Renderer content disposition encoding#7
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romsahel wants to merge 2 commits intoparser-rfc-2822-memory-optimizationfrom
Open
Renderer content disposition encoding#7romsahel wants to merge 2 commits intoparser-rfc-2822-memory-optimizationfrom
romsahel wants to merge 2 commits intoparser-rfc-2822-memory-optimizationfrom
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…ype parameters RFC 2047 encoded-words are forbidden in MIME parameter values. This change implements RFC 2231 encoding (charset'language'percent-encoded) for Content-Type and Content-Disposition parameters while preserving RFC 2047 for other headers.
Fix parser to correctly handle simple RFC 2231 extended parameters (e.g., filename*=UTF-8''value) by using `trim: true` in String.split.
romsahel
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Feb 4, 2026
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The parser currently only supported Parameter Value Continuations which has the format key*index="value"
Content-Type: message/external-body; access-type=URL;
URL*0="ftp://";
URL*1="cs.utk.edu/pub/moore/bulk-mailer/bulk-mailer.tar"
The non-continuation version is simply: key*="value"
romsahel
commented
Feb 4, 2026
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+177
| defp render_subtypes([{key, value} | subtypes], :rfc_2231) do | ||
| key = String.replace(key, "_", "-") | ||
| value = encode_header_value(value, :quoted_printable) | ||
|
|
||
| value = | ||
| if value =~ ~r/[\s()<>@,;:\\<\/\[\]?=]/ do | ||
| "\"#{value}\"" | ||
| else | ||
| value | ||
| end | ||
| if contains_non_ascii?(value) do | ||
| value = encode_header_value(value, :rfc_2231) | ||
| ["#{key}*=UTF-8''#{value}" | render_subtypes(subtypes, :rfc_2231)] | ||
| else | ||
| value = maybe_wrap_in_quotes(value) | ||
| ["#{key}=#{value}" | render_subtypes(subtypes, :rfc_2231)] | ||
| end | ||
| end |
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new handler is here.
The logic of the other handler is untouched
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Problem Statement
RFC 2047 defines "encoded-words" (the
=?UTF-8?Q?encoded-text?=format) for encoding non-ASCII characters in email headers. However, RFC 2047 explicitly states:This means:
Subject: =?UTF-8?Q?caf=C3=A9?=From: =?UTF-8?Q?Jos=C3=A9?= <jose@example.com>Content-Disposition: attachment; filename="=?UTF-8?Q?caf=C3=A9.pdf?="These headers must be encoded using the rfc2231 specifications:
parameter*=charset'language'encoded-value*) suffix when encoding is usedExamples:
Current implementation
In
lib/mail/renderers/rfc_2822.ex, therender_subtypes/1function is using RFC 2047 encoding for all header parameters.The proposed fix
Introduce a new
render_subtypesthat handles those headers (@rfc2231_headers) and apply the proper encoding when necessary (ie if it contains non-ASCII characters