# CVE-2026-73253: Mongoose: TLS Hostname Verification Bypass via Overly Permissive Wildcard Matching

> An on-path attacker can bypass TLS hostname verification in Mongoose to intercept and modify traffic using a wildcard certificate.

- **Severity:** Critical 9.1 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/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:** 20 Aug 2026
- **Updated:** 20 Aug 2026
- **Also known as:** DEBIAN-CVE-2026-73253, GHSA-jp6g-796f-39vp, JLSEC-2026-1377, UBUNTU-CVE-2026-73253

## What is CVE-2026-73253?

CVE-2026-73253 is a critical vulnerability in Cesanta Mongoose, published on 20 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Mongoose is an embedded web server and network library. Prior to version 7.22, an on-path network attacker with a wildcard certificate for a parent domain can impersonate deeper subdomains to a client using the built-in TLS stack. The mg_tls_verify_cert_san() and mg_tls_verify_cert_cn() functions in src/tls_builtin.c call mg_match(), whose wildcard can cross DNS label boundaries, so a pattern such as *.example.com can match foo.bar.example.com. The resulting hostname verification bypass permits interception and modification of TLS traffic. This issue is fixed in version 7.22.

Weakness: CWE-295 (Improper Certificate Validation)

## Which versions are affected by CVE-2026-73253?

It affects 1 product.


Products: Cesanta Mongoose.

## How do I fix CVE-2026-73253?

Upgrade to a fixed version.

## Is CVE-2026-73253 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-73253 exists.

## Other vulnerabilities in Cesanta Mongoose

- [CVE-2022-25299](https://insights.sbomify.com/CVE-2022-25299/): Arbitrary File Write (critical 9.8)
- [CVE-2020-25756](https://insights.sbomify.com/CVE-2020-25756/): Memory corruption in n/a (critical 9.8)
- [CVE-2019-19307](https://insights.sbomify.com/CVE-2019-19307/): Integer overflow in n/a (critical 9.8)
- [CVE-2019-12951](https://insights.sbomify.com/CVE-2019-12951/): Memory corruption in n/a (critical 9.8)
- [CVE-2018-20353](https://insights.sbomify.com/CVE-2018-20353/): Use after free in n/a (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-73253 and fix it if it is affected.

What it is: CVE-2026-73253 is a critical vulnerability in Cesanta Mongoose, published on 20 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-73253 exists.

Affected product: Cesanta Mongoose. 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-2026-73253/ (Markdown: https://insights.sbomify.com/CVE-2026-73253/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-73253
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-73253
- OSV: https://osv.dev/vulnerability/CVE-2026-73253

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-73253/
Summaries are generated and may contain mistakes; the official description above is the record.