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CVE-2026-5503 is a high-severity heap buffer overflow in wolfSSL’s Encrypted ClientHello (ECH) SNI handling, specifically involving TLSX_EchChangeSNI and TLSX_EchRestoreSNI. In the vulnerable logic, the ctx->extensions branch marked extensions as present even when TLSX_Find returned NULL. As a result, when no inner SNI was configured, TLSX_UseSNI could attach an attacker-controlled publicName value to the shared WOLFSSL_CTX. Cleanup in TLSX_EchRestoreSNI then failed because removal was gated on serverNameX != NULL, leaving the polluted SNI state in place. The inner ClientHello buffer was sized before this state pollution occurred but written afterward, so TLSX_SNI_Write could memcpy up to 255 bytes past the allocated heap buffer boundary. The issue was fixed in wolfSSL 5.9.1.
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No public exploit code observed for this vulnerability.
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Cross-references every affected SKU, including bundled OEM variants.
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