CVE-2021-3657Network attack vector

Memory corruption via oversized IMAP literal lengths

Published Feb 18, 2022 · Updated Aug 3, 2024

Buffer overflows in Isync Project isync before 1.4.4 allow remote attackers to corrupt memory via oversized IMAP literals. The mbsync IMAP parser converts an unrestricted literal length to an int, then uses int-sized lengths and offsets during allocation and downstream processing. A malicious or compromised IMAP server can trigger the overflows directly; code execution is unconfirmed, while external senders face mail-size and header-prediction constraints.

CVSS severity9.8
Critical
EPSS probability3.46%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Buffer overflows in Isync Project isync before 1.4.4 allow remote attackers to corrupt memory via oversized IMAP literals. The mbsync IMAP parser converts an unrestricted literal length to an int, then uses int-sized lengths and offsets during allocation and downstream processing. A malicious or compromised IMAP server can trigger the overflows directly; code execution is unconfirmed, while external senders face mail-size and header-prediction constraints.

The record

CVE
CVE-2021-3657
Published
Feb 18, 2022
Updated
Aug 3, 2024
Vendor
Isync Project
Product
isync
Classifications
CWE-119, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 18, 2022CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=isync 1.4.4

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Isync Project · isyncversion=isync 1.4.4

Published CVSS scores

9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-119Improper Restriction of Operations within the Bounds of a Memory Buffer
T1203Exploitation for Client Execution

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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