# CVE-2026-49440: Deno: Miller-Rabin Primality Test Allows Zero Rounds

> Deno's primality test incorrectly identifies certain composites as prime when checks are zero, potentially compromising cryptographic key generation.

- **Severity:** High 7.4 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 23 Jun 2026
- **Updated:** 25 Jun 2026
- **Also known as:** GHSA-9xg4-qhm4-g43w, JLSEC-2026-658

## What is CVE-2026-49440?

CVE-2026-49440 is a high-severity vulnerability in denoland deno, published on 23 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Deno is a JavaScript, TypeScript, and WebAssembly runtime. Prior to 2.8.1, node:crypto.checkPrime(candidate[, options][, callback]) and crypto.checkPrimeSync(candidate[, options]) ran no Miller-Rabin rounds at all when the caller left options.checks at its default of 0. In that mode, the only test applied to the candidate was trial division by the primes up to 17,863. Any composite whose smallest prime factor exceeds that bound — for example the product of two primes just above it, such as 17,881 × 17,891 — was reported as true ("probably prime"). The same divergence affected the lower-level op_node_check_prime / op_node_check_prime_bytes paths that the polyfill calls into. This vulnerability is fixed in 2.8.1.

Weakness: CWE-325 (Missing Cryptographic Step)

## Which versions are affected by CVE-2026-49440?

2 open source packages ship an affected version, including deno. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `deno` | crates.io | All versions | 2.8.1 |
| `deno` | crates.io | All versions | 2.8.1 |

Products: Deno Deno.

## How do I fix CVE-2026-49440?

Upgrade deno to 2.8.1 or later.

## Is CVE-2026-49440 being exploited?

There is no known exploitation of CVE-2026-49440 in the wild and no public exploit code.

## Other vulnerabilities in denoland deno

- [CVE-2023-28445](https://insights.sbomify.com/CVE-2023-28445/): Deno improperly handles resizable ArrayBuffer (critical 10.0)
- [CVE-2022-24783](https://insights.sbomify.com/CVE-2022-24783/): Sandbox bypass leading to arbitrary code execution in Deno (critical 10.0)
- [CVE-2021-32619](https://insights.sbomify.com/CVE-2021-32619/): Static imports inside dynamically imported modules do not adhere to permission checks (critical 9.8)
- [CVE-2026-22863](https://insights.sbomify.com/CVE-2026-22863/): Deno node:crypto doesn't finalize cipher (critical 9.2)
- [CVE-2024-27936](https://insights.sbomify.com/CVE-2024-27936/): Deno interactive permission prompt spoofing via improper ANSI stripping (high 8.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-49440 and fix it if it is affected.

What it is: CVE-2026-49440 is a high-severity vulnerability in denoland deno, published on 23 Jun 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-2026-49440 in the wild and no public exploit code.

Affected packages and fixed versions:
- deno (crates.io): affected all versions; fixed in 2.8.1
- deno (crates.io): affected all versions; fixed in 2.8.1

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-2026-49440/ (Markdown: https://insights.sbomify.com/CVE-2026-49440/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-49440
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-49440
- OSV: https://osv.dev/vulnerability/CVE-2026-49440

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-49440/
Summaries are generated and may contain mistakes; the official description above is the record.