CVE-2026-91403: 
Amazon Corretto JDK 취약점 분석 및 완화

CVE-2026-91403 is a critical vulnerability in the JSSE component of Oracle Java SE, Oracle GraalVM for JDK, and Oracle GraalVM Enterprise Edition, rooted in the distrust of compromised eMudhra intermediate CA certificates. The vulnerability allows unauthenticated network attackers to compromise the integrity and confidentiality of Java-based TLS communications. Affected versions include Oracle Java SE 8u511, 8u511-perf, 11.0.33, 17.0.21, 21.0.13, 25.0.5, and 27.0.1; Oracle GraalVM for JDK 17.0.21 and 21.0.13; and Oracle GraalVM Enterprise Edition 21.3.20. Disclosed on October 21, 2026, Oracle issued an out-of-band Security Alert the same day. The CVSS v3.1 base score is 9.1 (Critical) (Oracle Advisory, GitHub Advisory).

The root cause is that Java's JSSE trust store continued to trust eMudhra intermediate CA certificates whose private keys were found to be recoverable or otherwise compromised, classified under CWE-295 (Improper Certificate Validation), CWE-284 (Improper Access Control), and CWE-327 (Use of a Broken or Risky Cryptographic Algorithm) (GitHub Advisory). An attacker who can obtain or forge certificates signed by the compromised eMudhra intermediate CAs can conduct man-in-the-middle (MitM) attacks against Java TLS connections, bypassing certificate validation entirely. The initial Oracle/OpenJDK fix attempted to blocklist specific intermediate certificate fingerprints, but Sonatype reported this approach could be bypassed; Oracle subsequently released a follow-on fix (CVE-2026-114820) that removes the eMudhra root CA certificates from the Java cacerts trust store entirely (Red Hat Bugzilla). Exploitation is possible via any API or web service using the JSSE component, as well as sandboxed Java Web Start applications or applets loading untrusted code (Oracle Advisory).

Successful exploitation allows an unauthenticated remote attacker to perform MitM attacks against Java TLS connections, enabling unauthorized read access to all data accessible by the affected Java runtime (high confidentiality impact) and unauthorized creation, deletion, or modification of critical data (high integrity impact). Availability is not directly impacted. The vulnerability affects any Java deployment — server-side web services, client-side Java Web Start applications, and sandboxed applets — that relies on the Java trust store for TLS certificate validation, potentially exposing sensitive data in transit across a wide range of enterprise and consumer applications (Oracle Advisory, GitHub Advisory).

No confirmed weaponized exploit has been identified; a GitHub repository (Carl0gonzalez/java-security-update) was flagged but assessed as a security auditing/scanning tool rather than a true exploit (Feedly). Exploitation has been reported by cyberupdates365.com, though no in-the-wild attacks have been confirmed by authoritative sources. The EPSS score is approximately 0.39%, indicating a relatively low near-term exploitation probability. The vulnerability is rated as automatable (no user interaction required) with total technical impact per SSVC assessment. It is not currently listed in the CISA KEV catalog. Detection signatures are available via Qualys (IDs 389127, 6297900) and Nessus (ID 372634) (Red Hat Bugzilla).

1. Obtain or forge a certificate: Leverage the compromised eMudhra intermediate CA private keys (publicly known to be recoverable) to sign a fraudulent TLS certificate for a target domain.
2. Position for MitM: Place the attacker-controlled system in a network position between the Java client/server and the legitimate endpoint (e.g., via ARP spoofing, DNS poisoning, or rogue Wi-Fi access point).
3. Intercept TLS handshake: Present the forged certificate during the TLS handshake to the vulnerable Java runtime. Because the eMudhra intermediate CA is trusted in the Java cacerts store, the certificate passes validation without error.
4. Decrypt and manipulate traffic: With the MitM position established, decrypt, read, and optionally modify data in transit — including credentials, session tokens, API responses, or sensitive business data — before re-encrypting and forwarding to the legitimate endpoint.
5. Target high-value Java services: Focus on Java-based web services, enterprise middleware, or sandboxed Java Web Start applications that handle sensitive data and rely on JSSE for TLS validation (Oracle Advisory, Red Hat Bugzilla).

• Network: Unexpected TLS certificates in Java application traffic signed by eMudhra intermediate CAs (e.g., eMudhra Sub-CA for SSL, emsign intermediate CAs); TLS connections to known-good domains presenting certificates with eMudhra CA chain entries after patching.
• Logs: Java application logs showing TLS handshake completions with certificates issued under eMudhra CA hierarchy; JSSE debug output (-Djavax.net.debug=ssl) revealing eMudhra-chained certificates being accepted.
• File System: Presence of unpatched Java installations (versions listed as affected) on systems handling sensitive TLS traffic; absence of updated cacerts trust store reflecting eMudhra root removal.
• Process: Java processes establishing outbound TLS connections to unexpected endpoints that present eMudhra-signed certificates; anomalous data exfiltration patterns from Java-based services following TLS session establishment.

Oracle released an out-of-band Security Alert on October 21, 2026, providing Java updates that distrust the compromised eMudhra intermediate CA certificates; users should update to the latest patched Java version for their release train (Oracle Advisory). The initial fix (blocklisting specific intermediate certificate fingerprints) was found to be bypassable; Oracle subsequently issued a stronger fix under CVE-2026-114820 that removes eMudhra root CA certificates from the Java cacerts trust store — applying this follow-on fix is strongly recommended (Red Hat Bugzilla). Red Hat addressed the issue via RHSA-2026:88353 (OpenJDK 17.0.21) and RHSA-2026:89916 (OpenJDK 17.0.20.1), and Amazon Linux advisories ALAS2-2026-4040, ALAS2CORRETTO8-2026-026, and ALAS2023-2026-3316 are also available. As a workaround, administrators can manually remove eMudhra root certificates from the Java cacerts trust store using keytool -delete, and should restrict network access to Java applications where possible.

Oracle issued an emergency out-of-band Security Alert — unusual outside the quarterly CPU cycle — signaling the severity of the eMudhra CA compromise (Oracle Advisory). Sonatype published a notable blog post revealing that the initial fingerprint-blocklist fix could be bypassed, prompting Oracle to revise its mitigation guidance and issue a follow-on CVE (Red Hat Bugzilla). Security media including SecurityWeek, GBHackers, CyberSecurityNews, and The Hacker News covered the vulnerability, with SecurityWeek noting it as part of Oracle's broader October 2026 patching activity. Community discussion was active on Hacker News, Reddit (r/scala), Mastodon, and Bluesky, with developers debating the adequacy of certificate blocklisting versus root CA removal as a remediation strategy. The Elastic, Retool, and phoss-ap communities also raised concerns about impact on their Java-dependent products.