Heap corruption via malformed JSON escape skipping
Published Apr 30, 2026 · Updated Apr 30, 2026
Out-of-bounds heap write in Exim before 4.99.2 allows remote attackers to corrupt memory through malformed JSON in a message header. The JSON operator's scanner advances across backslash escapes incorrectly, allowing malformed header input to write beyond its heap buffer. Exploitation requires a configuration that applies JSON operators to externally supplied headers; resulting heap corruption can interrupt SMTP service and alter process memory.
Summary
What happened
Out-of-bounds heap write in Exim before 4.99.2 allows remote attackers to corrupt memory through malformed JSON in a message header. The JSON operator's scanner advances across backslash escapes incorrectly, allowing malformed header input to write beyond its heap buffer. Exploitation requires a configuration that applies JSON operators to externally supplied headers; resulting heap corruption can interrupt SMTP service and alter process memory.
The record
- CVE
- CVE-2026-40685
- Published
- Apr 30, 2026
- Updated
- Apr 30, 2026
- Vendor
- LibSPF2 Project
- Product
- Exim
- Classifications
- CWE-787, CWE-684, T1499.004
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Apr 30, 2026CVE publishedPublication date reported by the CVE source.
- Apr 30, 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.
- Affected versionversion=0 <4.99.2
What conditions does exploitation require?
What is affected?
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
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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