CVE-2026-43859Network attack vector

IMAP authentication failure via NUL-truncated MD5 digest

Published May 4, 2026 · Updated May 4, 2026

Improper NUL handling in Mutt before 2.3.2 allows remote attackers to disrupt IMAP CRAM-MD5 authentication for long secrets. In imap/auth_cram.c, hmac_md5 passes the 16-byte binary md5_buffer result to strfcpy, which treats an embedded NUL as a terminator and also reserves one byte for string termination. The defect is reached when the IMAP secret exceeds 64 bytes, producing a truncated HMAC secret and failed authentication rather than code execution or data disclosure.

CVSS severity3.7
Low
EPSS probability0.16%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper NUL handling in Mutt before 2.3.2 allows remote attackers to disrupt IMAP CRAM-MD5 authentication for long secrets. In imap/auth_cram.c, hmac_md5 passes the 16-byte binary md5_buffer result to strfcpy, which treats an embedded NUL as a terminator and also reserves one byte for string termination. The defect is reached when the IMAP secret exceeds 64 bytes, producing a truncated HMAC secret and failed authentication rather than code execution or data disclosure.

The record

CVE
CVE-2026-43859
Published
May 4, 2026
Updated
May 4, 2026
Vendor
Mutt
Product
Mutt
Classifications
CWE-158
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 4, 2026CVE publishedPublication date reported by the CVE source.
  2. May 4, 2026Record 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=0 <2.3.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mutt · Muttversion=0 <2.3.2

Published CVSS scores

4.8SUSECVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
6.3SUSECVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N
3.7MITRECVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-158Improper Neutralization of Null Byte or NUL Character

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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