GHSA-frrj-87jh-2772

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](https://github.com/user-attachments/files/28203698/INT-bgp-gobgp-confed-empty-aspath-panic.zip)

```bash bash ./poc/run.sh ```

```text 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 ```001-fix.diff 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