CVE-2026-43798

Executive Summary

CVE-2026-43798 is a critical, pre-authentication remote code execution vulnerability in Apple's swift-nio-ssh library, a Swift implementation of the SSH protocol built on the SwiftNIO event-driven networking framework. A single crafted SSH message can trigger an out-of-bounds stack write of attacker-controlled length and content against any application built on swift-nio-ssh versions prior to 0.14.1. The vulnerability carries a CVSS score of 9.8, indicating maximum severity, and requires no authentication or user interaction to exploit. Any organization running Swift-based network services that expose SSH functionality built on swift-nio-ssh should treat this as a priority remediation item.

Technical Analysis

Vulnerability Class: CWE-121 (Stack-based Buffer Overflow)

Affected Component: swift-nio-ssh, versions prior to 0.14.1

Attack Vector: Unauthenticated network-based

Vulnerability Mechanism:

The NVD description states that a single crafted SSH message gives an unauthenticated network attacker an out-of-bounds stack write of attacker-controlled length and content. This describes a classic stack buffer overflow condition, where the vulnerable code path in swift-nio-ssh fails to validate the length of incoming message data before copying it to a fixed-size stack buffer. Because both the length and the content are attacker-controlled, the attacker can achieve arbitrary stack corruption.

The SSH protocol is message-oriented, with each message containing a length header followed by the message payload. The vulnerability manifests during the parsing or deserialization of an incoming SSH message. If the parsing code trusts the declared message length without verifying it against an internal buffer's capacity, a malicious message can cause the SSH library to write beyond the bounds of a stack-allocated buffer.

Attack Surface:

The attack surface is any network-exposed service that integrates swift-nio-ssh. This includes SSH servers, SSH client applications that connect to malicious servers, port forwarding utilities, and any custom network service using SSH-based tunneling. Since the vulnerability is triggered purely by receiving a crafted message, both the server side (receiving connections from arbitrary attackers) and the client side (connecting to a malicious SSH server) are exploitable.

Code-Level Analysis:

Specific function names, class names, or file paths are not disclosed in the NVD description. The vulnerability description only refers to the swift-nio-ssh library as a whole. The insufficient technical detail available prevents pinpointing the exact parsing function responsible; however, the flaw resides in the SSH message framing/deserialization layer.

Affected Versions: All swift-nio-ssh versions prior to 0.14.1 are presumed vulnerable, although the NVD data does not enumerate a specific version range beyond the fix version.

Exploitation Scenarios

Steps:

1. Reconnaissance/Target Identification: An attacker performs network scanning to identify services running SSH on non-standard ports or services using SSH protocol for tunneling. Since swift-nio-ssh is a library, the vulnerable service may not respond like a traditional OpenSSH server — it may lack standard SSH banners or respond with custom application protocols. More advanced attackers may fingerprint services by sending crafted SSH banners or observing handshake behavior that differs from conventional SSH implementations.

2. Triggering the Flaw: The attacker establishes a TCP connection to the target service and sends a single, specially crafted SSH message. The malicious message declares a payload length that exceeds the actual allocated stack buffer size in the swift-nio-ssh message parser. No SSH handshake completion or authentication is required — the flaw triggers during message parsing, which occurs early in the connection lifecycle. This means the attack is entirely pre-authentication.

3. Payload/Execution Method: At the vulnerability-class level, the out-of-bounds stack write allows the attacker to overwrite stack metadata — typically the saved return address, frame pointer, or adjacent local variables on the call stack. By controlling both the content and length of the overflow, the attacker can redirect execution flow to attacker-controlled shellcode. Modern mitigations (stack canaries, ASLR, NX) may complicate exploitation, but a length-and-content-controlled stack overflow of this nature is highly likely to be exploitable with sufficient development effort, particularly on platforms where swift-nio-ssh is deployed on Linux or macOS without full exploit mitigations enabled. The attacker may also use return-oriented programming (ROP) chains instead of direct shellcode injection.

4. Post-Exploitation Impact: Successful exploitation yields code execution with the privileges of the application process. For a typical SSH server, this means remote code execution as the service account, potentially root if the server runs privileged. The attacker can then pivot to internal networks, install persistence mechanisms, exfiltrate data, or move laterally. Since the write is of attacker-controlled length and content, partial exploitation could also be used for denial-of-service (crashing the process via stack corruption) even if full code execution is not achieved.

Note: No public PoC is available; the attack chain is inferred from the vulnerability class and NVD description. Exploit code availability is marked as "NO" in the CVE data despite PoC being marked "Public" in general — this contradiction suggests the PoC claim may refer to a proof-of-concept writeup or technical description rather than working exploit code.

Impact Assessment

CVSS Breakdown (Score: 9.8, Critical):

• Attack Vector: Network (AV:N) — Exploitable remotely over the network.
• Attack Complexity: Low (AC:L) — No special conditions or race conditions required.
• Privileges Required: None (PR:N) — Pre-authentication exploitation.
• User Interaction: None (UI:N) — No victim interaction required.
• Confidentiality Impact: High (C:H) — Full information disclosure via code execution.
• Integrity Impact: High (I:H) — Full system compromise possible.
• Availability Impact: High (A:H) — Complete denial of service or system takeover.

Business Impact:

• Affected Systems: Swift-based SSH implementations, including custom SSH servers and embedded network devices built on SwiftNIO. This includes macOS and Linux server environments running Swift applications with SSH capabilities.
• Blast Radius: Applications using swift-nio-ssh as a client are also at risk (connecting to malicious servers). The supply chain impact is significant: any downstream application that embeds this library inherits the vulnerability.
• EPSS Analysis: EPSS score of 0.00337 (26.4th percentile) suggests low current exploitation probability in the wild; however, the score can change rapidly once working exploit code circulates. The pre-authentication nature and 9.8 CVSS score make this a high-attention CVE for security teams despite the low current EPSS.
• CISA KEV Status: Not currently listed on CISA's Known Exploited Vulnerabilities catalog.

Threat Actor Interest: High. Critical pre-auth RCE vulnerabilities in widely-used protocol libraries historically attract rapid exploit development from both criminal and nation-state actors. The supply chain nature of a library vulnerability amplifies the potential blast radius.

Detection Guidance

The following detection rules target anomaly detection around the SSH message parsing phase. Because the vulnerability is triggered by a single crafted message before authentication, detection focuses on malformed SSH messages, unusually large declared packet lengths, or connection patterns consistent with exploit attempts.

Sigma Rule

title: Suspicious SSH Message Length Anomaly - Possible CVE-2026-43798 Exploitation
id: 91f6a7e2-5c93-4f2d-9a10-4c6bde51d8a2
status: experimental
description: Detects SSH messages with anomalous packet length declarations that may indicate an attempt to trigger CVE-2026-43798 (out-of-bounds write in swift-nio-ssh)
author: Valtersit Threat Intelligence
date: 2026/01/15
logsource:
  category: network_connection
  product: zeek
  service: ssh
detection:
  selection_message_length_query:
    # Beacon/initial SSH message (KEXINIT) typically does not exceed 65535 bytes
    # Attackers may declare lengths far above protocol maximums
    zeek.ssh.length|gte: 65535
    zeek.ssh.auth_success: false
  selection_short_connection:
    # Exploit triggers within the first packet exchange - connection may end abruptly
    zeek.ssh.duration|lt: 0.5s
  condition: selection_message_length_query or selection_short_connection
level: high
falsepositives:
  - Legitimate large SSH banner announcements (rare)
  - Non-standard SSH implementations sending protocol extensions
fields:
  - src_ip
  - dst_ip
  - zeek.ssh.length
tags:
  - attack.t1190
  - cve.2026-43798
  - cwe.121

YARA Rule

rule CVE_2026_43798_SSH_Stack_Overflow_Indicators
{
    meta:
        author = "Valtersit Threat Intelligence"
        description = "Detects binary patterns associated with crafted SSH messages exploiting CVE-2026-43798 stack buffer overflow in swift-nio-ssh"
        date = "2026-01-15"
        cve = "CVE-2026-43798"
        cwe = "CWE-121"
        score = 70
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-43798"
    strings:
        // SSH protocol packet header: 4 bytes of length field
        // Suspiciously large declared length (0xFFFFFFFF or near-max values)
        $large_packet_len_1 = { FF FF FF FC }
        $large_packet_len_2 = { FF FF FF FD }
        $large_packet_len_3 = { FF FF FF FE }
        $large_packet_len_4 = { FF FF FF FF }
        
        // SSH version exchange strings that are anomalously long
        $long_ssh_ident = /SSH-\d\.\d-[^\r\n]{200,}/
        
        // Repeated SSH message type 20 (KEXINIT) with padding bytes
        $repeated_kexinit = { 14 00 00 00 00 00 00 00 00 00 }
        
        // NOP-sled-like padding patterns often embedded in overflow payloads
        $nop_sled = { 90 90 90 90 90 90 90 90 }
    condition:
        uint32be(0) > 100000 and  // Unusually large declared length
        any of them
}

Suricata IDS Rule

alert tcp any any -> any 22 (msg:"CVE-2026-43798 - Possible SwiftNIO SSH Stack Overflow Exploitation Attempt"; flow:to_server,established; dsize:>1500; content:"|00 00 00|"; offset:0; depth:8; content:"|14|"; within:10; byte_test:4,>,:50000,0,string,dec,relative; threshold:type both, track by_src, count 1, seconds 60; classtype:attempted-admin; sid:20264379801; rev:1; metadata:created_at 2026_01_15, cve CVE_2026_43798, cwe CWE_121, mitre_tactic TA0001, mitre_technique T1190;)

alert tcp any any -> any any (msg:"CVE-2026-43798 - Anomalous SSH Packet Length Declared Pre-Authentication"; flow:to_server,established; content:"SSH-"; nocase; within:100; pcre:"/^.{4}\xff\xff\xff\xfe/i"; classtype:attempted-admin; sid:20264379802; rev:1; metadata:created_at 2026_01_15, cve CVE_2026_43798, mitre_tactic TA0001, mitre_technique T1190;)

Elastic Detection

// Detection for anomalous SSH connections that may indicate CVE-2026-43798 exploitation attempts
// Detects SSH connections with unusual duration patterns or large packet sizes pre-authentication

sequence by source.ip
  [network where event.category == "network" and 
   network.protocol == "ssh" and 
   event.action == "connection_attempted"]
  [network where event.category == "network" and 
   network.protocol == "ssh" and 
   network.bytes > 100000 and 
   event.action == "connection_failed"]

Splunk SPL Query

index=network sourcetype=firewall OR sourcetype=proxy 
| search dest_port=22
| eval packet_size=if(isnull(bytes_in),0,bytes_in)
| where packet_size > 65535
| eval pre_auth = if(auth_status="failed" OR auth_status="NOT_AVAILABLE",1,0)
| where pre_auth=1
| table _time, src_ip, dest_ip, dest_port, bytes_in, bytes_out, auth_status
| sort - _time
| stats count by src_ip, dest_ip
| where count > 5

Wazuh OSSEC Rule

<group name="ssh,attack,">
  <rule id="100438" level="12">
    <decoded_as>json</decoded_as>
    <field name="event_type">ssh</field>
    <field name="ssh.message_length">\d+</field>
    <regex>ssh.message_length: ([5-9][0-9]{5}|[0-9]{7,})</regex>
    <description>CVE-2026-43798 potential exploitation: Anomalous SSH message length detected (>50000 bytes)</description>
    <group>cve-2026-43798,gpg13,attack.t1190,</group>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
  
  <rule id="100439" level="10">
    <decoded_as>json</decoded_as>
    <field name="event_type">ssh</field>
    <description>CVE-2026-43798 SSH connection terminated abruptly post-handshake - possible crash from overflow</description>
    <group>cve-2026-43798,</group>
    <if_sid>100438</if_sid>
    <field name="connection_duration">0\.[0-9]+</field>
    <options>no_full_log</options>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 2/4 (CLAIMED — a single source (NVD) claims the patch; not independently confirmed by vendor advisory or multiple independent sources. Security teams should verify the fix in their actual deployment before relying on it.)

Immediate Mitigations

1. Upgrade immediately: Update swift-nio-ssh to version 0.14.1 or later, which addresses this vulnerability. This is the only known remediation.
2. Restrict network access: If immediate patching is not possible, restrict network access to SSH services built on swift-nio-ssh. Use network ACLs or firewall rules to limit which source IPs can connect to the affected service.
3. Isolate affected services: If the service is internet-facing, consider placing it behind a reverse proxy or VPN gateway until it can be patched.
4. Monitor aggressively: Deploy the detection rules above to identify potential exploitation attempts.

Patch/Upgrade Steps

1. Update the swift-nio-ssh package dependency in your Swift Package Manager manifest (Package.swift) to specify version 0.14.1 or later.
2. Rebuild and redeploy all applications that depend on swift-nio-ssh.
3. Verify the deployed version using package resolution (swift package show-dependencies or equivalent).
4. Validate that no regression occurred in SSH functionality post-upgrade.

Long-Term Hardening

1. Stack hardening: Ensure compilation flags enable stack canaries (-fstack-protector-strong or equivalent for Swift builds).
2. Address Space Layout Randomization (ASLR): Confirm the target platform applies ASLR to Swift binaries.
3. Non-Executable Stack: Verify NX (no-execute) is enforced on the deployment platform.
4. Network segmentation: Keep SSH management interfaces on dedicated administrative networks rather than general production segments.
5. Dependency monitoring: Subscribe to security advisories for swift-nio-ssh and related SwiftNIO projects to receive early notification of future vulnerabilities.
6. Supply chain review: Audit applications for any embedded copies of swift-nio-ssh that may not be centrally managed.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-43798 — Official CVE record with NVD description and CVSS scoring.
2. Apple SwiftNIO SSH GitHub Repository: https://github.com/apple/swift-nio-ssh — Source repository containing the fix in version 0.14.1 (release notes for 0.14.1 should confirm the security fix).
3. CWE-121 Reference: https://cwe.mitre.org/data/definitions/121.html — MITRE CWE documentation for stack-based buffer overflows, the vulnerability class.
4. CISA KEV Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog — Confirms this CVE is NOT currently listed in CISA's Known Exploited Vulnerabilities catalog.
5. EPSS Data: https://www.first.org/epss/ — FIRST.org EPSS scoring (score: 0.00337, 26.4th percentile), indicating low current in-the-wild exploitation probability.

Known Threat Actor Campaigns: None reported at the time of this report. No exploit-in-the-wild evidence has been documented, and the CVE is not on CISA's KEV list. The patch is only claimed by a single source (NVD) and has not been independently verified. Organizations should monitor for vendor advisories, proof-of-concept disclosures, and updated EPSS scores in the coming weeks as the vulnerability receives more analysis.

Report generated by Valtersit Threat Intelligence. This document is for informational purposes and should inform — not replace — organizational vulnerability management processes.