CVE-2025-3875 is a vulnerability in Mozilla Thunderbird email client affecting versions prior to 128.10.1 and 138.0.1. The root cause is Thunderbird's flawed parsing of the From header in incoming emails. Specifically, if the mail server accepts an invalid From address format, such as a malformed string like "Spoofed Name ", Thunderbird interprets the actual sender address as spoofed@example.com or another attacker-controlled address. This parsing flaw enables an attacker to craft emails that appear to originate from arbitrary senders, effectively bypassing sender verification mechanisms within the client. The vulnerability does not require any privileges or user interaction and can be exploited remotely by sending a specially crafted email. The impact is primarily on confidentiality, as recipients may be misled about the true origin of messages, increasing the risk of phishing, fraud, or social engineering attacks. The vulnerability is tracked under CWE-290 (Authentication Bypass by Spoofing) and has a CVSS v3.1 base score of 7.5, indicating high severity. No public exploits have been reported yet, but the flaw's nature makes it a serious concern for organizations relying on Thunderbird for secure communications. Mozilla has released fixed versions 128.10.1 and 138.0.1 to address this issue, though patch links were not provided in the source information.

For European organizations, this vulnerability poses a significant risk to email trust and security. Spoofed sender addresses can facilitate phishing campaigns, business email compromise (BEC), and social engineering attacks targeting employees, partners, and customers. Confidential information could be disclosed if users are tricked into responding to spoofed emails or clicking malicious links. The integrity of email communications is undermined, potentially damaging organizational reputation and leading to financial losses. Since Thunderbird is widely used in both public and private sectors across Europe, especially in government, finance, and education, the impact could be broad. Attackers exploiting this flaw can bypass sender verification without needing authentication or user interaction, increasing the likelihood of successful attacks. Although availability is not directly affected, the indirect consequences of successful spoofing attacks can disrupt operations and require costly incident response efforts.

European organizations should immediately upgrade all Thunderbird clients to versions 128.10.1 or 138.0.1 or later to remediate this vulnerability. Until patches are applied, implement strict email filtering rules that detect and quarantine messages with malformed From headers or suspicious sender addresses. Deploy and enforce email authentication standards such as SPF, DKIM, and DMARC to help identify and block spoofed emails at the gateway level. Train users to recognize signs of spoofed emails and encourage verification of unexpected or unusual requests via alternative communication channels. Network-level protections like advanced threat protection (ATP) and sandboxing can help detect malicious payloads delivered through spoofed emails. Regularly audit and monitor email logs for anomalies indicative of spoofing attempts. Coordinate with email service providers to ensure proper validation and rejection of invalid From headers. Finally, maintain an incident response plan specifically addressing email spoofing and phishing scenarios.