{"description":"Trending threats, MITRE ATT\u0026CK coverage, and detection metadata. Fed continuously.","feed_url":"https://feed.craftedsignal.io/tags/cve-2026-42013/feed.json","home_page_url":"https://feed.craftedsignal.io/","items":[{"_cs_actors":[],"_cs_cpes":[],"_cs_cves":[{"cvss":8.2,"id":"CVE-2026-42013"}],"_cs_exploited":false,"_cs_has_poc":false,"_cs_poc_references":[],"_cs_products":["gnutls"],"_cs_severities":["high"],"_cs_tags":["certificate validation","spoofing","man-in-the-middle","gnutls","CVE-2026-42013"],"_cs_type":"threat","_cs_vendors":["Red Hat"],"content_html":"\u003cp\u003eCVE-2026-42013 describes a certificate validation bypass vulnerability within the gnutls library. The vulnerability occurs when gnutls encounters an oversized Subject Alternative Name (SAN) during certificate validation. Instead of properly rejecting the certificate, the validation process incorrectly falls back to checking the Common Name (CN) field. This fallback behavior allows a remote attacker to potentially bypass certificate validation. An attacker could exploit this flaw to perform spoofing or man-in-the-middle attacks by presenting a certificate with a valid CN but a manipulated SAN. This vulnerability was published on 2026-05-26.\u003c/p\u003e\n\u003ch2 id=\"attack-chain\"\u003eAttack Chain\u003c/h2\u003e\n\u003col\u003e\n\u003cli\u003eAttacker crafts a malicious certificate with an oversized Subject Alternative Name (SAN) field.\u003c/li\u003e\n\u003cli\u003eThe attacker sets the Common Name (CN) field in the certificate to a value they wish to impersonate (e.g., a legitimate domain).\u003c/li\u003e\n\u003cli\u003eThe attacker initiates a TLS connection to a target server or client using the crafted certificate.\u003c/li\u003e\n\u003cli\u003eThe gnutls library on the target attempts to validate the presented certificate.\u003c/li\u003e\n\u003cli\u003eDue to the oversized SAN, the gnutls library fails to properly process the SAN field.\u003c/li\u003e\n\u003cli\u003eThe gnutls library incorrectly falls back to validating the CN field.\u003c/li\u003e\n\u003cli\u003eThe CN field matches the expected value, and the gnutls library incorrectly considers the certificate valid.\u003c/li\u003e\n\u003cli\u003eThe attacker successfully bypasses certificate validation, enabling potential spoofing or man-in-the-middle attacks.\u003c/li\u003e\n\u003c/ol\u003e\n\u003ch2 id=\"impact\"\u003eImpact\u003c/h2\u003e\n\u003cp\u003eSuccessful exploitation of CVE-2026-42013 allows a remote attacker to bypass certificate validation, potentially leading to spoofing or man-in-the-middle attacks. This could allow the attacker to intercept sensitive data, inject malicious content, or compromise the confidentiality and integrity of communications. The CVSS v3.1 base score is 8.2, indicating a high severity.\u003c/p\u003e\n\u003ch2 id=\"recommendation\"\u003eRecommendation\u003c/h2\u003e\n\u003cul\u003e\n\u003cli\u003eApply the necessary patches or updates provided by Red Hat to address CVE-2026-42013 on systems using the affected gnutls library.\u003c/li\u003e\n\u003cli\u003eMonitor network traffic for TLS connections using certificates with unusually large SAN fields, as these could indicate exploitation attempts. Consider implementing a network connection rule targeting connections utilizing certificates with large SAN sizes.\u003c/li\u003e\n\u003cli\u003eDeploy the Sigma rule \u003ccode\u003eDetect GnuTLS Certificate Validation Bypass - Large SAN\u003c/code\u003e to identify potential exploitation attempts based on process execution patterns and network connections.\u003c/li\u003e\n\u003c/ul\u003e\n","date_modified":"2026-05-26T22:18:53Z","date_published":"2026-05-26T22:18:53Z","id":"https://feed.craftedsignal.io/briefs/2026-05-gnutls-cert-bypass/","summary":"A vulnerability in gnutls (CVE-2026-42013) allows a remote attacker to bypass certificate validation by providing an oversized Subject Alternative Name (SAN), causing the validation process to fall back to the Common Name (CN) field, potentially leading to spoofing or man-in-the-middle attacks.","title":"CVE-2026-42013: gnutls Certificate Validation Bypass via Oversized SAN","url":"https://feed.craftedsignal.io/briefs/2026-05-gnutls-cert-bypass/"}],"language":"en","title":"CraftedSignal Threat Feed — CVE-2026-42013","version":"https://jsonfeed.org/version/1.1"}