CVE-2026-4602 is a vulnerability identified in the jsrsasign JavaScript cryptographic library, specifically affecting versions before 11.1.1. The root cause lies in the ext/jsbn2.js file, where the library incorrectly handles negative exponents during modular exponentiation operations performed by the modPow function. Modular exponentiation is fundamental in cryptographic algorithms such as RSA and DSA for operations like signature verification. The vulnerability allows an attacker to supply a negative exponent, which leads to an incorrect calculation of modular inverses. This results in the failure of signature verification processes, effectively allowing an attacker to forge or bypass digital signatures. The flaw does not require any privileges, authentication, or user interaction, and can be exploited remotely over the network. The CVSS 4.0 vector (AV:N/AC:L/AT:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:P) reflects a high impact on the integrity of affected systems with no confidentiality or availability impact. Although no public exploits have been observed, the vulnerability poses a severe risk to any system relying on jsrsasign for cryptographic validation, including web applications, APIs, and client-side cryptographic operations. The issue was reserved and published in March 2026, and the recommended fix is upgrading to jsrsasign version 11.1.1 or later, where the numeric conversion logic has been corrected to properly handle negative exponents.

The primary impact of CVE-2026-4602 is the compromise of cryptographic signature verification integrity. Systems relying on jsrsasign for validating digital signatures, such as authentication tokens, software updates, or secure communications, may be tricked into accepting forged or tampered data. This can lead to unauthorized access, privilege escalation, or execution of malicious code under the guise of trusted signatures. Since the vulnerability can be exploited remotely without authentication or user interaction, it significantly increases the attack surface for threat actors. Organizations using jsrsasign in client-side or server-side applications risk data integrity breaches, undermining trust in their security mechanisms. The vulnerability does not directly affect confidentiality or availability but indirectly threatens these by enabling further attacks that rely on signature bypass. The lack of known exploits in the wild currently reduces immediate risk, but the high CVSS score and ease of exploitation make rapid patching critical to prevent future attacks.

To mitigate CVE-2026-4602, organizations should immediately upgrade all instances of jsrsasign to version 11.1.1 or later, where the numeric conversion flaw has been fixed. For environments where immediate upgrade is not feasible, consider implementing input validation to reject negative exponents passed to modPow or wrapping the function to sanitize inputs. Conduct a thorough audit of all cryptographic operations relying on jsrsasign to identify potential exposure points. Employ defense-in-depth by combining cryptographic signature verification with additional integrity checks or multi-factor authentication where applicable. Monitor network traffic and application logs for anomalous calls to cryptographic functions that may indicate exploitation attempts. Educate development teams about secure usage of cryptographic libraries and the risks of improper numeric handling. Finally, maintain an up-to-date inventory of all software dependencies to ensure timely patching of similar vulnerabilities in the future.