CVE-2026-49440: Deno Prime Number Validation Flaw Allows Composite Numbers to Pass as Prime

Technical summary

CVE-2026-49440 affects Deno's Node.js crypto polyfill implementation of primality testing. The checkPrime(candidate[, options][, callback]) and checkPrimeSync(candidate[, options]) functions default options.checks to 0, which causes them to bypass all Miller-Rabin probabilistic primality testing rounds. The only validation performed is trial division against primes up to 17,863. Composite numbers whose smallest prime factor exceeds 17,863—such as products of two primes just above that threshold (e.g., 17,881 × 17,891)—are incorrectly classified as probable primes. The same defect exists in the lower-level op_node_check_prime and op_node_check_prime_bytes operational paths. This vulnerability was resolved in Deno 2.8.1.

Business impact

Organizations relying on Deno for cryptographic operations face serious risk if they use the affected checkPrime() functions with default parameters. Weak or composite prime acceptance could lead to generation of cryptographically inadequate keys, potentially allowing attackers to compromise encryption schemes, digital signatures, and other security-critical protocols. The impact is particularly severe in applications performing key generation, TLS/SSL operations, or any cryptographic function that assumes true primality of its inputs.

Affected systems

Deno versions prior to 2.8.1 are affected. This includes all releases that shipped with the vulnerable prime-checking implementation in the node:crypto polyfill. Organizations should verify their exact Deno version and upgrade to 2.8.1 or later.

Exploitability

The vulnerability requires no special privileges or user interaction. An attacker who can influence the selection of prime candidates—or who observes keys generated by vulnerable code—can exploit this flaw remotely. However, successful exploitation requires the application to actually use checkPrime() with default settings; applications that explicitly configure options.checks to a non-zero value are unaffected. The network-accessible nature (AV:N) and lack of interaction requirements (UI:N) increase the attack surface for exposed services.

Remediation

Upgrade to Deno 2.8.1 or later immediately. This patch restores proper Miller-Rabin primality testing. After upgrading, regenerate any cryptographic keys that were generated or validated with affected versions. Verify that your application does not pass default options to checkPrime() or checkPrimeSync() without understanding the security implications; consider explicitly setting options.checks to an appropriate value (e.g., 64 for 2^-128 confidence) in all new code.

Patch guidance

Update Deno to version 2.8.1 or later using your standard dependency management process. Verify the upgrade by checking the runtime version: deno --version should report 2.8.1 or higher. Test your cryptographic operations in a staging environment to confirm no unexpected behavior. Coordinate the upgrade with any key rotation or re-generation of security-critical material, as keys generated under the vulnerable version should be considered suspect.

Detection guidance

Search application logs and source code for calls to checkPrime(), checkPrimeSync(), or any use of the node:crypto module for primality testing. Monitor for Deno version numbers in deployment manifests and container images. If your infrastructure includes cryptographic key generation, audit the generation timestamps against the Deno version deployed at those times. Consider implementing post-upgrade validation of newly generated keys against external primality verification if the keys are used in high-security contexts.

Why prioritize this

This vulnerability scores HIGH (CVSS 7.4) due to its direct impact on cryptographic integrity (High impact on Confidentiality and Integrity), remote exploitability, and the absence of authentication requirements. While the bug requires that an application actually use the affected function with default settings, the consequences of cryptographic compromise are severe. Organizations using Deno for any cryptographic purposes should prioritize this patch.

Risk score, explained

The CVSS 3.1 score of 7.4 reflects a network-accessible vector (AV:N), high complexity attack (AC:H, since exploitation requires influence over prime selection or discovery of weak keys), no privilege requirement (PR:N), and high impact on confidentiality and integrity (C:H, I:H). The absence of availability impact (A:N) prevents a higher score. The 'H' complexity accounts for the attacker's need to either control the prime candidate input or to detect and exploit weakly generated keys in the wild.

Frequently asked questions