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Date: Fri, 3 Dec 2021 12:24:32 +0100
From: Oswald Buddenhagen <>
Subject: CVE-2021-3657: multiple buffer overflows in isync/mbsync


A flaw was found in mbsync versions prior to 1.4.4. Due to inadequate
handling of extremely large (>=2GiB) IMAP literals, malicious or
compromised IMAP servers, and hypothetically even external email
senders, could cause several different buffer overflows, which could
conceivably be exploited for remote code execution.


upgrade to the freshly released v1.4.4 available from , or apply the 
matching attached patch. note that while a patch for v1.3.x is provided, 
no upstream release will be made any more.


i'm not sure it's actually possible to pull off RCE with these. a
non-server attacker would be additionally impaired by message size
limitations and being unable to predict the exact size of the headers
stored in the box, so they would likely need an account on the same
liberally configured system, apart from knowing to target mbsync.

View attachment "CVE-2021-3657-buffer-overflows-on-big-1.4.patch" of type "text/x-diff" (7190 bytes)

View attachment "CVE-2021-3657-buffer-overflows-on-big-1.3.patch" of type "text/x-diff" (5489 bytes)

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