What causes OpenDKIM to incorrectly validate DKIM signatures for emails with long headers?

OpenDKIM can incorrectly validate DKIM signatures for emails with long headers due to a combination of canonicalization discrepancies, header modifications during transit, and specific software bugs. Issues often arise from OpenDKIM's inconsistent handling of whitespace, line folding, and character encoding in complex or mime-encoded long headers, sometimes deviating from standard RFCs or the signing server's process. Furthermore, alterations made by various message transfer agents, email clients, or security solutions after an email has been signed can change long headers, leading to a mismatch with the original signature. This problem is compounded by known, long-standing bugs related to specific header types like 'List-Unsubscribe' and vulnerabilities concerning excessively long header names.

OpenDKIM can incorrectly validate DKIM signatures for emails containing long headers due to a range of factors, primarily stemming from its sensitive interpretation of header formatting. This often involves discrepancies in how OpenDKIM performs canonicalization, particularly for long words within mime-encoded headers like 'List-Unsubscribe', or when handling varied whitespace and line folding. Furthermore, modifications made to these headers by various intermediaries-such as Message Transfer Agents (MTAs), email clients, or security solutions like spam filters and antivirus software-after an email has been signed, can alter the header content, leading to a mismatch with the original signature. These issues are compounded by known, long-standing bugs within OpenDKIM itself related to header parsing and unfolding, which can cause legitimate emails to fail validation, even in the latest software versions.

Incorrect DKIM validation by OpenDKIM for emails with long headers often results from the intricate interplay of several factors, including the precise handling of header canonicalization, unintended modifications during email transit, and specific internal software defects. OpenDKIM's strict interpretation of header formatting can lead to validation failures when even minor differences exist between the signed header's original form and its state during verification. This is particularly true for complex, multi-line headers where whitespace, line folding, or character encoding variations can cause mismatches. Furthermore, various intermediate systems, such as mail servers or security solutions, may inadvertently alter these headers after the DKIM signature has been applied, further complicating accurate validation. Persistent software bugs within OpenDKIM also contribute to these false negatives, making robust email authentication challenging for senders utilizing long header fields.

OpenDKIM's occasional failure to correctly validate DKIM signatures for emails with long headers stems from a combination of specific software bugs, its sensitivity to varied header formatting, and potential deviations from internet standards. These issues include a known bug in older versions that mishandled relaxed canonicalization for headers with mixed tabs and spaces, as well as a general sensitivity to how email clients format and fold lengthy headers. Furthermore, OpenDKIM's internal parsing or 'unfolding' of these complex headers may not always perfectly align with RFC specifications, leading to canonical form mismatches. In severe cases, vulnerabilities like buffer overflows triggered by excessively long header names can completely disrupt the validation process, marking legitimate emails as invalid.