# CVE-2026-50721: IKEv1 Denial of Service via RSA-SHA1 (PKCS#1 Version 1.5 Encrypted) authentication payload

> Remote attackers can cause denial of service in Libreswan by sending malformed IKEv1 packets with specific RSA signatures, requiring small public exponents.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 2 Jul 2026
- **Updated:** 7 Jul 2026
- **Also known as:** AZL-91773, CLSA-2026-1784306624, DEBIAN-CVE-2026-50721, GHSA-93pj-2pmf-2x7p, UBUNTU-CVE-2026-50721

## What is CVE-2026-50721?

CVE-2026-50721 is a high-severity denial of service vulnerability in The Libreswan Project libreswan, published on 2 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Libreswan, via the function RSA_authenticate_hash_signature_raw_rsa(), did not correctly verify the length of the authentication hash when the SIG payload of an IKEv1 packet was encoded using PKCS #1 RSA Encryption as per RFC 2313. A remote attacker can use a variation on the Bleichenbacher attack to forge the SIG payload when small public exponents are being used (e.g., e=3), which could lead to impersonation. Additionally, a remote attacker, by encoding a shorter than expected hash in the SIG payload, could trigger an assertion leading to denial-of-service. The daemon aborts and restarts; continued exploitation causes sustained denial of service. Remote code execution is not possible. X.509 certificate verifications of remote IKE peers are not affected.

Weakness: CWE-617 (Reachable Assertion), CWE-347 (Improper Verification of Cryptographic Signature)

## Which versions are affected by CVE-2026-50721?

It affects 1 product.


Products: Libreswan Libreswan.

## How do I fix CVE-2026-50721?

Apply the vendor's patch.

## Is CVE-2026-50721 being exploited?

There is no known exploitation of CVE-2026-50721 in the wild and no public exploit code.

## Other vulnerabilities in The Libreswan Project libreswan

- [CVE-2013-7283](https://insights.sbomify.com/CVE-2013-7283/): Race condition in n/a (critical 9.3)
- [CVE-2026-12413](https://insights.sbomify.com/CVE-2026-12413/): IKEv2 Denial of Service via malformed fragmentation (high 7.5)
- [CVE-2026-50722](https://insights.sbomify.com/CVE-2026-50722/): IKEv2 Denial of Service via RSA-SHA1 (PKCS#1 RSASSA-PKCS1-v1_5) authentication payload (high 7.5)
- [CVE-2023-30570](https://insights.sbomify.com/CVE-2023-30570/): Denial of service in n/a (high 7.5)
- [CVE-2023-2295](https://insights.sbomify.com/CVE-2023-2295/): Denial of service in libreswan (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-50721 and fix it if it is affected.

What it is: CVE-2026-50721 is a high-severity denial of service vulnerability in The Libreswan Project libreswan, published on 2 Jul 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-2026-50721 in the wild and no public exploit code.

Affected product: The Libreswan Project libreswan. 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-2026-50721/ (Markdown: https://insights.sbomify.com/CVE-2026-50721/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-50721
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-50721
- OSV: https://osv.dev/vulnerability/CVE-2026-50721

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-50721/
Summaries are generated and may contain mistakes; the official description above is the record.