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Threadjacking is the practice of hijacking an existing email thread by inserting unrelated content or topics into an ongoing conversation. This disruptive behavior derails the original discussion, confuses recipients, and is commonly used by spammers to bypass filters by piggybacking on legitimate email threads.
Threadjacking poses significant risks to email security and communication efficiency. For organizations, it can lead to important messages being overlooked when buried under unrelated content. Critical project updates, client communications, or time-sensitive decisions may get lost in hijacked threads, leading to missed deadlines and miscommunication. From a security perspective, threadjacking is a common vector for phishing attacks and malware distribution. By inserting malicious links into trusted conversation threads, attackers can trick recipients into clicking harmful content they would otherwise ignore. This technique is particularly effective in business email compromise (BEC) attacks where criminals impersonate colleagues or vendors. Threadjacking also damages email deliverability and sender reputation. When spam or malicious content appears in otherwise legitimate threads, it can trigger spam complaints and cause email providers to flag the entire thread or associated domains. Organizations that experience frequent threadjacking may find their legitimate emails landing in spam folders.
Threadjacking exploits the way email clients group messages into conversations. When someone replies to an email, the client uses headers like In-Reply-To and References to chain messages together. Threadjackers take advantage of this by replying to existing threads with completely unrelated content, making their messages appear as part of a legitimate conversation. Spammers and malicious actors use threadjacking to bypass spam filters that might otherwise block their messages. Since the email appears to be part of an ongoing trusted conversation, filters are less likely to flag it. They may gain access to threads by compromising an account in the conversation, intercepting forwarded messages, or being CC'd on replies that include the original thread. In corporate environments, threadjacking can happen unintentionally when employees use the reply function to start new conversations simply because it's faster than composing a new email. This creates confusion as unrelated topics get mixed into existing discussions, making it difficult to search for and reference important information later.
Threadjacking involves inserting unrelated content into existing legitimate email threads, while email spoofing means forging the sender address to impersonate someone else. Threadjacking exploits conversation threading, whereas spoofing exploits trust in the sender's identity. Both techniques are often used together in sophisticated attacks.
Look for sudden topic changes in the middle of a conversation, unexpected attachments or links, changes in writing style or tone, and requests that seem out of context. Also check if the sender's email address matches previous messages in the thread and verify any unusual requests through a separate communication channel.
Advanced email security solutions can detect some threadjacking attempts by analyzing message context, link reputation, and behavioral patterns. However, threadjacking that exploits compromised legitimate accounts is harder to detect automatically. A combination of technical controls and user awareness training provides the best defense.
Yes, even unintentional threadjacking can cause significant problems. It makes email archives difficult to search, confuses recipients about discussion topics, and can cause important information to be overlooked. It also wastes time as recipients must sort through irrelevant content to find what they need.
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