CVE-2026-58317 Overview
CVE-2026-58317 is an Unsigned to Signed Conversion Error [CWE-196] in the TTSSH2 plugin of Tera Term, an open-source terminal emulator maintained by the TeraTerm Project. When Tera Term initiates an SSH connection to a server controlled by an attacker, the flaw triggers an out-of-bounds read or write. This memory safety issue can leak the contents of adjacent memory regions to the remote server. It can also cause Tera Term to behave unexpectedly or terminate abnormally, disrupting user sessions.
Critical Impact
Attackers operating a malicious SSH server can trigger out-of-bounds memory access in Tera Term clients, exposing adjacent memory to the server and causing abnormal termination.
Affected Products
- Tera Term (TTSSH2 plugin) by TeraTerm Project
- SSH client functionality within Tera Term
- Any workstation initiating SSH connections through the vulnerable TTSSH2 component
Discovery Timeline
- 2026-07-17 - CVE-2026-58317 published to the National Vulnerability Database
- 2026-07-17 - Last updated in NVD database
Technical Details for CVE-2026-58317
Vulnerability Analysis
The vulnerability resides in the TTSSH2 plugin, which handles Secure Shell (SSH) protocol operations for Tera Term. An unsigned-to-signed conversion error causes the plugin to misinterpret size or length values during SSH protocol parsing. When a value expected as a small positive integer is reinterpreted as a large negative or unexpectedly large number, subsequent buffer arithmetic becomes unsound.
Exploitation requires user interaction: the victim must initiate an SSH connection to a server the attacker controls. Once connected, the malicious server sends crafted protocol messages that trigger the faulty conversion. The result is an out-of-bounds read or write against memory adjacent to the intended buffer.
The consequences are twofold. First, memory contents outside the intended buffer may be transmitted back to the attacker-controlled server, leaking process memory. Second, corrupted memory operations can cause the Tera Term process to crash or behave in unexpected ways.
Root Cause
The root cause is improper type handling when converting between unsigned and signed integer representations [CWE-196]. Buffer size calculations and loop bounds derived from these values become incorrect, allowing memory access outside allocated regions.
Attack Vector
The attack vector is network-based but requires the victim to initiate an outbound SSH session to a server controlled by the attacker. Social engineering, phishing links, or typo-squatted hostnames are plausible delivery methods. No authentication is required on the attacker side because the flaw is triggered during the SSH handshake or early protocol exchange handled by TTSSH2.
No verified proof-of-concept code is publicly available at the time of publication. The vulnerability mechanism is described in the Tera Term Security Advisory and the JVN #65294474 Advisory.
Detection Methods for CVE-2026-58317
Indicators of Compromise
- Unexpected termination or crash reports involving ttxssh.dll or the Tera Term process on user workstations
- Outbound SSH connections from user endpoints to unknown or newly registered domains and IP addresses
- Windows Error Reporting entries for Tera Term referencing access violations during SSH session establishment
Detection Strategies
- Inventory endpoints running Tera Term and identify installed TTSSH2 plugin versions to locate unpatched hosts
- Correlate process crash telemetry for ttermpro.exe with preceding outbound SSH connection events
- Monitor SSH client connections to servers outside the approved administrative infrastructure list
Monitoring Recommendations
- Log all outbound TCP/22 traffic and enrich with destination reputation data
- Alert on Tera Term process crashes and correlate with recent SSH connection attempts to external hosts
- Track software inventory changes to detect installations of outdated Tera Term versions
How to Mitigate CVE-2026-58317
Immediate Actions Required
- Update Tera Term to the fixed release identified in the Tera Term Security Advisory
- Restrict SSH client usage to approved internal destinations through egress filtering on TCP/22
- Educate users to verify SSH server hostnames and fingerprints before connecting to unfamiliar systems
Patch Information
The TeraTerm Project has published fix guidance in the Tera Term Security Advisory and the corresponding JVN #65294474 Advisory. Administrators should apply the vendor-supplied update to the TTSSH2 plugin on all endpoints running Tera Term.
Workarounds
- Avoid initiating SSH connections to untrusted or unverified servers until the patch is applied
- Use an alternative, patched SSH client for connections to servers outside the trusted administrative boundary
- Enforce host-based firewall rules that limit outbound SSH to a known allowlist of destination addresses
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
