What is the CVSS score of CVE-2026-40685?

CVE-2026-40685 has a CVSS 3.1 base score of 6.5 (Medium). CVSS vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:H. EPSS exploitation probability: 0.0%.

Is there a patch available for CVE-2026-40685?

Yes, a patch is available for CVE-2026-40685. Update the affected software to the latest version immediately.

Exim CVE-2026-40685

EUVD-2026-26443 MEDIUM

Incorrect Provision of Specified Functionality (CWE-684)

2026-04-30 mitre

Buffer Overflow Suse

6.5

CVSS 3.1 · NVD

Severity by source

NVD PRIMARY

6.5 MEDIUM

AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:H

SUSE

MEDIUM

qualitative

Primary rating from NVD.

CVSS VectorNVD

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:H

Attack Vector

Network

Attack Complexity

High

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

Low

Availability

High

Lifecycle Timeline

Patch released

May 01, 2026 - 17:51 nvd

Patch available

Apr 30, 2026 - 23:02 EUVD

Analysis Generated

Apr 30, 2026 - 22:00 vuln.today

EUVD ID Assigned

Apr 30, 2026 - 21:45 euvd

EUVD-2026-26443

Analysis Generated

Apr 30, 2026 - 21:45 vuln.today

CVE Published

Apr 30, 2026 - 00:00 nvd

MEDIUM 6.5

DescriptionCVE.org

In Exim before 4.99.2, when JSON lookup is enabled, an out-of-bounds heap write can occur when a JSON operator encounters malformed JSON in an untrusted header, because of an incorrect implementation of \ skipping.

AnalysisAI

Out-of-bounds heap write in Exim before 4.99.2 allows unauthenticated remote attackers to cause denial of service and potentially corrupt memory when the JSON lookup feature is enabled and malformed JSON is present in untrusted email headers, due to incorrect backslash escape sequence handling in the JSON operator.

Technical ContextAI

Exim is a mail transport agent (MTA) that processes email headers and message content. The vulnerability exists in Exim's JSON lookup functionality, which parses JSON-formatted data within email processing contexts. The root cause (CWE-684: Incorrect Type Conversion or Cast) stems from improper implementation of backslash escape sequence skipping in the JSON operator-specifically, the parser fails to correctly handle malformed escape sequences, leading to out-of-bounds writes into heap memory. When JSON lookup is enabled and an untrusted email header contains specially crafted malformed JSON, the parser writes beyond allocated buffer boundaries, corrupting heap structures. The vulnerability is triggered during mail message processing, a network-facing operation in typical MTA deployments.

RemediationAI

Upgrade Exim to version 4.99.2 or later to apply the vendor-released patch. The upstream fix is confirmed by commit 9fdc057e71b87c87a0d3d2288b2810a0efaaba57 in the Exim repository. For organizations unable to patch immediately, the primary mitigation is to disable the JSON lookup feature in the Exim configuration file unless it is actively required for mail processing logic-edit the configuration to remove or comment out JSON lookup operators and restart the service. As a compensating control, restrict email routing through the vulnerable server to trusted internal sources only and implement strict rate-limiting on mail submission ports to reduce exposure window. However, these controls do not eliminate the vulnerability if JSON lookup remains enabled. Detailed assessment and patch availability information is available at https://exim.org/static/doc/security/cve-2026-04.1/CVE2026-40685.assessment and the Exim security advisory at https://exim.org/static/doc/security/CVE-2025-40685.txt.