CVE-2026-87081 in Net-IDN-UTS46
Summary
by MITRE • 09/22/2026
Net::IDN::UTS46 versions before 2.590 for Perl allow CPU exhaustion via quadratic punycode encoding of an overlong label before the length check in to_ascii.
to_ascii punycode encodes each label and only then applies the 63-byte DNS limit. encode_punycode in both backends follows the sample implementation in RFC 3492, whose outer loop runs once per distinct non-ASCII code point and scans the whole input each round, so a label of distinct non-ASCII characters costs the square of its length before the limit rejects it. Every ASCII conversion in the distribution, including domain_to_ascii and email_to_ascii, goes through to_ascii.
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Analysis
by VulDB Data Team • 09/22/2026
The vulnerability identified in Net::IDN::UTS46 versions prior to 2.590 represents a significant resource exhaustion flaw rooted in the order of operations within its ASCII conversion logic. This Perl module is designed to handle Internationalized Domain Names by converting Unicode labels into Punycode, which allows them to be transmitted over DNS infrastructure that only supports ASCII characters. The core technical deficiency lies in the implementation of the to_ascii function, where the process of encoding a domain label as Punycode occurs before validating whether the resulting string adheres to standard length constraints. Specifically, the module applies the 63-byte limit for DNS labels after the computationally expensive encoding step has already been executed, rather than checking input lengths or bounds prior to processing.
The operational mechanism behind this vulnerability exploits the algorithmic complexity of Punycode encoding as defined in RFC 3492 and referenced by UTS46 standards. The encode_punycode function, utilized by both available backends within the distribution, follows a sample implementation where the outer loop iterates once for each distinct non-ASCII code point found in the input string. Crucially, during each iteration of this loop, the algorithm scans the entire input sequence to identify characters and update state variables. This design choice results in quadratic time complexity relative to the number of unique non-ASCII characters in a label. Consequently, an attacker can craft a domain name with a long sequence of distinct Unicode characters that exceeds standard length limits but is not rejected until after the encoding process has completed its exhaustive computation.
This architectural flaw allows for CPU exhaustion attacks against any service relying on Net::IDN::UTS46 for DNS resolution or email address validation functions such as domain_to_ascii and email_to_ascii. Since every ASCII conversion in the distribution passes through to_ascii, a malicious actor can submit specially crafted inputs that trigger this quadratic behavior without being immediately blocked by length checks. The system will consume disproportionate CPU resources attempting to encode these overlong labels before finally rejecting them for violating DNS label limits. This creates a reliable vector for Denial of Service attacks, potentially degrading the performance or availability of web servers, mail transfer agents, or any application that processes internationalized domain names using this library.
From a classification perspective, this vulnerability aligns with CWE-400, which describes Uncontrolled Resource Consumption, specifically manifesting as CPU exhaustion due to inefficient algorithmic handling of untrusted input length and complexity. In the context of the MITRE ATT&CK framework, this flaw facilitates resource hijacking techniques where an adversary leverages application logic flaws to consume system resources, thereby impacting availability without necessarily compromising confidentiality or integrity. The lack of early validation for label lengths before processing is a common pattern in legacy text-processing libraries that prioritize strict adherence to encoding specifications over performance optimization against adversarial inputs.
To mitigate this vulnerability, organizations must upgrade Net::IDN::UTS46 to version 2.590 or later, where the developers have addressed the ordering of validation and encoding steps. Implementing input sanitization at the application layer is also recommended as a defense-in-depth measure. Developers should ensure that domain names and email addresses are validated for length constraints before being passed to IDN conversion functions. Additionally, integrating rate limiting and resource monitoring can help detect and mitigate potential abuse attempts while patching efforts are underway. Ensuring that all dependencies in the software supply chain are updated is critical, as this vulnerability affects multiple entry points within the distribution including email address processing.