GoBGP: BGP OPEN capability parser may read capability values outside declared CapLen boundaries

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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.

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10 Sep 2026 21:56:13 UTC

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15 Sep 2026 15:10:15 UTC

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23 Oct 2026 15:48:27 UTC

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CVSS 3.1 · 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

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.

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Sep 10, 2026, 9:56:13 PM (timezone not stated)

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Sep 15, 2026, 3:10:15 PM (timezone not stated)

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11 Nov 2026 13:05:02 UTC

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EUVD CVSS 3.1 · 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

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.

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10 Sep 2026 22:16:57 UTC

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16 Sep 2026 13:42:46 UTC

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25 Oct 2026 12:17:38 UTC

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CVSS 3.1 · [email protected] · Secondary · 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

### Summary GoBGP contains 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. ### Details The issue is in the BGP OPEN capability parser under: - `pkg/packet/bgp/bgp.go` - `pkg/packet/bgp/validate.go` - The BGP OPEN optional parameter capability format includes a capability code, a capability length field, and a capability value. Each concrete capability decoder should only parse bytes inside the declared capability value boundary. In affected versions, the generic capability parser records the declared `CapLen`, but several concrete capability decoders continue parsing from the full remaining capability buffer after advancing past the two-byte capability header. Conceptually, the vulnerable pattern is: ```go data = data[2:] // decoder reads from data without first limiting it to CapLen ### PoC The following parser-level proof of concept demonstrates the issue without requiring a full BGP session or a running `bgpd` instance. The malformed capability uses: - Capability Code: `65` (`BGP_CAP_FOUR_OCTET_AS_NUMBER`) - Declared `CapLen`: `0` - Four following bytes: `00 00 fd e8` Although the capability declares an empty value, affected versions parse the following four bytes as the 4-octet AS value `65000`. ### Impact A remote peer that can send a malformed BGP OPEN message to a GoBGP instance may cause capability values to be parsed from outside their declared `CapLen` boundaries. In the 4-octet AS capability case, this may affect: - peer AS validation; - capability negotiation; - interpretation of malformed OPEN messages; - acceptance or rejection decisions during BGP session establishment. This issue does not appear to be arbitrary memory corruption, remote code execution, or information disclosure. It is a protocol parser boundary validation issue that can affect BGP OPEN validation semantics.

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9 Jul 2026 23:20:57 UTC

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24 Jul 2026 19:11:27 UTC

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24 Oct 2026 13:19:41 UTC

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GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

Publisher rating · MODERATE

GoBGP: BGP OPEN capability parser may read capability values outside declared CapLen boundaries in github.com/osrg/gobgp

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24 Jul 2026 18:35:55 UTC

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24 Jul 2026 19:00:27 UTC

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24 Oct 2026 13:19:41 UTC

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OSV · upstream source license applies

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