CVE-2026-103415 TLS 1.3 Client - Buffer Overflow

TLS 1.3 clients copy the server's cookie from HelloRetryRequest into the second ClientHello without validating cookie size, causing a buffer overflow before certificate authentication, letting malicious servers overwrite memory.

CVSS base score 9.3 /10
Critical
CISA KEV
Not listed
Public exploit
None known
Authentication
Not required
View the NVD record ↗

Details

Type
Buffer Overflow
Impact
Remote Code Execution, Denial Of Service
CVSS vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Published
14 October 2026
Updated
15 October 2026
Weakness
CWE-787

Vendor fix

Update to the latest TLS 1.3 client implementation that validates cookie size before copying.

Patching closes this CVE. It does not tell you which of your systems run the affected version, or what else is exposed alongside them.

References

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