GoBGP confederation validation panics on empty AS_PATH attribute

Found through variant analysis based on CVE-2026-41643

Summary

GoBGP 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.

Affected

• Project: gobgp
• Repo: https://github.com/osrg/gobgp
• Pinned ref: c24629411ba49f160d9dc09126f418218127e016

Root cause

An 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.

Reproduction

INT-bgp-gobgp-confed-empty-aspath-panic.zip

bash ./poc/run.sh

TRIGGERED: confed empty AS_PATH validation panic: runtime error: index out of range

The 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.

Impact

A 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.

Suggested fix

diff --git a/pkg/packet/bgp/validate.go b/pkg/packet/bgp/validate.go
index 2237afb..f07f4fa 100644
--- a/pkg/packet/bgp/validate.go
+++ b/pkg/packet/bgp/validate.go
@@ -159,6 +159,9 @@ func ValidateAttribute(a PathAttributeInterface, rfs map[Family]BGPAddPathMode,
 	case *PathAttributeAsPath:
 		if isEBGP {
 			if isConfed {
+				if len(p.Value) == 0 {
+					return false, NewMessageError(eCode, eSubCodeMalformedAspath, nil, "empty AS_PATH for confederation eBGP")
+				}
 				if segType := p.Value[0].GetType(); segType != BGP_ASPATH_ATTR_TYPE_CONFED_SEQ {
 					return false, NewMessageError(eCode, eSubCodeMalformedAspath, nil, fmt.Sprintf("segment type is not confederation seq (%d)", segType))
 				}

Resources

• https://github.com/osrg/gobgp/blob/c24629411ba49f160d9dc09126f418218127e016/pkg/packet/bgp/bgp.go#L11533-L11540
• https://github.com/osrg/gobgp/blob/c24629411ba49f160d9dc09126f418218127e016/pkg/packet/bgp/validate.go#L159-L164