Introduction
A single crafted BGP packet can crash the BGP agent on affected Arista EOS switches, dropping every BGP session on the device and disrupting routing until the process comes back. Arista rates the flaw CVSS 9.8 and cautions that a sophisticated attacker might turn the underlying stack overflow into arbitrary code execution, which makes it relevant to any network that relies on EOS for routing.
Arista Networks builds EOS, the network operating system behind its switching and routing portfolio for data center and AI, campus and routing environments. The company entered the WAN routing market in 2023. IDC reported that Arista held 13.4% of the total Ethernet switch market and 18.7% of the data center Ethernet switch segment in 2Q26, in a quarter where Arista's Ethernet switch revenue reached $2.5 billion. With that footprint, a bug in a core EOS routing agent reaches a large share of modern data center fabrics.
Technical information
Root cause
Arista classifies CVE-2026-121289 as CWE-121, a stack-based buffer overflow, in the multi-agent BGP agent of EOS. When the agent processes a specially crafted packet, memory on the stack is corrupted and the agent terminates with a segmentation fault (signal 11, SIGSEGV). Arista tracks the issue internally as BUG 1927959 and disclosed it in Security Advisory 0208 (revision 1.0, November 10, 2026).
EOS is closed source, and Arista has not published a code diff or described the change. Its interim guidance does point at the likely location: Arista recommends discarding received BGP path attribute type 23, the Tunnel Encapsulation Attribute. Our reading is that the vulnerable code parses Tunnel Encapsulation Attributes and copies attribute data into a fixed-size stack buffer without adequately bounding the copy. For this bug class, the usual fix is to check the encoded length of each attribute and nested sub-TLV against both the destination buffer size and the bytes remaining in the message before copying, and to handle malformed attributes safely. That is an educated inference from the advisory, not a confirmed description of BUG 1927959.
The CVSS v3.1 vector is:
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
The vector rates the attack as network reachable, low complexity, with no privileges or user interaction required, and high impact to confidentiality, integrity and availability.
Preconditions
Two conditions must both hold for a device to be exposed:
- BGP is configured and running.
- EOS is using the multi-agent routing protocol model. Devices running the ribd model are not exposed.
Arista also notes that only an established peering session triggers the vulnerable code path.
Attack flow
Based on the advisory and Arista's interim guidance, exploitation would proceed roughly as follows:
- The target runs an affected EOS release with BGP configured under the multi-agent model.
- The attacker sends traffic over an established BGP peering session with the device.
- The crafted input, which Arista's workaround suggests is carried in a Tunnel Encapsulation Attribute (type 23), reaches the vulnerable parsing code.
- Data is written past the bounds of a stack buffer, corrupting memory and causing the BGP agent to segfault.
- The agent terminates, all BGP sessions on the device drop, and routing is disrupted until ProcMgr restarts the agent and sessions reconverge.
- Arista warns that a sophisticated attacker may be able to control the overflow well enough to achieve arbitrary code execution instead of a crash. No public demonstration of this outcome exists.
Impact
The confirmed outcome is a BGP agent crash and the loss of every BGP session on the device. The advisory does not quantify recovery time. Repeated delivery of the crafted input could, in principle, keep the agent cycling. The code-execution scenario remains a vendor warning rather than a demonstrated result, and should be reported that way.
Patch information
Arista fixes CVE-2026-121289 through new EOS releases. There is no separate patch file and no published code diff. NVD lists Security Advisory 0208 as the only vendor reference for this CVE.
Fixed releases by train:
- 4.37.x: fixed from the first release (4.37.0F and later).
- 4.36.x: 4.36.2F and earlier are vulnerable. Fixed in 4.36.3M and later.
- 4.35.x: 4.35.6M and earlier are vulnerable. Fixed in 4.35.7M and later.
- 4.34.x: 4.34.8M and earlier are vulnerable. Fixed in 4.34.9M and later.
- 4.33.x: 4.33.10M and earlier are vulnerable. Fixed in 4.33.11M and later.
- End of Support trains (4.25.1F up to 4.33.0F): all releases are vulnerable, with no fix. Move to a supported train.
Planning notes:
- The fix is limited to supported trains. NVD's affected-version data covers every release from 4.25.1F up to, but not including, 4.33.0F, and none of those trains receives a fixed build.
- Some earlier EOS advisories offered a SWIX hotfix extension loadable onto a running image. Advisory 0208 states plainly that no hotfixes are available, so a full image upgrade is required.
- Arista recommends going to the latest release in the chosen train rather than just the first fixed build, to pick up other accumulated fixes.
- Until upgrades are complete, Arista recommends discarding received BGP Tunnel Encapsulation Attributes (path attribute type 23).
To verify a device, run show version and compare the EOS release with the list above. If the device is at or above the first fixed build for its train, it has the fix regardless of hardware platform.
The unreviewed GitHub Advisory Database entry GHSA-rrpp-332g-xvjw lists both affected and patched versions as "Unknown" because its auto import did not pull in the version data. Treat Arista's advisory as the authoritative list.
Detection methods
Arista has not released Snort, Suricata, Sigma or YARA signatures for this CVE, and we found no verified third-party signature. Public detection guidance comes from Security Advisory 0208 and covers exposure checks and a log-based indicator.
1. Check exposure
The advisory gives two CLI checks:
switch>show ip bgp summary switch>show running-config all | inc service routing protocols service routing protocols model multi-agent
If the summary shows established neighbors and the running configuration shows the multi-agent model, the vulnerable code paths are active. If the output is % BGP is not running, or the model is ribd, the device is not exposed. Running both checks across the fleet alongside the version list tells you which switches to prioritize.
2. Log-based indicator: BGP agent crash
Arista says an unexpected crash or restart of the BGP agent may indicate exploitation. ProcMgr writes a message like this to show logging:
Jan 1 00:00:41 switch ProcMgr: %PROCMGR-6-PROCESS_TERMINATED: 'Bgp' (PID=<pid>, status=-11) has terminated.
Key fields:
- The PROCESS_TERMINATED event is EOS's process manager reporting that a managed agent died.
'Bgp'names the BGP agent affected by this CVE.- The status of negative 11 corresponds to signal 11 (SIGSEGV), consistent with stack memory corruption rather than a clean administrative restart.
To search historical logs from the EOS bash shell, the advisory provides:
grep "PROCESS_TERMINATED: 'Bgp'" /var/log/eos
3. Operationalizing at scale
Forward EOS syslog to your SIEM and alert on the PROCESS_TERMINATED: 'Bgp' string, treating segfault terminations as the highest priority. Because the crash drops every BGP session on the device, a genuine match should coincide with simultaneous neighbor flaps on that switch and on its peers. Correlating the ProcMgr message with peer-side session drops helps separate a real crash from routine maintenance.
4. Hunting lead (inference)
Arista's interim workaround of discarding type 23 attributes suggests the crafted input arrives in the Tunnel Encapsulation Attribute. If you collect BGP UPDATE messages with a BGP monitoring or packet capture tool, look for type 23 attributes from peers that should not be sending them, particularly just before a Bgp crash. Since only an established peering session reaches the vulnerable path, focus on unexpected or newly established peers. This is an inference. Arista has not published a network signature.
Limitations
A successful code-execution exploit that did not crash the agent would leave no PROCESS_TERMINATED entry, so the log indicator reliably catches the crash outcome but may miss a stealthy compromise. Conversely, a BGP crash alone is not proof of exploitation, since other bugs can crash the agent. Confirm each match with crash artifacts and Arista TAC.
Exploitation status
Arista states it is not aware of any malicious exploitation in customer networks. The OffSeq Threat Radar tracker labels the CVE as not known to be exploited, reports no matching attack traffic on its sensors, and names no threat actor. One sensor set is not proof of universal absence. One upstream patch check claimed the CVE appears in the CISA Known Exploited Vulnerabilities catalog, but none of the sources we reviewed supports that claim.
Affected systems and versions
Vulnerable configuration: an affected EOS release with BGP configured and the multi-agent routing protocol model enabled.
Vulnerable EOS releases:
- 4.36.x: 4.36.2F and earlier
- 4.35.x: 4.35.6M and earlier
- 4.34.x: 4.34.8M and earlier
- 4.33.x: 4.33.10M and earlier
- End of Support trains: all releases from 4.25.1F up to, but not including, 4.33.0F (no fix available)
- 4.37.x: not affected from 4.37.0F onward
Affected products listed by Arista include EOS-based fixed-configuration and modular switches, CloudEOS, cEOS lab, vEOS lab and CloudVision eXchange. Devices using the ribd routing model, or without BGP running, are not exposed.
Vendor security history
Arista operates a Product Security Incident Response Team that says it continuously monitors emerging attack vectors, uses different advisory publication schedules, and publishes vulnerabilities with associated solutions through its advisory channels. Earlier in 2026, Arista published Security Advisory 0142 (dated August 10, 2026) for a separate issue involving EOS routing features. Some prior EOS advisories offered SWIX hotfixes, which Advisory 0208 does not. These sources document a formal disclosure process but are not sufficient on their own to rate Arista's overall security record or compare it with other vendors.
References
- NVD: CVE-2026-121289
- CVE Record: CVE-2026-121289
- Arista Security Advisory 0208
- Arista Security Advisory 0208 (PDF)
- GitHub Advisory Database: GHSA-rrpp-332g-xvjw
- OffSeq Threat Radar: CVE-2026-121289
- Arista Security Advisory 0142
- Arista: Device Hardening and Vulnerability Management
- Arista: Company Overview
- Arista: Quick Facts
- IDC: Ethernet Switch Market, 2Q26
