Forge: Denial of Service via infinite loop in BigInteger.modInverse() with negative input (incomplete fix of CVE-2026-33891)

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

2.9 · CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS

• CVE-2026-118781: 0.12% estimated exploitation activity in the next 30 days; percentile 1.75 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary `BigInteger.prototype.modInverse()` in node-forge's bundled jsbn library enters an infinite loop when the value or the modulus is negative, for example when a negative public exponent is passed to `forge.pki.rsa.generateKeyPair()`. This is an incomplete fix of CVE-2026-33891 (GHSA-5m6q-g25r-mvwx), which handled only a zero value. Fixed in node-forge 1.4.1.

Details `bnModInverse()` (`lib/jsbn.js`, exposed as `forge.jsbn.BigInteger.prototype.modInverse`) implements the binary extended Euclidean algorithm (Handbook of Applied Cryptography, Algorithm 14.61), which is defined for positive integers. With a negative operand the loop cannot terminate: `rShiftTo` is an arithmetic shift, so a negative value never moves towards zero, and the loop's exit condition (`u` reaching zero) is never met. The calling thread spins at 100% CPU indefinitely; in Node.js this blocks the event loop. Memory does not grow. node-forge 1.4.0 fixed CVE-2026-33891 (GHSA-5m6q-g25r-mvwx) by returning…

Published by the source

30 Oct 2026, 23:02 UTC

Last modified by the source

30 Oct 2026, 23:15 UTC

Source last checked

31 Oct 2026, 06:00 UTC

1. Revision 2 · amended · 31 Oct 2026

   Amended from osv record GHSA-h6cr-wjv9-3cpp

2. Revision 1 · created · 30 Oct 2026

   Published from ghsa record GHSA-h6cr-wjv9-3cpp

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# Patch Next advisory brief: CVE-2026-118781

Advisory: Forge: Denial of Service via infinite loop in BigInteger.modInverse() with negative input (incomplete fix of CVE-2026-33891)
IDs: CVE-2026-118781, GHSA-h6cr-wjv9-3cpp
Record: https://patchnext.com/advisories/cve-2026-118781 (revision 2, source checked 31 Oct 2026, 06:00 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 2.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: node-forge 1.4.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm node-forge: affected < 1.4.1; fixed in 1.4.1

## Suggested inspection (read-only)
- `npm ls node-forge --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on node-forge.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-h6cr-wjv9-3cpp (GitHub-reviewed advisory GHSA-h6cr-wjv9-3cpp lists affected ranges: npm node-forge >=0 <=1.4.0)
- Verified: https://osv.dev/vulnerability/GHSA-h6cr-wjv9-3cpp (OSV record GHSA-h6cr-wjv9-3cpp lists affected ranges: npm node-forge >=0 <1.4.1 (fixed))

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