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#6276 Improve message parsing for signed message #6277

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f8f8396
feat: improve message parsing for signed message
martgil Aug 21, 2026
d66639c
fix: unsafe non-null return values for getSubjectFromNode
martgil Aug 21, 2026
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83 changes: 62 additions & 21 deletions extension/js/common/core/mime.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -60,6 +60,7 @@ export type MimeProccesedMsg = {
type SendingType = 'to' | 'cc' | 'bcc';

export class Mime {
private static readonly MAX_SIGNED_CONTENT_DEPTH = 50;

@sosnovsky sosnovsky Aug 21, 2026

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there is similar recursive call in

flowcrypt-browser/extension/js/common/message-renderer.ts

Line 618 in 78f34e3

if (!sigResult?.match && signedContentInDecryptedData) {

which can cause extension lag for nested signed content. we'll need to implement similar fix there, but let's leave it for separate PR

@martgil martgil Aug 21, 2026

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Got it - Will remember this one and fix it on separate PR after this one.

public static processBody(decoded: MessageBody): MsgBlock[] {
const blocks: MsgBlock[] = [];
if (decoded.text) {
Expand Down Expand Up @@ -198,15 +199,15 @@ export class Mime {
};
return await new Promise((resolve, reject) => {
try {
parser.onend = async () => {
parser.onend = () => {
try {
for (const name of Object.keys(parser.node.headers)) {
mimeContent.headers[name] = parser.node.headers[name][0].value;
}
mimeContent.rawSignedContent = Mime.retrieveRawSignedContent([parser.node]);
if (!mimeContent.subject && mimeContent.rawSignedContent) {
const rawSignedContentDecoded = await Mime.decode(Buf.fromUtfStr(mimeContent.rawSignedContent));
mimeContent.subject = rawSignedContentDecoded.subject;
const signedContentNode = Mime.retrieveSignedContentNode([parser.node]);
if (!mimeContent.subject && signedContentNode) {
mimeContent.subject = Mime.getSubjectFromNode(signedContentNode);
}
for (const node of Object.values(leafNodes)) {
const nodeType = Mime.getNodeType(node);
Expand Down Expand Up @@ -356,33 +357,73 @@ export class Mime {
return { ...result, from };
}

private static retrieveRawSignedContent(nodes: MimeParserNode[]): string | undefined {
private static isSignedContentNode(node: MimeParserNode): boolean {
/* eslint-disable no-underscore-dangle */
if (node._isMultipart === 'signed') {
return true;
}
return (
node._isMultipart === 'mixed' &&
node._childNodes !== false &&
node._childNodes.length === 2 &&
(Mime.getNodeType(node._childNodes[1]) === 'application/pgp-signature' || node._childNodes[1].contentType?.params?.name === 'signature.asc')
);
/* eslint-enable no-underscore-dangle */
}

private static retrieveSignedContentNode(nodes: MimeParserNode[], depth = 0): MimeParserNode | undefined {
if (depth > Mime.MAX_SIGNED_CONTENT_DEPTH) {
return undefined;
}
for (const node of nodes) {
/* eslint-disable no-underscore-dangle */
if (!node._childNodes || !node._childNodes.length) {
continue; // signed nodes tend contain two children: content node, signature node. If no node, then this is not pgp/mime signed content
}
const isSigned = node._isMultipart === 'signed';
const isMixedWithSig =
node._isMultipart === 'mixed' &&
node._childNodes.length === 2 &&
(Mime.getNodeType(node._childNodes[1]) === 'application/pgp-signature' || node._childNodes[1].contentType?.params?.name === 'signature.asc');
if (isSigned || isMixedWithSig) {
// PGP/MIME signed content uses <CR><LF> as in // use CR-LF https://tools.ietf.org/html/rfc3156#section-5
// however emailjs parser will replace it to <LF>, so we fix it here
let rawSignedContent = node._childNodes[0].raw.replace(/\r?\n/g, '\r\n');
if (rawSignedContent.endsWith('--')) {
// end of boundary without a mandatory newline
rawSignedContent += '\r\n'; // emailjs wrongly leaves out the last newline, fix it here
}
return rawSignedContent;
if (Mime.isSignedContentNode(node)) {
return node._childNodes[0];
}
return Mime.retrieveRawSignedContent(node._childNodes);
return Mime.retrieveSignedContentNode(node._childNodes, depth + 1);
/* eslint-enable no-underscore-dangle */
}
/* eslint-enable no-underscore-dangle */
return undefined;
}

private static retrieveRawSignedContent(nodes: MimeParserNode[]): string | undefined {
const signedContentNode = Mime.retrieveSignedContentNode(nodes);
if (!signedContentNode) {
return undefined;
}
// PGP/MIME signed content uses <CR><LF> as in // use CR-LF https://tools.ietf.org/html/rfc3156#section-5
// however emailjs parser will replace it to <LF>, so we fix it here
let rawSignedContent = signedContentNode.raw.replace(/\r?\n/g, '\r\n');
if (rawSignedContent.endsWith('--')) {
// end of boundary without a mandatory newline
rawSignedContent += '\r\n'; // emailjs wrongly leaves out the last newline, fix it here
}
return rawSignedContent;
}

private static getSubjectFromNode(node: MimeParserNode): string | undefined {
let current = node;
let fallbackSubject: string | undefined;
for (let depth = 0; depth <= Mime.MAX_SIGNED_CONTENT_DEPTH; depth++) {
const currentSubject = current.headers.subject?.[0]?.value;
if (!Mime.isSignedContentNode(current)) {
return currentSubject || fallbackSubject;
}
fallbackSubject = currentSubject || fallbackSubject;
/* eslint-disable no-underscore-dangle */
const childNodes = current._childNodes;
if (childNodes === false || !childNodes.length) {
return currentSubject || fallbackSubject;
}
current = childNodes[0];
/* eslint-enable no-underscore-dangle */
}
return fallbackSubject;
}

// eslint-disable-next-line @typescript-eslint/no-explicit-any
private static createAttachmentNode(attachment: Attachment, encodeType: MimeEncodeType): any {
// todo: MimeBuilder types
Expand Down
147 changes: 147 additions & 0 deletions test/source/tests/browser-unit-tests/unit-Mime.js
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Original file line number Diff line number Diff line change
Expand Up @@ -311,3 +311,150 @@ BROWSER_UNIT_TEST_NAME(`Mime attachment file name issue 3352`);
}
return 'pass';
})();

BROWSER_UNIT_TEST_NAME(`Mime.decode parses nested signed message exactly once`);
(async () => {
const nestedSignedMime = [
'Content-Type: multipart/signed; protocol="application/pgp-signature"; micalg="pgp-sha256"; boundary="b0"',
'MIME-Version: 1.0',
'',
'--b0',
'Content-Type: multipart/signed; protocol="application/pgp-signature"; micalg="pgp-sha256"; boundary="b1"',
'',
'--b1',
'Content-Type: multipart/signed; protocol="application/pgp-signature"; micalg="pgp-sha256"; boundary="b2"',
'',
'--b2',
'Content-Type: text/plain',
'Subject: nested subject',
'',
'hello nested world',
'--b2',
'Content-Type: application/pgp-signature',
'',
'sig2',
'--b2--',
'--b1',
'Content-Type: application/pgp-signature',
'',
'sig1',
'--b1--',
'--b0',
'Content-Type: application/pgp-signature',
'',
'sig0',
'--b0--',
].join('\r\n');

const countDecodeCalls = async mime => {
const originalDecode = Mime.decode;
let decodeCalls = 0;
Mime.decode = (...args) => {
decodeCalls++;
return originalDecode(...args);
};
try {
return { decoded: await Mime.decode(mime), decodeCalls };
} finally {
Mime.decode = originalDecode;
}
};

const nestedResult = await countDecodeCalls(nestedSignedMime);
if (nestedResult.decodeCalls !== 1) {
throw Error(`Mime.decode was called ${nestedResult.decodeCalls} times for a nested signed message, expecting exactly 1`);
}
if (nestedResult.decoded.subject !== 'nested subject') {
throw Error(`unexpected subject '${nestedResult.decoded.subject}', expecting 'nested subject'`);
}

const controlMime = [
'Content-Type: multipart/mixed; boundary="m0"',
'MIME-Version: 1.0',
'Subject: control',
'',
'--m0',
'Content-Type: multipart/mixed; boundary="m1"',
'',
'--m1',
'Content-Type: multipart/mixed; boundary="m2"',
'',
'--m2',
'Content-Type: text/plain',
'',
'hello',
'--m2',
'Content-Type: application/octet-stream',
'',
'data',
'--m2--',
'--m1',
'Content-Type: application/octet-stream',
'',
'data',
'--m1--',
'--m0',
'Content-Type: application/octet-stream',
'',
'data',
'--m0--',
].join('\r\n');

const controlResult = await countDecodeCalls(controlMime);
if (controlResult.decodeCalls !== 1) {
throw Error(`Mime.decode was called ${controlResult.decodeCalls} times for an unsigned control message, expecting exactly 1`);
}
if (controlResult.decoded.subject !== 'control') {
throw Error(`unexpected subject '${controlResult.decoded.subject}', expecting 'control'`);
}
return 'pass';
})();

BROWSER_UNIT_TEST_NAME(`Mime.decode extracts subject from Thunderbird-style signed content`);
(async () => {
const thunderbirdSignedMime = [
'Content-Type: multipart/signed; protocol="application/pgp-signature"; micalg="pgp-sha256"; boundary="tb"',
'MIME-Version: 1.0',
'',
'This is an OpenPGP/MIME signed message.',
'',
'--tb',
'Content-Type: multipart/alternative; boundary="alt"',
'Subject: Encrypted Subject: hello thunderbird',
'',
'--alt',
'Content-Type: text/plain',
'',
'plain body',
'--alt',
'Content-Type: text/html',
'',
'<html><body>html body</body></html>',
'--alt--',
'--tb',
'Content-Type: application/pgp-signature',
'',
'sig',
'--tb--',
].join('\r\n');

const originalDecode = Mime.decode;
let decodeCalls = 0;
Mime.decode = (...args) => {
decodeCalls++;
return originalDecode(...args);
};
let decoded;
try {
decoded = await Mime.decode(thunderbirdSignedMime);
} finally {
Mime.decode = originalDecode;
}
if (decodeCalls !== 1) {
throw Error(`Mime.decode was called ${decodeCalls} times for a signed message, expecting exactly 1`);
}
if (decoded.subject !== 'Encrypted Subject: hello thunderbird') {
throw Error(`unexpected subject '${decoded.subject}', expecting 'Encrypted Subject: hello thunderbird'`);
}
return 'pass';
})();
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