Exim: Use After Free
CVE-2026-45185 is a critical-severity use after free vulnerability in Exim, with a CVSS 3.1 base score of 9.8 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 4.99.3 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 1.27% chance of exploitation within 30 days, higher than 70% of scored CVEs.
Critical severity: a successful attack would have severe impact.
What to do
- Update Exim to 4.99.3 or a later version.
- Read the advisory on code.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
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. Scored by CNA (mitre.org).
Official description
Exim before 4.99.3, in certain GnuTLS configurations, has a remotely reachable use-after-free in the BDAT body parsing path. It is triggered when a client sends a TLS close_notify mid-body during a CHUNKING transfer, followed by a final cleartext byte on the same TCP connection. This can lead to heap corruption. An unauthenticated network attacker exploiting this vulnerability could execute arbitrary code.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Use After Free
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.
How developers prevent this class of flaw
- Choose a language that provides automatic memory management.
- When freeing pointers, be sure to set them to NULL once they are freed. However, the utilization of multiple or complex data structures may lower the usefulness of this strategy.
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: total).
- 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/2026/05/12/4Mailing List, Third Party Advisory
- xbow.com/blog/dead-letter-cve-2026-45185-xbow-found-rce-eximThird Party Advisory
- openwall.com/lists/oss-security/2026/05/12/25Mailing List, Third Party Advisory
Other references
- exim.orgProduct
- news.ycombinator.com/item?id=48111748Issue Tracking
Other vulnerabilities in Exim
- 7.5 High severity
- 7.8 High severity
- 9.1 Critical severity
- 9.8 Critical severity
- 7.5 High severity
- 9.8 Critical severity
Recent Use After Free vulnerabilities
- 7.0 High severity
- 7.8 High severity
- 7.5 High severity
- 7.5 High severity
- 8.8 High severity
- 8.8 High severity
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