CVE-2026-4602: Jsrsasign Numeric Conversion Vulnerability

CVE-2026-4602 Overview

CVE-2026-4602 is a cryptographic vulnerability affecting the jsrsasign JavaScript cryptographic library, a widely-used package for RSA signing and verification in Node.js applications. Versions of jsrsasign before 11.1.1 are vulnerable to Incorrect Conversion between Numeric Types (CWE-681) due to improper handling of negative exponents in the ext/jsbn2.js file. An attacker can exploit this flaw by calling modPow with a negative exponent, forcing the computation of incorrect modular inverses and ultimately breaking signature verification mechanisms.

Critical Impact

This vulnerability allows attackers to bypass signature verification by exploiting incorrect modular inverse calculations, potentially undermining the cryptographic integrity of applications relying on jsrsasign for digital signature operations.

Affected Products

• jsrsasign versions prior to 11.1.1
• Node.js applications using vulnerable jsrsasign packages
• Web applications implementing RSA signature verification with affected library versions

Discovery Timeline

• 2026-03-23 - CVE-2026-4602 published to NVD
• 2026-03-23 - Last updated in NVD database

Technical Details for CVE-2026-4602

Vulnerability Analysis

The vulnerability exists within the ext/jsbn2.js file of the jsrsasign library, specifically in how the modPow function processes negative exponent values. The cryptographic library fails to properly validate or handle negative exponents during modular exponentiation operations, which is a fundamental operation in RSA cryptography.

When a negative exponent is passed to the modPow function, the library performs incorrect numeric type conversions that result in erroneous modular inverse calculations. This mathematical error propagates through the signature verification process, causing valid signatures to potentially fail verification while allowing crafted invalid signatures to pass verification checks.

The flaw represents a classic Incorrect Conversion between Numeric Types issue where the library's assumptions about input ranges are violated, leading to undefined cryptographic behavior.

Root Cause

The root cause of CVE-2026-4602 lies in the ext/jsbn2.js module's failure to properly validate exponent parameters before performing modular exponentiation. The modPow function does not include boundary checks for negative values, and when such values are processed, the underlying BigInteger arithmetic produces mathematically incorrect results.

The library assumes that exponent values will always be non-negative integers, as is standard for RSA operations. However, without explicit validation, malicious or unexpected negative inputs bypass this assumption and corrupt the mathematical operations essential for cryptographic integrity.

Attack Vector

The attack vector for this vulnerability is network-based, requiring no user interaction or privileges. An attacker can exploit this vulnerability through the following mechanism:

The vulnerability can be triggered by supplying crafted input that causes the modPow function to receive a negative exponent value. In a signature verification context, this could occur through:

1. Crafting malicious signature data that, when processed, results in negative exponent calculations
2. Directly calling library functions with manipulated parameters in applications that expose such interfaces
3. Exploiting downstream application logic that passes untrusted data to cryptographic operations

Successful exploitation allows attackers to forge signatures or cause denial of service by disrupting cryptographic operations. Technical details and proof-of-concept information are available in the GitHub Gist PoC.

Detection Methods for CVE-2026-4602

Indicators of Compromise

• Unexpected signature verification failures in applications using jsrsasign
• Log entries showing cryptographic operation errors or exceptions in jsbn2.js
• Anomalous modular arithmetic results or BigInteger operation failures
• Signs of signature forgery attempts or authentication bypass in application logs

Detection Strategies

• Implement Software Composition Analysis (SCA) scanning to identify jsrsasign versions below 11.1.1 in your dependency tree
• Monitor application logs for cryptographic exceptions originating from the jsrsasign library
• Deploy runtime application self-protection (RASP) to detect unexpected cryptographic operation patterns
• Use static code analysis to identify direct calls to modPow with potentially untrusted input

Monitoring Recommendations

• Enable detailed logging for all cryptographic operations within applications using jsrsasign
• Set up alerts for authentication or signature verification anomalies that could indicate exploitation attempts
• Monitor npm audit and dependency vulnerability reports for jsrsasign-related advisories
• Track application behavior for unusual patterns in RSA signature processing

How to Mitigate CVE-2026-4602

Immediate Actions Required

• Update jsrsasign to version 11.1.1 or later immediately
• Audit your package.json and package-lock.json files to identify all instances of jsrsasign dependencies
• Review transitive dependencies that may include vulnerable jsrsasign versions
• Implement input validation to reject negative exponent values at application boundaries

Patch Information

The jsrsasign project has addressed this vulnerability in version 11.1.1. The fix includes proper validation of exponent parameters to prevent negative values from being processed by the modPow function. The specific commit addressing this issue is available in the GitHub Commit Update.

Additional technical discussion regarding the fix can be found in the GitHub Pull Request Discussion. The Snyk Vulnerability Report provides supplementary vulnerability tracking information.

Workarounds

• If immediate patching is not possible, implement application-level input validation to ensure exponent values are non-negative before passing to jsrsasign functions
• Consider temporarily replacing jsrsasign with an alternative cryptographic library that is not affected by this vulnerability
• Restrict access to endpoints or functionality that rely on jsrsasign signature operations until patching is complete

# Update jsrsasign to patched version
npm update jsrsasign@11.1.1

# Verify installed version
npm list jsrsasign

# Audit dependencies for vulnerabilities
npm audit