CVE-2021-31251
Description
An authentication bypass in telnet server in BF-430 and BF431 232/422 TCP/IP Converter, BF-450M and SEMAC from CHIYU Technology Inc allows obtaining a privileged connection with the target device by supplying a specially malformed request and an attacker may force the remote telnet server to believe that the user has already authenticated.
AI Insight
LLM-synthesized narrative grounded in this CVE's description and references.
An authentication bypass in CHIYU BF-430, BF-431, BF-450M, and SEMAC telnet servers allows an unauthenticated attacker to gain privileged access via a specially malformed request.
Vulnerability
An authentication bypass vulnerability exists in the telnet server of CHIYU Technology BF-430, BF-431, BF-450M 232/422 TCP/IP Converter devices, and the SEMAC device. The server incorrectly handles a malformed request, believing the user has already authenticated. This affects firmware versions prior to the latest update (as of June 2021) [1].
Exploitation
An unauthenticated attacker with network access to the target device's telnet service can send a specially crafted malformed request. No prior authentication or user interaction is required. The server processes this request and treats the connection as authenticated, granting immediate access [1].
Impact
Successful exploitation allows the attacker to obtain a privileged telnet session with the device. This can lead to full compromise of the device, including unauthorized configuration changes, data exfiltration, and potential pivoting to other internal systems [1].
Mitigation
The vendor has not publicly disclosed a specific firmware release that fixes this issue. The available references do not mention a patched version. It is recommended to contact CHIYU Technology for updated firmware and to restrict telnet access via network segmentation or firewall rules [1].
AI Insight generated on May 27, 2026. Synthesized from this CVE's description and the cited reference URLs; citations are validated against the source bundle.
Affected products
5- CHIYU Technology Inc/TCP/IP Converterdescription
Patches
0No patches discovered yet.
Vulnerability mechanics
Root cause
"An overflow or state-handling flaw during telnet protocol negotiation allows the server to skip authentication and treat the connection as already authenticated."
Attack vector
An unauthenticated attacker on the network sends a specially malformed telnet request to the target device. During the telnet protocol negotiation, the server fails to properly handle the negotiation state — at the 4th TCP request the server "jumps to the next state and believes that the user has already authenticated" [ref_id=1]. This bypasses the telnet authentication banner and grants the attacker direct access to the privileged configuration menu without supplying valid credentials [ref_id=1]. The attack requires no authentication and can be performed remotely over the network.
Affected code
The telnet server component in BF-430, BF-431, BF-450M, and SEMAC devices from CHIYU Technology Inc. The advisory states the flaw occurs "during the negotiation of the telnet protocol" [ref_id=1]. No specific source file or function name is provided in the reference.
What the fix does
The vendor's mitigation was to disable the telnet service entirely in the latest firmware version [ref_id=1]. No source-level patch is published; the advisory notes that "the telnet service was disabled in order to solve this issue" [ref_id=1]. Users are instructed to install the latest firmware to remediate the vulnerability.
Preconditions
- networkThe target device must have the telnet service enabled and reachable over the network.
- authNo authentication or prior access is required.
Generated on May 25, 2026. Inputs: CWE entries + fix-commit diffs from this CVE's patches. Citations validated against bundle.
References
3- gitbook.seguranca-informatica.pt/cve-and-exploits/cves/chiyu-iot-devicesmitrex_refsource_MISC
- seguranca-informatica.pt/dancing-in-the-iot-chiyu-devices-vulnerable-to-remote-attacks/mitrex_refsource_MISC
- www.chiyu-tech.com/msg/message-Firmware-update-87.htmlmitrex_refsource_CONFIRM
News mentions
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