Summary
OpenDKIM, a widely used open-source implementation for DKIM signing and verification, can sometimes incorrectly validate DKIM signatures, especially for emails with unusually long headers. This issue stems from how OpenDKIM handles header canonicalization, a process that normalizes email headers before signing or verification. A known bug, observed since 2014, leads to discrepancies in how such headers are processed, causing valid signatures to be deemed invalid by other systems (or vice versa, if OpenDKIM is the signing entity).
Key findings
- Header canonicalization: OpenDKIM has been observed to incorrectly canonicalize headers containing exceptionally long words or those that undergo immediate folding after the header name.
- Longstanding bug: This specific canonicalization bug in OpenDKIM dates back to at least 2014, and despite efforts, patches have not fully resolved the issue in all versions.
- Impact on validation: If an email is signed by an OpenDKIM instance affected by this bug, its DKIM validation will likely fail on recipient systems that do not exhibit the same bug, and vice versa.
- Deliverability implications: False DKIM validation failures can lead to DMARC rejections, increased bounce rates, and overall negative impacts on email deliverability.
Key considerations
- Software version: While a significant bug, ensure your OpenDKIM instance is running the latest available version, as some improvements or workarounds might have been introduced.
- Header structure: Pay close attention to email header construction, particularly for MIME-encoded content or unusually long values, which are prone to triggering this canonicalization issue.
- Monitor DMARC reports: Regularly review DMARC aggregate reports to identify sources of DKIM failures and pinpoint specific issues related to your sending domains.
- Test email authentication: Use an email deliverability tester to proactively check your DKIM, SPF, and DMARC authentication before sending campaigns.
- Canonicalization awareness: A foundational understanding of DKIM canonicalization (simple/relaxed) can help diagnose and prevent these types of signature validation problems.
What email marketers say
Email marketers are often the first to notice the effects of DKIM validation failures, such as diminished inbox placement or increased spam flagging. While they may not directly manage mail server configurations, the downstream impact on campaign performance is significant. They emphasize the frustration of authentication issues that appear subtle but can lead to widespread deliverability problems, underscoring the importance of robust technical setup and continuous monitoring.
Key opinions
- Deliverability impact: Marketers confirm that invalid DKIM signatures directly contribute to emails landing in spam folders or being outright rejected by recipient servers.
- Debugging complexity: Identifying the precise technical reason for DKIM validation failures can be challenging for marketers without a deep understanding of mail transfer agent (MTA) operations.
- DMARC reliance: Many marketers rely on DMARC reports as a primary indicator to surface underlying authentication issues, even if the technical details of the failure are obscure.
- Focus on inbox placement: The ultimate goal for marketers is inbox placement and engagement, making any authentication failure a critical barrier to campaign success.
Key considerations
- Leverage authentication tools: Regularly use email deliverability tools to check the validation status of your DKIM, SPF, and DMARC records.
- Communicate with IT: Establish clear communication channels with your technical team or email service provider (ESP) to promptly diagnose and resolve authentication issues.
- Monitor metrics: Keep a close watch on bounce rates, spam complaint rates, and Google Postmaster Tools data for any signs of authentication failures.
- Stay informed: While technical implementation may not be your direct responsibility, understanding common causes of email authentication failures can help in troubleshooting.
Marketer view
A marketer from Email Geeks suggests that the issue of OpenDKIM producing false validation failures for long headers is quite significant. This highlights a critical bug that can impede email deliverability.
Marketer view
A marketer from Email Geeks humorously questions if the latest version of OpenDKIM is being used. This implies that keeping software updated is a common troubleshooting step for such issues.
What the experts say
Email deliverability experts frequently encounter the nuances of MTA configurations and authentication protocols like DKIM. They understand that specific software implementations, such as OpenDKIM, can harbor subtle bugs that lead to widespread validation failures, even when general configurations appear to be correct. This specific issue highlights the critical challenge of canonicalization, especially when dealing with long or complex header lines, which can deviate from expected standards and cause legitimate emails to fail authentication.
Key opinions
- Canonicalization accuracy: Experts pinpoint incorrect canonicalization of email headers as a primary technical reason for DKIM validation failures.
- Software-specific bugs: They identify that specific software versions or implementations, like OpenDKIM, can contain bugs that subtly alter how signatures are generated or verified.
- Impact on DMARC: Experts note that even minor software bugs leading to invalid DKIM signatures can directly result in DMARC enforcement failures, causing email rejections or quarantines.
- Patching importance: The consensus is that applying patches and regular software updates are crucial for addressing known authentication vulnerabilities and maintaining deliverability.
Key considerations
- RFC compliance: Ensure your mail server's DKIM implementation strictly adheres to RFC specifications, especially regarding header folding and canonicalization, to guarantee interoperability.
- Regular testing: Implement robust testing of email authentication across various recipient systems to uncover subtle discrepancies, especially since tools and Postmaster reports can conflict.
- Source code vigilance: For critical email systems, it is vital to audit source code or stay updated on known bugs and fixes in open-source projects like OpenDKIM via their official issue trackers.
- Backup authentication: While addressing root causes, ensure your SPF and DMARC records are configured optimally to provide redundancy and prevent total email deliverability loss.
Expert view
An expert from Email Geeks discovered an interesting phenomenon where OpenDKIM was giving false validation failures. This finding pointed to a significant issue within the system.
Expert view
An expert from Email Geeks indicates that if a header has a very long word at the beginning, especially with MIME encoding, the header folding might occur immediately after the header name. This specific structure can cause canonicalization problems.
What the documentation says
Official documentation for DKIM (RFC 6376) and OpenDKIM lays out the precise mechanisms for signing and verifying email messages. It meticulously defines canonicalization methods (simple and relaxed) for both headers and bodies, detailing how white space and line folding should be handled to ensure consistent signatures. Any deviation from these specifications, even subtle ones within software implementations, can lead to validation failures, making adherence to these standards critical for proper email authentication.
Key findings
- RFC 6376 specifies canonicalization: The DKIM standard precisely defines how email headers and bodies must be prepared (canonicalized) for signing and verification to prevent message modifications from invalidating the signature.
- Header folding rules: Internet Message Format RFCs (like RFC 5322) specify how long header fields can be 'folded' onto multiple lines, which is a critical aspect for their correct canonicalization and subsequent signing.
- Discrepancy impact: Any inconsistency between how a DKIM signer canonicalizes a header and how a verifier canonicalizes the same header will inevitably result in a signature mismatch.
- OpenDKIM's adherence: OpenDKIM is an open-source implementation designed to facilitate DKIM, and its operational behavior should ideally align perfectly with RFC standards to ensure broad interoperability.
Key considerations
- Strict RFC adherence: Developers and administrators should prioritize strict adherence to RFCs in their email systems to guarantee proper DKIM validation and broad interoperability.
- Testing edge cases: Documentation often implicitly or explicitly emphasizes the importance of testing diverse email formats, particularly those with complex or long headers, to uncover subtle canonicalization issues.
- Monitor release notes: Regularly reviewing release notes and bug trackers for OpenDKIM and related email software can provide crucial insights into known issues, their impact, and any available resolutions or workarounds.
- Configuration parameters: Understanding specific OpenDKIM configuration parameters related to canonicalization methods and header handling can help administrators fine-tune their setup to avoid validation failures.
Technical article
The Mail-in-a-Box GitHub issue discusses how using relaxed or simple canonicalization can lead to invalid DKIM signatures for long header lines. This highlights a known vulnerability in how DKIM processes certain header formats.
Technical article
RFC 6376, in its canonicalization section, states that a canonicalization algorithm defines the method used to prepare a message for signing and verification. This preparation is essential to prevent subtle modifications to the message from invalidating the signature.
Related resources
10 resources
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