CVE-2026-40686

In Exim before 4.99.2, when utf8 operators are enabled, there is an out-of-bounds read if large UTF-8 trailing characters are present (malformed UTF-8 header data). Information might be divulged within an error message produced during handling of an unrelated e-mail message.

• CVE-2026-40686: out-of-bounds read in the GETUTF8INC macro in src/expand.c when processing malformed UTF-8 in expansion operators with utf8 enabled, potentially disclosing heap data via SMTP rejection messages.

• SECURITY UPDATE: heap read out-of-bounds in UTF-8 expansion
  • debian/patches/CVE-2026-40686.patch: harden ${from_utf8:} expansion operator against malformed UTF-8 trailing bytes.
  • CVE-2026-40686
• SECURITY UPDATE: SPA authenticator buffer hardening
  • debian/patches/CVE-2026-40687.patch: zero spa_base64_to_bits output buffer and replace static 1024-byte buffers in unicodeToString, strToUnicode, toString with dynamic store_get allocations sized to input.
  • CVE-2026-40687

• CVE-2026-40686: out-of-bounds heap read in GETUTF8INC macro when utf8 operators process malformed input; may exfiltrate heap data via SMTP rejection messages

Timo Longin discovered that Exim incorrectly handled certain SMTP messages in PIPELINING/CHUNKING configurations. A remote attacker could possibly use this issue to perform SMTP smuggling. This issue only affected Ubuntu 14.04 LTS. (CVE-2023-51766)

It was discovered that Exim incorrectly handled certain malformed JSON data in headers. A remote attacker could possibly use this issue to crash Exim, resulting in a denial of service, or execute arbitrary code. This issue only affected Ubuntu 20.04 LTS. (CVE-2026-40685)

It was discovered that Exim incorrectly handled certain malformed UTF-8 headers. A remote attacker could possibly use this issue to obtain sensitive information. This issue only affected Ubuntu 20.04 LTS. (CVE-2026-40686)

It was discovered that Exim incorrectly handled certain SPA resources. A remote attacker could possibly use this issue to crash Exim, resulting in a denial of service, or obtain sensitive information. This issue only affected Ubuntu 20.04 LTS. (CVE-2026-40687)

It was discovered that Exim incorrectly handled certain CHUNKING transfers in some GnuTLS configurations. A remote attacker could possibly use this issue to crash Exim, resulting in a denial of service, or execute arbitrary code. This issue only affected Ubuntu 20.04 LTS. (CVE-2026-45185)

Warisjeet Singh discovered that Exim incorrectly handled certain proxy connections in builds with proxy support enabled. A remote attacker could possibly use this issue to obtain sensitive information. This issue only affected Ubuntu 16.04 LTS, Ubuntu 18.04 LTS, and Ubuntu 20.04 LTS. (CVE-2026-48840)