication bypass, or code execution is claimed.\n\n## Suggested fix\n```001-fix.diff\ndiff --git a/pkg/packet/bgp/validate.go b/pkg/packet/bgp/validate.go\nindex 2237afb..f07f4fa 100644\n--- a/pkg/packet/bgp/validate.go\n+++ b/pkg/packet/bgp/validate.go\n@@ -159,6 +159,9 @@ func ValidateAttribute(a PathAttributeInterface, rfs map[Family]BGPAddPathMode,\n \tcase *PathAttributeAsPath:\n \t\tif isEBGP {\n \t\t\tif isConfed {\n+\t\t\t\tif len(p.Value) == 0 {\n+\t\t\t\t\treturn false, NewMessageError(eCode, eSubCodeMalformedAspath, nil, \"empty AS_PATH for confederation eBGP\")\n+\t\t\t\t}\n \t\t\t\tif segType := p.Value[0].GetType(); segType != BGP_ASPATH_ATTR_TYPE_CONFED_SEQ {\n \t\t\t\t\treturn false, NewMessageError(eCode, eSubCodeMalformedAspath, nil, fmt.Sprintf(\"segment type is not confederation seq (%d)\", segType))\n \t\t\t\t}\n```\n\n## Resources\n- https://github.com/osrg/gobgp/blob/c24629411ba49f160d9dc09126f418218127e016/pkg/packet/bgp/bgp.go#L11533-L11540\n- https://github.com/osrg/gobgp/blob/c24629411ba49f160d9dc09126f418218127e016/pkg/packet/bgp/validate.go#L159-L164",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/osrg/gobgp/v4"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "4.7.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 4.6.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/osrg/gobgp/security/advisories/GHSA-frrj-87jh-2772"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/osrg/gobgp"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-129" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-09T23:21:05Z",
    "nvd_published_at" : null
  }
}