Exim: Incorrect Provision of Specified Functionality
CVE-2026-40684 is a high-severity incorrect provision of specified functionality vulnerability in Exim, 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 version 4.99.2 as the first without the flaw, so updating Exim to that release or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.36% 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 Exim to 4.99.2 or a later version.
- Read the advisory on exim.org for workarounds and exact fixed builds.
- 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 | 5.9 | Medium | CNA (mitre.org) |
| CVSS 3.1 | 7.5 | High | NVD |
Official description
In Exim before 4.99.2, on systems using musl libc (not glibc), an attacker can crash the connection instance when malformed DNS data is present in PTR records. This is caused by a dn_expand oddity in octal printing.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Incorrect Provision of Specified Functionality
The code does not function according to its published specifications, potentially leading to incorrect usage.
How developers prevent this class of flaw
- Ensure that your code strictly conforms to specifications.
More Incorrect Provision of Specified Functionality vulnerabilities (CWE-684)
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
Patches and release notes
Third-party and government advisories
- openwall.com/lists/oss-security/2026/04/30/21Mailing List, Third Party Advisory
Other references
Other vulnerabilities in Exim
- 7.5 High severity
- 7.8 High severity
- 9.8 Critical severity
- 9.1 Critical severity
- 9.8 Critical severity
- 9.8 Critical severity
Recent Incorrect Provision of Specified Functionality vulnerabilities
- 7.5 High severity
- 9.8 Critical severity
- 7.4 High severity
- 9.1 Critical severity
- 7.5 High severity
- 9.8 Critical severity
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