CVE-2026-110870 in Nodemailer
Summary
by MITRE • 10/15/2026
Nodemailer from 3.0.0 before 10.0.13 contains an improper input validation vulnerability in its address parser that fails to strip RFC 5322 comments inside angle-bracket addresses. Attackers can supply recipients like a display name with a parenthesized comment in the domain, so illegal characters reach envelope.to and RCPT TO, causing rejected delivery or parser interpretation conflicts that bypass domain allow-lists.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified in Nodemailer versions prior to 10.0.13 stems from an improper input validation mechanism within its email address parsing logic. Specifically, the library fails to properly strip RFC 5322 comments when processing addresses enclosed in angle brackets. According to RFC 5322, comments can be included anywhere in a mailbox specification by enclosing them in parentheses, such as [email protected] (comment). While this syntax is valid for display purposes or metadata within email headers, it introduces significant risks if the raw string containing these comments is passed directly to lower-level SMTP envelope commands without sanitization. The flaw allows attackers to inject parenthesized content into domain fields that are subsequently used in critical delivery operations.
This technical deficiency leads to a scenario where illegal characters and unintended data structures reach the RCPT TO command of the Simple Mail Transfer Protocol (SMTP). When an application relies on Nodemailer to construct email envelopes, it may inadvertently include these unsanitized comments as part of the recipient address string sent to the mail transfer agent. This can cause immediate delivery failures due to syntax errors rejected by strict SMTP servers that do not permit such formatting in envelope recipients. More critically, this behavior creates opportunities for parser interpretation conflicts and logic bypasses. If an application employs domain allow-listing or validation mechanisms that operate on the raw input string rather than a normalized version of it, attackers can exploit the comment structure to obscure malicious domains or inject unauthorized recipient addresses.
The operational impact of this vulnerability is twofold: reliability degradation and potential security policy evasion. On one hand, applications may experience intermittent delivery failures as mail servers reject messages with malformed envelope recipients containing embedded comments. This disrupts business continuity for services dependent on reliable email communication. On the other hand, if the application logic uses the unsanitized address string to check against a whitelist of allowed domains, an attacker could potentially craft addresses where the comment contains a malicious domain or bypasses regex-based filters designed to block specific patterns. For instance, if a filter checks for exact matches without accounting for RFC 5322 syntax nuances, it might incorrectly validate a request that ultimately results in sending mail to an unauthorized destination via SMTP envelope manipulation.
From a classification perspective, this issue aligns with CWE-20 Improper Input Validation and CWE-74 Injection into Smtp Commands under the broader category of injection vulnerabilities. It also relates to ATT&CK technique T1583 Acquire Infrastructure: Domains, as it could facilitate unauthorized email relay or phishing campaigns by bypassing domain restrictions. The root cause lies in the assumption that address strings are safe for direct use in SMTP commands without stripping non-essential syntactic elements defined by RFC standards.
To mitigate this vulnerability, developers must upgrade Nodemailer to version 10.0.13 or later, where the address parser has been corrected to properly strip RFC 5322 comments before passing addresses to the envelope construction logic. Additionally, applications should implement defense-in-depth strategies by validating and sanitizing recipient addresses at the application layer using strict regular expressions that only permit alphanumeric characters, hyphens, underscores, periods, and plus signs in domain parts. It is crucial to ensure that any user-supplied input intended for email fields undergoes normalization processes consistent with RFC 5322 standards before being handed off to mail libraries or SMTP clients. Regular security audits of email handling code should also verify that envelope recipients are derived from cleaned data sources rather than raw user inputs.