What does the SpamAssassin rule FONT_INVIS_MSGID test for?

9Marketer opinions 3Expert opinions 3Technical articles 2Resources

Summary

The SpamAssassin rule FONT_INVIS_MSGID is a metarule designed to identify spam emails that attempt to bypass filters by using a combination of invisible text and a suspicious or manipulated Message-ID. The 'suspicious' Message-ID often deviates from expected formats and patterns, especially in relation to the sender and received headers, indicating an attempt to conceal the message's true nature. This rule is part of a broader strategy to detect manipulated message headers aimed at evading spam detection.

Key findings

Invisible Text: The rule detects the presence of text rendered invisible or nearly so, often through techniques like white text on a white background or extremely small fonts.
Suspicious Message-ID: It identifies Message-IDs that are not consistent with expected formats or sender patterns, suggesting manipulation or forgery.
Spam Filter Evasion: The combined use of invisible text and a suspicious Message-ID is a deliberate attempt to bypass spam filters and deliver unwanted messages.
Meta Rule: FONT_INVIS_MSGID is a meta rule that activates only when both invisible text and a suspicious Message-ID are detected.
Rule Availability: While the rule exists, older SpamAssassin versions may not have the most current definitions, making updates important.

Key considerations

Sender Authentication: Validate that the Message-ID aligns with the sending domain and expected patterns to ensure legitimacy.
Header Analysis: Thorough analysis of email headers is crucial for identifying inconsistencies and suspicious patterns.
Evolving Techniques: Spammers are constantly developing new methods to evade detection, so continuous adaptation and updating of spam filters are necessary.
Context Matters: Consider the overall context of the email to avoid false positives. While the rule is a strong indicator, it is not always definitive.

What email marketers say
9Marketer opinions

The SpamAssassin rule FONT_INVIS_MSGID is designed to identify spam emails that attempt to bypass filters by using hidden text (such as white text on a white background or very small fonts) combined with a manipulated or suspicious Message-ID. This combination is a common tactic used by spammers to evade detection and appear legitimate.

Key opinions

Hidden Text Detection: The rule specifically looks for instances of invisible or nearly invisible text within the email content.
Suspicious Message-ID: It also checks for Message-IDs that deviate from expected patterns or appear to be forged, indicating manipulation.
Spam Filter Evasion: The primary goal of using these techniques is to bypass spam filters and deliver unwanted messages.
Common Spam Tactic: The combination of hidden text and a manipulated Message-ID is a frequently observed tactic in spam campaigns.

Key considerations

Rule Versions: Older versions of SpamAssassin may have different rule sets, so it's important to use the most up-to-date version for accurate detection.
Sender Authentication: A valid Message-ID should align with the sending domain or service; discrepancies can indicate potential spam.
Context Matters: While the presence of hidden text and a suspicious Message-ID is a strong indicator, the context of the message should also be considered to avoid false positives.

Marketer view

Email marketer from EmailVendorSelection says that the FONT_INVIS_MSGID rule is triggered when a message contains hidden text (like white text on a white background) and the message ID looks suspicious. This rule is designed to catch spammers who try to evade detection by making their messages appear legitimate.

June 2023 - EmailVendorSelection

Marketer view

Email marketer from MailChannels explains that it identifies messages where there's an attempt to hide text (making it invisible or very small) combined with a message ID that doesn't match the typical pattern for the sending domain or service.

February 2024 - MailChannels

Marketer view

Email marketer from EmailDeliverabilityGuru explains that it is a rule that looks for messages where there's an attempt to hide text combined with a message ID that doesn't match the typical pattern for the sending domain.

November 2023 - EmailDeliverabilityGuru

Marketer view

Email marketer from Reddit user states the FONT_INVIS_MSGID rule identifies emails trying to bypass filters by hiding text and using a fake message ID.

October 2023 - Reddit

Marketer view

Email marketer from Stack Overflow user explains that the rule is triggered when spam filters detect potentially hidden text and a seemingly spoofed message ID, which are both common spam tactics.

April 2023 - Stack Overflow

Marketer view

Marketer from Email Geeks shares that spamassassin rules are still available in the archives, but the rules you can check are for versions 3.3 and below.

June 2024 - Email Geeks

Marketer view

Email marketer from Reddit user shares that this rule looks for messages where the sender is trying to hide content using techniques like tiny fonts or white-on-white text, combined with a forged or unusual message ID.

December 2021 - Reddit

Marketer view

Email marketer from EmailGeek shares that the FONT_INVIS_MSGID rule is specifically designed to detect emails that use invisible text in combination with a manipulated message ID to bypass spam filters.

March 2025 - EmailGeek

Marketer view

Email marketer from Email Security Forum responds that it's designed to catch spammers who use hidden text to bypass content filters and manipulate the message ID to appear legitimate.

June 2024 - Email Security Forum

What the experts say
3Expert opinions

The SpamAssassin rule FONT_INVIS_MSGID is a metarule designed to detect spam emails that employ a combination of invisible text and a suspicious Message-ID. This combination indicates an attempt to bypass spam filters by concealing the true nature of the message and avoiding content-based detection. The 'suspicious' Message-ID often deviates from expected formats, especially in relation to the sender and received headers.

Key opinions

Invisible Text: The rule identifies emails where text is intentionally hidden from the recipient, often using techniques like white text on a white background or extremely small fonts.
Suspicious Message-ID: It also checks for Message-IDs that appear abnormal or inconsistent with the sender's domain or expected format, raising suspicion of forgery.
Filter Bypass: The ultimate goal of using these techniques is to circumvent spam filters and deliver unwanted messages to the recipient's inbox.
Metarule: The rule is a 'metarule', meaning it's triggered by a combination of other factors (invisible text AND suspicious message-ID).

Key considerations

Unexpected Format: A suspicious Message-ID could indicate an unexpected format for the sender given the received headers, requiring careful analysis of email headers.
Intentional Concealment: The use of invisible text is almost always indicative of malicious intent to hide content from the user and/or automated filters.
Header Analysis: Understanding the expected format and origin of Message-IDs is crucial in determining their legitimacy and identifying potential spam.

Expert view

Expert from Word to the Wise responds that the rule is triggered when a message contains invisible text and a suspicious Message-ID. This suggests an attempt to hide the true nature of the message and bypass spam filters.

January 2022 - Word to the Wise

Expert view

Expert from Email Geeks explains that FONT_INVIS_MSGID is a metarule that activates when a message has invisible text and the message-ID appears suspicious, potentially due to an unexpected format for the sender given the received headers.

April 2022 - Email Geeks

Expert view

Expert from Spam Resource explains that FONT_INVIS_MSGID identifies emails employing techniques like using white text on a white background or very small fonts to hide content from the recipient, a tactic often used to bypass spam filters.

November 2024 - Spam Resource

What the documentation says
3Technical articles

The SpamAssassin rule FONT_INVIS_MSGID is designed to identify spam messages that attempt to evade detection through a combination of techniques: rendering text invisible or nearly invisible, and using a suspicious Message-ID format. This rule is part of a larger strategy to detect manipulated message headers aimed at bypassing spam filters.

Key findings

Invisible Text: The rule detects messages containing text rendered invisible, often through techniques like using white text on a white background.
Suspicious Message-ID: The rule identifies Message-IDs that deviate from expected formats and norms, particularly when considered in conjunction with specific sender patterns.
Evasion Technique: The combination of invisible text and a suspicious Message-ID is a deliberate attempt to evade spam detection mechanisms.
Broader Strategy: This rule is one component of a more comprehensive approach to identifying manipulated messages and preventing spam from reaching inboxes.

Key considerations

Sender Patterns: Analyzing sender patterns in conjunction with Message-ID formats is crucial for accurately identifying suspicious messages.
Combination of Factors: The presence of both invisible text and a suspicious Message-ID significantly increases the likelihood of a message being spam.
Constant Evolution: Spam techniques are constantly evolving, so spam filters must adapt to identify new methods of evasion.

Technical article

Documentation from SpamAssassin Source Code reveals that the rule checks for combinations of invisible characters (e.g., white text on white background) and Message-IDs that deviate from expected norms, especially in conjunction with specific sender patterns.

October 2024 - SpamAssassin Source Code

Technical article

Documentation from GTUBE explains that the FONT_INVIS_MSGID rule is part of a broader strategy to identify messages that attempt to evade detection by concealing text and manipulating message headers.

January 2024 - GTUBE

Technical article

Documentation from Apache SpamAssassin Wiki explains that FONT_INVIS_MSGID identifies messages that contain text rendered invisible (or nearly so) and have a suspicious Message-ID format, often indicative of spam.

September 2024 - Apache SpamAssassin Wiki