CVE-2026-41643 — v4
HIGHCVE-2026-41643 is a high-severity (CVSS 7.5) CWE-129 vulnerability in github.com/osrg/gobgp/v4. A fix is available for github.com/osrg/gobgp/v4 — see the affected versions and patch details below.
GoBGP: Remote Denial of Service (Panic) in UpdatePathAttrs4ByteAs via Malformed BGP UPDATE
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-41643.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-41643 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,567 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
github.com/osrg/gobgp/v4Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Summary
A remote Denial of Service (DoS) vulnerability exists in GoBGP where a malformed BGP UPDATE message can trigger a runtime error: index out of range panic. This occurs during the processing of 4-byte AS attributes when the message structure causes an internal slice index shift that is not properly handled.
Details
The vulnerability is located in internal/pkg/table/message.go within the UpdatePathAttrs4ByteAs function.
When GoBGP processes a BGP UPDATE message containing both an AS_PATH and an AS4_PATH attribute, it attempts to merge or validate them to support 4-byte AS numbers. If the attributes are ordered such that AS4_PATH (Type 17) appears before AS_PATH (Type 2), and the AS4_PATH is deemed invalid/malformed, the code attempts to remove the AS4_PATH attribute from the msg.PathAttributes slice. It appears the crash happens due to an index shift in msg.PathAttributes:
#GoBGP v4.2.0
// Line 112: If AS4_PATH precedes AS_PATH, the deletion causes all subsequent attributes to shift left.
msg.PathAttributes = append(msg.PathAttributes[:as4AttrPos], msg.PathAttributes[as4AttrPos+1:]...)
// Line 206: The stale asAttrPos index is used here.
//The function continues to use the stale index (asAttrPos) to update the AS_PATH. Since the slice length has decreased, accessing the old index leads to a panic.
msg.PathAttributes[asAttrPos] = bgp.NewPathAttributeAsPath(newIntfParams)
This deletion causes all subsequent attributes in the slice to shift left by one position. However, the function continues to use the original asAttrPos index (calculated before the deletion) to access or modify the AS_PATH attribute later at Line 206. Because the slice is now shorter, the "stale" index points out of bounds, triggering a panic and crashing the entire GoBGP process.
PoC
Environment: * GoBGP version: 4.2.0 Configuration: Passive peering enabled. Reproduction Steps: Configure GoBGP with a neighbor (e.g., 192.168.31.195). Send a specially crafted BGP UPDATE hex payload:
ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
00 22 02
00 00 # Withdrawn Routes Length
00 0b # Total Path Attribute Length
40 11 01 02 # AS4_PATH (Type 17, Len 1, Val 02) - Triggers the bug
40 02 04 ff ff de ad # AS_PATH (Type 2, Len 4)
The GoBGP process will immediately crash with: panic: runtime error: index out of range [1] with length 1.
Impact
Vulnerability Type: Remote Denial of Service (DoS). Impacted Users: Any GoBGP deployment (v4.2.0 and earlier) that accepts BGP UPDATE messages from peers. Since this crash occurs in the FSM (Finite State Machine) handling loop, a single malicious peer or a malformed route propagated through a transit provider can consistently crash the BGP daemon, leading to a complete loss of routing capabilities.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/osrg/gobgp/v4 | all versions | 4.3.0go get github.com/osrg/gobgp/v4@v4.3.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/osrg/gobgp/v4, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/osrg/gobgp/v4 to 4.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-41643 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-41643 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-41643. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
Frequently Asked Questions
Is CVE-2026-41643 in your dependencies?
O3 Security finds CVE-2026-41643 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.