CVE-2026-106069 in Node.js
Summary
by MITRE • 10/23/2026
A flaw in Node.js TLS hostname verification can cause a server certificate to be validated against a different IP address than the one the client connected to, when the host is a non-canonical IPv4 literal with a leading zero, such as `0127.0.0.1`.
`tls.checkServerIdentity()` normalizes the host following the WHATWG URL rules, which read a leading-zero octet as octal (`0127.0.0.1` becomes `87.0.0.1`), while the connection is made through the operating system resolver, which on some platforms such as macOS reads it as decimal (`127.0.0.1`). A server at the connected address can then be authenticated with a certificate issued for another IP address.
This vulnerability affects Node.js **26.x**, **24.x**, and **22.x**.
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Analysis
by VulDB Data Team • 10/23/2026
The identified vulnerability in Node.js represents a critical discrepancy in TLS hostname verification logic, specifically affecting versions 26.x, 24.x, and 22.x. The core issue stems from an inconsistency between how the application layer normalizes hostnames for certificate validation and how the underlying operating system resolves network connections when dealing with non-canonical IPv4 literals containing leading zeros. This flaw allows a server to present a valid TLS certificate that does not match the actual IP address of the connection, thereby bypassing standard identity verification mechanisms designed to prevent man-in-the-middle attacks.
The technical root cause lies in the divergent parsing rules applied by different components within the execution environment. When Node.js performs hostname validation via its internal tls.checkServerIdentity function, it adheres to WHATWG URL Standard specifications for normalization. Under these rules, an IPv4 address with a leading zero in any octet is interpreted as an octal number. Consequently, an input such as 0127.0.0.1 is mathematically converted to its decimal equivalent, which is 87.0.0.1. However, the actual network connection established by Node.js relies on the operating system's socket resolver. On several platforms, including macOS and potentially others depending on configuration or version, IPv4 literals with leading zeros are treated as standard decimal addresses rather than octal ones. This means that while the client connects to 127.0.0.1 (the loopback address), the TLS verification logic checks the server's certificate against 87.0.0.1.
This mismatch creates a significant security gap where an attacker controlling a legitimate service at IP address 87.0.0.1 can authenticate successfully when a client attempts to connect via localhost using the octal notation. The server presents its own valid certificate, which matches 87.0.0.1, and because Node.js normalizes the input host to this same value during verification, the connection is deemed secure despite the physical connection being established with a different entity at 127.0.0.1. This scenario effectively undermines the trust model of TLS by allowing certificate validation against an identity that does not correspond to the actual network endpoint reached by the client.
From a classification perspective, this vulnerability aligns with CWE-295 Improper Certificate Validation and CWE-697 Incorrect Comparison During SSL/TLS Handshake within the Common Weakness Enumeration framework. In terms of adversary tactics, it relates to MITM (Man-in-the-Middle) attacks where an attacker exploits protocol implementation flaws to intercept encrypted traffic without detection by clients that rely on hostname verification for security assurance. The ATT&CK technique associated with this behavior is T1557 Adversary-in-the-Middle, specifically leveraging misconfiguration or logic errors in cryptographic implementations rather than active network interception alone.
The operational impact of this flaw is severe for applications relying on strict TLS validation to ensure data integrity and confidentiality. If an application connects to a service using octal IPv4 literals without additional safeguards, it may inadvertently establish secure channels with unintended servers. This could lead to the exfiltration of sensitive credentials or proprietary data to malicious actors who control the IP address derived from the octal interpretation. Furthermore, this issue highlights broader risks in software that depends on external standards for parsing network identifiers while relying on OS-level behaviors for actual connectivity, creating a persistent attack surface if not explicitly addressed through code hardening.
Mitigation strategies should focus on preventing the use of non-canonical IPv4 literals with leading zeros in application configurations and source code. Developers must ensure that all hostnames passed to TLS functions are canonical decimal representations before validation occurs. Additionally, organizations running affected Node.js versions should prioritize upgrading to patched releases where this normalization logic has been corrected or aligned with OS behavior. For environments unable to upgrade immediately, implementing strict input sanitization to reject octal-formatted IP addresses and enforcing explicit certificate pinning for critical services can provide compensating controls against exploitation of this specific verification bypass.