# CVE-2026-23943: Pre-auth SSH DoS via unbounded zlib inflate

> Unbounded zlib inflation in Erlang OTP SSH allows pre-authentication attackers to cause memory exhaustion and denial of service via compression bombs.

- **Severity:** Medium 6.9 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/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:** 13 Mar 2026
- **Updated:** 24 Jul 2026
- **Also known as:** AZL-79877, DEBIAN-CVE-2026-23943, EEF-CVE-2026-23943, GHSA-c836-qprm-jw9r, UBUNTU-CVE-2026-23943

## What is CVE-2026-23943?

CVE-2026-23943 is a medium-severity denial of service vulnerability in Erlang OTP, published on 13 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Improper Handling of Highly Compressed Data (Compression Bomb) vulnerability in Erlang OTP ssh (ssh_transport modules) allows Denial of Service via Resource Depletion.

The SSH transport layer advertises legacy zlib compression by default and inflates attacker-controlled payloads pre-authentication without any size limit, enabling reliable memory exhaustion DoS.

Two compression algorithms are affected:

* zlib: Activates immediately after key exchange, enabling unauthenticated attacks
* zlib@openssh.com: Activates post-authentication, enabling authenticated attacks

Each SSH packet can decompress ~255 MB from 256 KB of wire data (1029:1 amplification ratio). Multiple packets can rapidly exhaust available memory, causing OOM kills in memory-constrained environments.

This vulnerability is associated with program files lib/ssh/src/ssh_transport.erl and program routines ssh_transport:decompress/2, ssh_transport:handle_packet_part/4.

This issue affects OTP from OTP 17.0 before OTP 28.4.1, OTP 27.3.4.9 and OTP 26.2.5.18, corresponding to ssh from 3.0.1 before 5.5.1, 5.2.11.6 and 5.1.4.14.

Weakness: CWE-409 (Improper Handling of Highly Compressed Data (Data Amplification))

## Which versions are affected by CVE-2026-23943?

It affects 2 products.


Products: Erlang Erlang\/Otp, Erlang Erlang\/Ssh.

## How do I fix CVE-2026-23943?

Apply the vendor's patch.

## Is CVE-2026-23943 being exploited?

There is no known exploitation of CVE-2026-23943 in the wild and no public exploit code.

## Other vulnerabilities in Erlang OTP

- [CVE-2022-37026](https://insights.sbomify.com/CVE-2022-37026/): Auth bypass in n/a (critical 9.8)
- [CVE-2016-10253](https://insights.sbomify.com/CVE-2016-10253/): Memory corruption in n/a (critical 9.8)
- [CVE-2026-55953](https://insights.sbomify.com/CVE-2026-55953/): TLS 1.2 and DTLS client accepts unoffered anonymous cipher suite, bypassing server authentication (critical 9.1)
- [CVE-2026-49759](https://insights.sbomify.com/CVE-2026-49759/): Stack buffer overflow in SCTP error cause parsing in inet_drv allows remote VM crash (high 8.8)
- [CVE-2026-58227](https://insights.sbomify.com/CVE-2026-58227/): TLS/DTLS denial of service via unbounded recursion on cross-signed peer certificate chain (high 8.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-23943 and fix it if it is affected.

What it is: CVE-2026-23943 is a medium-severity denial of service vulnerability in Erlang OTP, published on 13 Mar 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-2026-23943 in the wild and no public exploit code.

Affected product: Erlang OTP. 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-2026-23943/ (Markdown: https://insights.sbomify.com/CVE-2026-23943/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23943
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23943
- OSV: https://osv.dev/vulnerability/CVE-2026-23943

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23943/
Summaries are generated and may contain mistakes; the official description above is the record.