CVE-2026-91403: 
Amazon Corretto JDK 脆弱性分析と緩和策

CVE-2026-91403 is a critical vulnerability in the JSSE (Java Secure Socket Extension) component of Oracle Java SE, Oracle GraalVM for JDK, and Oracle GraalVM Enterprise Edition, rooted in the compromise of eMudhra intermediate CA certificates. The vulnerability allows unauthenticated, network-accessible attackers to compromise affected Java runtimes via multiple protocols, resulting in high confidentiality and integrity impacts. 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. It was disclosed on October 21, 2026, with Oracle issuing an out-of-band Security Alert the same day. The CVSS v3.1 base score is 9.1 (Critical) (Oracle Advisory, GitHub Advisory).

The vulnerability is classified under CWE-295 (Improper Certificate Validation), CWE-284 (Improper Access Control), and CWE-327 (Use of a Broken or Risky Cryptographic Algorithm). The root cause is that Java's JSSE component trusted a set of eMudhra intermediate CA certificates whose private keys were found to be weak or recoverable, enabling an attacker to forge certificates trusted by the JVM. An attacker with a fraudulently obtained or forged certificate signed by a compromised eMudhra intermediate CA could intercept or tamper with TLS-protected communications accepted by vulnerable Java runtimes. The initial Oracle fix attempted to blocklist specific intermediate certificate fingerprints, but Sonatype researchers discovered this approach could be bypassed, leading Oracle to subsequently issue a follow-on fix (CVE-2026-114820) that removes the eMudhra root CA certificates entirely from the Java trust store (Red Hat Bugzilla, Oracle Advisory). The vulnerability can be exploited via web services supplying data to JSSE APIs, or through sandboxed Java Web Start applications and applets loading untrusted code from the internet (GitHub Advisory).

Successful exploitation allows an unauthenticated remote attacker to perform unauthorized creation, deletion, or modification of critical data accessible to Oracle Java SE, GraalVM for JDK, or GraalVM Enterprise Edition, as well as unauthorized read access to all data accessible by these products. In practice, this means an attacker can conduct man-in-the-middle (MitM) attacks against TLS connections made by vulnerable Java applications, enabling interception of sensitive data (credentials, session tokens, API keys) and injection of malicious content into trusted communication channels. Availability is not directly impacted, but the confidentiality and integrity risks are rated High, affecting any Java-based service or client that relies on TLS trust validation (Oracle Advisory, GitHub Advisory).

The vulnerability is rated as "easily exploitable" by Oracle, requiring no authentication and no user interaction over the network. The EPSS score is approximately 0.39%, reflecting moderate near-term exploitation probability. A GitHub repository (Carl0gonzalez/java-security-update) was flagged as a potential PoC but was assessed as a security auditing/simulation tool rather than a true exploit. A separate GitHub repository (emirwtz/CVE-2026-91403) was also identified. Exploitation has been reported by cyberupdates365.com, though no confirmed in-the-wild weaponized exploitation has been independently verified. The vulnerability is not currently listed in the CISA KEV catalog based on available data. Notably, the initial patch was found bypassable (reported by Sonatype), prompting a follow-on fix under CVE-2026-114820 (Red Hat Bugzilla, GitHub Advisory).

1. Obtain or forge a certificate: Leverage the compromised eMudhra intermediate CA private key (which was found to be weak/recoverable) to sign a fraudulent TLS certificate for a target domain trusted by vulnerable Java applications.
2. Position for MitM: Set up a network-level man-in-the-middle position between the target Java client/server and its intended TLS endpoint — for example, via ARP spoofing, DNS poisoning, or BGP hijacking on a shared network segment.
3. Present the forged certificate: When the vulnerable Java runtime initiates a TLS handshake, present the attacker-controlled certificate signed by the compromised eMudhra intermediate CA. The vulnerable JSSE component will validate and accept this certificate as trusted.
4. Intercept or tamper with traffic: With TLS terminated at the attacker's proxy, decrypt and read sensitive data (credentials, tokens, PII) or inject malicious payloads into the communication stream before re-encrypting and forwarding to the legitimate endpoint.
5. Target sandboxed environments (alternative): For Java Web Start or applet deployments, serve untrusted code signed with the forged certificate to bypass sandbox trust checks, potentially enabling code execution within the Java sandbox context (Oracle Advisory, Red Hat Bugzilla).

• Network: Unexpected TLS certificates presented to Java clients that chain to eMudhra intermediate or root CAs; TLS connections to known-good endpoints terminating at unexpected IP addresses; anomalous certificate serial numbers or validity periods in network traffic.
• Logs: Java application logs showing TLS handshake completions with certificates issued by eMudhra CAs (e.g., eMudhra, emsign in issuer fields); unexpected certificate validation errors or warnings in application logs after patching.
• File System: Presence of eMudhra-related certificates in the Java cacerts trust store ($JAVA_HOME/lib/security/cacerts) on unpatched systems; unexpected modifications to blocked.certs or cacerts files.
• Process: Java processes initiating outbound TLS connections to unusual endpoints; Java Web Start or applet processes loading content from untrusted or newly registered domains (Red Hat Bugzilla, Oracle Advisory).

Oracle released an out-of-band Security Alert on October 21, 2026, providing updated Java versions that distrust the compromised eMudhra intermediate CA certificates. However, the initial fix (blocklisting intermediate certificate fingerprints) was found bypassable by Sonatype researchers; Oracle subsequently issued a stronger fix under CVE-2026-114820 that removes eMudhra root CA certificates from the Java trust store entirely. Organizations should update to the latest patched Java releases and apply the CVE-2026-114820 fix as well. Red Hat issued multiple errata (RHSA-2026:88348, 88350, 88351, 88353, 89916, 90912) for Red Hat Build of OpenJDK; RHEL OpenJDK RPM packages are addressed via the ca-certificates package fix for CVE-2026-114406. As a workaround, administrators can manually remove eMudhra root CA certificates from the Java cacerts trust store using keytool -delete, and restrict network access to Java applications where possible (Oracle Advisory, Red Hat Bugzilla).

Oracle issued an emergency out-of-band Security Alert — a relatively rare occurrence — underscoring the severity of the eMudhra CA compromise. Sonatype publicly disclosed that the initial Oracle fix (fingerprint-based blocklist) was bypassable, prompting Oracle to release a follow-on fix and a new CVE (CVE-2026-114820), which drew significant attention in the Java security community (Red Hat Bugzilla). Security researchers and bloggers widely covered the incident, with commentary on Hacker News, Mastodon, Bluesky, and Medium highlighting the broader trust implications of CA key compromise for the Java ecosystem. JetBrains, Elastic (Logstash), AWS, Google Cloud, Gradle, and other vendors issued their own advisories or updates in response. The incident was dubbed part of "Black October 2026" by some security outlets, reflecting the concentration of high-severity disclosures that month.