Postfix: Off-by-one Error
CVE-2026-43964 is a high-severity off-by-one error vulnerability in Postfix, with a CVSS 3.1 base score of 7.5 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists versions 3.8.16, 3.9.10, 3.10.9 as the first without the flaw, so updating Postfix to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.51% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update Postfix to 3.8.16 or 3.9.10 or 3.10.9, whichever matches your release line, or a later version.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- NoneNo data is disclosed.
- Integrity
- NoneNothing can be modified.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 3.7 | Low | CNA (mitre.org) |
| CVSS 3.1 | 7.5 | High | NVD |
| CVSS 3.1 | 7.5 | High | CNA |
Official description
Postfix before 3.8.16, 3.9 before 3.9.10, and 3.10 before 3.10.9 sometimes allows a buffer over-read and process crash via an enhanced status code that lacks text after the third number.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Off-by-one Error
A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value.
How developers prevent this class of flaw
- When copying character arrays or using character manipulation methods, the correct size parameter must be used to account for the null terminator that needs to be added at the end of the array. Some examples of functions susceptible to this weakness in C include strcpy(), strncpy(), strcat(), strncat(), printf(), sprintf(), scanf() and sscanf().
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Third-party and government advisories
- mail-archive.com/[email protected]/msg00110.htmlMailing List, Third Party Advisory
- openwall.com/lists/oss-security/2026/05/04/30Mailing List, Third Party Advisory
Other references
- access.redhat.com/errata/RHSA-2026:25930
- access.redhat.com/errata/RHSA-2026:25932
- access.redhat.com/errata/RHSA-2026:26205
- access.redhat.com/errata/RHSA-2026:50963
- access.redhat.com/errata/RHSA-2026:50964
- access.redhat.com/errata/RHSA-2026:51034
- access.redhat.com/errata/RHSA-2026:51436
- access.redhat.com/errata/RHSA-2026:57174
- access.redhat.com/security/cve/CVE-2026-43964
- bugzilla.redhat.com/show_bug.cgi?id=2466488
- security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-43964.json
Recent Off-by-one Error vulnerabilities
- 7.1 High severity
- 8.1 High severity
- 7.8 High severity
- 7.1 High severity
- 7.8 High severity
- 8.4 High severity
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