CVE-2019-15237 in Roundcube Webmail
Summary
by MITRE
Roundcube Webmail through 1.3.9 mishandles Punycode xn-- domain names, leading to homograph attacks.
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Analysis
by VulDB Data Team • 04/30/2026
The vulnerability identified as CVE-2019-15237 affects Roundcube Webmail versions prior to 1.3.9 and represents a critical security flaw in domain name handling that enables homograph attacks. This issue arises from the webmail application's improper management of Punycode encoded domain names, specifically those beginning with the xn-- prefix used for internationalized domain names. The flaw allows attackers to exploit the visual similarity between ASCII characters and Unicode characters from different scripts, creating deceptive email addresses that appear legitimate but direct to malicious servers.
The technical root cause stems from Roundcube's insufficient validation and normalization of domain names during email address processing. When processing email addresses containing Punycode domains, the application fails to properly canonicalize or validate these domain names against their Unicode equivalents. This creates a scenario where an attacker can register a domain name using Unicode characters that visually resemble common ASCII domains, such as using Cyrillic characters that look identical to Latin characters in certain fonts. The vulnerability specifically impacts the email address parsing and verification mechanisms within the Roundcube interface, allowing malicious actors to craft addresses that appear trustworthy to users who may not notice the subtle visual differences.
The operational impact of this vulnerability extends beyond simple phishing attacks, as it enables sophisticated social engineering campaigns that can bypass traditional email security measures. Users may inadvertently trust emails from addresses that appear to come from legitimate sources but actually route to attacker-controlled servers. This flaw particularly affects organizations that rely heavily on email communication and have users who may not be trained to recognize subtle visual differences in internationalized domain names. The attack vector is particularly dangerous because it can be exploited through various means including email spoofing, malicious attachments, or even credential harvesting through deceptive login prompts that appear to be from trusted domains.
Organizations utilizing Roundcube Webmail versions prior to 1.3.9 should immediately implement mitigation strategies including updating to the patched version, implementing additional email filtering rules, and conducting user awareness training on recognizing suspicious email addresses. The vulnerability aligns with CWE-1004 weakness category related to security-relevant CWEs that are commonly exploited, and it maps to ATT&CK technique T1566.001 for spearphishing via email. Network administrators should also consider implementing DNS-based filtering solutions and monitoring for suspicious domain name patterns that may indicate attempts to exploit this vulnerability. The fix implemented in Roundcube 1.3.9 addresses the core issue by properly handling Punycode domain names through enhanced validation and normalization processes that ensure domain name consistency between Unicode and ASCII representations, thereby preventing the homograph attack vectors that were previously possible.