CVE-2019-10732 in KMail
Summary
by MITRE
In KDE KMail 5.2.3, an attacker in possession of S/MIME or PGP encrypted emails can wrap them as sub-parts within a crafted multipart email. The encrypted part(s) can further be hidden using HTML/CSS or ASCII newline characters. This modified multipart email can be re-sent by the attacker to the intended receiver. If the receiver replies to this (benign looking) email, they unknowingly leak the plaintext of the encrypted message part(s) back to the attacker.
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Analysis
by VulDB Data Team • 09/08/2025
The vulnerability identified as CVE-2019-10732 represents a sophisticated email security flaw within KDE KMail version 5.2.3 that exploits the handling of encrypted email content through crafted multipart email structures. This vulnerability specifically targets the way email clients process and display encrypted content, creating a scenario where legitimate encrypted messages can be manipulated to reveal their plaintext content during normal email interactions. The flaw operates by allowing attackers to embed legitimate encrypted email parts within malicious multipart email structures, effectively creating a covert channel for information leakage.
The technical implementation of this vulnerability leverages the multipart email format specification while exploiting how KMail processes nested email structures. Attackers can craft emails where encrypted content appears as sub-parts within a larger multipart message, using HTML/CSS styling or ASCII newline characters to obscure the encrypted components from casual inspection. This obfuscation technique makes the malicious email appear benign to users who might not notice the hidden encrypted content. When victims interact with such emails by replying, the email client's handling of the multipart structure inadvertently exposes the plaintext of the embedded encrypted messages to the attacker.
The operational impact of this vulnerability extends beyond simple information disclosure, creating a persistent threat vector for attackers who can systematically harvest plaintext content from encrypted communications. The attack requires the attacker to first obtain legitimate encrypted emails, which they can then manipulate and retransmit to victims. When victims reply to these crafted messages, they unknowingly participate in the attack by sending back the plaintext of the embedded encrypted content, effectively creating a feedback loop for information leakage. This vulnerability specifically affects the secure communication protocols that users rely on for protecting sensitive information, undermining the fundamental purpose of encryption.
The security implications of CVE-2019-10732 align with CWE-200, which addresses information exposure, and demonstrates how improper handling of email content can lead to information leakage through seemingly benign interactions. The vulnerability also relates to ATT&CK technique T1566, which covers spearphishing attacks that can be enhanced through sophisticated content manipulation. Organizations using KDE KMail 5.2.3 are particularly vulnerable as the attack does not require sophisticated technical skills from the attacker beyond knowledge of email format manipulation, making it a significant risk for any environment where encrypted email communications are common. The vulnerability represents a critical failure in email client security that affects the trust model of encrypted communications.
Mitigation strategies for this vulnerability primarily involve upgrading to patched versions of KDE KMail, as the issue was resolved through proper handling of multipart email structures and improved validation of encrypted content within nested email formats. Security administrators should also implement email filtering solutions that can detect and block suspicious multipart email patterns, while users should be educated about the risks of replying to unfamiliar or suspicious emails. Additionally, organizations can consider implementing email security gateways that perform deeper inspection of multipart email structures to identify potentially malicious content manipulation. The vulnerability underscores the importance of proper email client security implementation and highlights the need for continuous security testing of email handling components, particularly those involved in encryption and content processing.