Raw vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:L/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:XCVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.
Summary
CVE-2026-5503 is a medium-severity Out-of-bounds Write (CWE-787) vulnerability in Wolfssl Wolfssl. Its CVSS base score is 6.9 (Medium).
Operationally, exploitation aligns with the MITRE ATT&CK technique Security Account Manager (T1003.002); ranked at the 38th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SA-15 (Development Process, Standards, and Tools) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
CVE-2026-5503, published on 2026-04-09, is a buffer overflow vulnerability (CWE-787) in the wolfSSL cryptographic library, affecting its TLS Encrypted Client Hello (ECH) implementation. The flaw resides in the TLSX_EchChangeSNI function, where the ctx->extensions branch sets extensions unconditionally even when TLSX_Find returns NULL. This allows TLSX_UseSNI to attach an attacker-controlled publicName to the shared WOLFSSL_CTX when no inner SNI is configured. TLSX_EchRestoreSNI then fails to clean it up due to removal being gated on serverNameX != NULL, causing the inner ClientHello to be sized before the pollution but written after it, resulting in TLSX_SNI_Write performing a memcpy 255 bytes past the allocation boundary.
The vulnerability carries a CVSS v3.1 base score of 9.1 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H), indicating network-accessible exploitation with low attack complexity, no privileges or user interaction required, and unchanged scope. Remote attackers can trigger the buffer overflow by crafting a malicious ClientHello with ECH extensions, potentially achieving high integrity and availability impacts such as arbitrary code execution, memory corruption, or denial of service on affected wolfSSL deployments.
Mitigation is addressed in wolfSSL pull request #10102 at https://github.com/wolfSSL/wolfssl/pull/10102, which security practitioners should review and apply to vulnerable versions.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-21234
Vulnerability Data
In TLSX_EchChangeSNI, the ctx->extensions branch set extensions unconditionally even when TLSX_Find returned NULL. This caused TLSX_UseSNI to attach the attacker-controlled publicName to the shared WOLFSSL_CTX when no inner SNI was configured. TLSX_EchRestoreSNI then failed to clean it up because its…
more
removal was gated on serverNameX != NULL. The inner ClientHello was sized before the pollution but written after it, causing TLSX_SNI_Write to memcpy 255 bytes past the allocation boundary.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Developer testing and evaluation (including fuzzing and bounds checks) finds out-of-bounds write flaws before deployment.
Requiring documented secure-development standards and tools can mandate bounds-checked coding practices that avoid the weakness.
Input validation can structurally reject or sanitize data that would otherwise trigger an out-of-bounds write.
Memory-protection mechanisms limit the exploitability and blast radius of a successful out-of-bounds write.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Vulnerability scanning and recording can discover out-of-bounds write flaws so they can be remediated.
Patching or replacing vulnerable software directly eliminates known instances of this coding weakness.
Secure-development practices (static analysis, bounds checking, code review) are the primary means of preventing out-of-bounds writes.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.