# CVE-2026-5503: out-of-bounds write in TLSX_EchChangeSNI via attacker-controlled publicName

> An out-of-bounds write in wolfSSL's TLS extension handling allows attackers to corrupt memory, potentially leading to code execution or crashes.

- **Severity:** Medium 6.9 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 9 Apr 2026
- **Updated:** 10 Apr 2026
- **Also known as:** DEBIAN-CVE-2026-5503, GHSA-65xm-pfx9-g5p3, JLSEC-2026-731, UBUNTU-CVE-2026-5503

## What is CVE-2026-5503?

CVE-2026-5503 is a medium-severity memory corruption vulnerability in Wolfssl Wolfssl, published on 9 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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 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.

Weakness: CWE-787 (Out-of-bounds Write)

## Which versions are affected by CVE-2026-5503?

It affects 1 product.


Products: Wolfssl Wolfssl.

## How do I fix CVE-2026-5503?

Apply the vendor's patch.

## Is CVE-2026-5503 being exploited?

There is no known exploitation of CVE-2026-5503 in the wild and no public exploit code.

## Other vulnerabilities in Wolfssl Wolfssl

- [CVE-2024-5991](https://insights.sbomify.com/CVE-2024-5991/): Buffer overread in domain name matching (critical 10.0)
- [CVE-2021-37155](https://insights.sbomify.com/CVE-2021-37155/): Security feature bypass in n/a (critical 9.8)
- [CVE-2020-36177](https://insights.sbomify.com/CVE-2020-36177/): Memory corruption in n/a (critical 9.8)
- [CVE-2014-2896](https://insights.sbomify.com/CVE-2014-2896/): Memory corruption in n/a (critical 9.8)
- [CVE-2014-2897](https://insights.sbomify.com/CVE-2014-2897/): Memory corruption in n/a (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-5503 and fix it if it is affected.

What it is: CVE-2026-5503 is a medium-severity memory corruption vulnerability in Wolfssl Wolfssl, published on 9 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-5503 in the wild and no public exploit code.

Affected product: Wolfssl Wolfssl. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-5503/ (Markdown: https://insights.sbomify.com/CVE-2026-5503/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-5503
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-5503
- OSV: https://osv.dev/vulnerability/CVE-2026-5503

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-5503/
Summaries are generated and may contain mistakes; the official description above is the record.