CVE-2026-49837 in GoBGP
Summary
by MITRE • 09/11/2026
GoBGP is an open source Border Gateway Protocol (BGP) implementation in the Go Programming Language. Versions prior to 4.6.0 contain a BGP OPEN capability parsing issue where several concrete capability decoders may parse data from the full remaining capability buffer instead of the slice bounded by the declared capability length, `CapLen`. A malformed BGP OPEN message can cause bytes from a following capability to be interpreted as part of the current capability. The most security-relevant case is the 4-octet AS capability, where a capability with `CapLen == 0` may cause the parser to read bytes from the following capability as the 4-octet AS value. This parsed value may later affect peer AS validation during BGP session establishment. Version 4.6.0 patches the issue.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 09/11/2026
GoBGP is a widely deployed open-source implementation of the Border Gateway Protocol written in the Go programming language, serving as critical infrastructure for internet routing decisions. Versions prior to 4.6.0 contain a significant parsing vulnerability within the BGP OPEN message handling logic that stems from improper validation of capability length fields. The core technical flaw involves several concrete capability decoders failing to restrict their data consumption to the slice bounded by the declared capability length, known as CapLen. Instead of stopping after reading the specified number of bytes for a specific capability, these parsers continue consuming data from the remaining buffer. This behavior allows an attacker crafting a malformed BGP OPEN message to inject arbitrary byte sequences that are misinterpreted as part of the current capability structure rather than being treated as separate or invalid data.
The most critical manifestation of this flaw occurs during the parsing of the 4-octet Autonomous System (AS) number capability, which is fundamental for establishing peer relationships in BGP sessions. When a malicious actor sends an OPEN message containing a capability with a CapLen value set to zero, the parser incorrectly proceeds to read subsequent bytes from the buffer as part of this AS capability field. Consequently, data belonging to following capabilities or padding fields is erroneously interpreted as the 4-octet AS identifier for the peer router. This misinterpretation directly impacts the operational integrity of BGP session establishment because the parsed value influences peer AS validation logic. If the extracted bytes form a valid-looking but incorrect AS number, it may bypass certain security checks designed to prevent route leaks or unauthorized peering arrangements, potentially allowing an attacker to manipulate routing tables by spoofing their identity as a trusted neighbor with a specific autonomous system identifier.
From a classification perspective, this vulnerability aligns with CWE-20 Improper Input Validation and CWE-134 Use of Externally-Controlled Format String in contexts where buffer boundaries are not respected during parsing operations. In terms of the MITRE ATT&CK framework for enterprise security, this flaw facilitates Initial Access through Protocol Manipulation, specifically leveraging BGP session establishment mechanisms to inject malicious routing information or disrupt service availability by causing unexpected state transitions or validation failures on the receiving router. The impact extends beyond simple denial of service; it introduces a risk of route hijacking if the misparsed AS number allows an attacker to claim authority over prefixes they do not own, leading to traffic interception or blackholing depending on how downstream routers accept and propagate these routes.
Mitigation strategies primarily involve upgrading GoBGP to version 4.6.0 or later, where this parsing logic has been corrected to strictly adhere to declared capability lengths. For environments unable to immediately patch the software, network operators should implement strict ingress filtering using BGP community tags or prefix lists that validate AS paths and reject OPEN messages with anomalous capability structures. Additionally, deploying deep packet inspection solutions capable of analyzing BGP control plane traffic can help detect malformed OPEN messages before they reach vulnerable routers. Continuous monitoring for unusual BGP session establishment patterns, particularly those involving unexpected capabilities or zero-length fields in critical attributes like the AS number, provides an additional layer of defense against exploitation attempts targeting this specific parsing flaw.