Adobe Patches Reader Zero-Day Exploited for Months

Adobe on Saturday released emergency patches for a critical Acrobat and Reader zero-day that has been exploited in the wild for several months. The vulnerability has been assigned the CVE identifier CVE-2026-34621 and a CVSS score of 9.6. According to the software giant, the flaw stems from improperly controlled modifications to prototype attributes and can be exploited to execute arbitrary code. Acrobat and Reader for Windows and macOS are affected. Patches are included in version 26.001.21411 of Acrobat DC and Acrobat Reader DC, and versions 24.001.30362 and 24.001.30360 of Acrobat 2024. Adobe’s advisory confirms that  CVE-2026-34621 has been exploited in the wild. The company has credited Haifei Li for reporting the vulnerability. Li is a reputable researcher who has worked at Fortinet, McAfee, Microsoft, and Check Point. He is the founder of Expmon, a sandbox system designed to detect file-based exploits. Li came across the zero-day while analyzing a sophisticated PDF exploit uploaded to Expmon. The exploit he identified was designed to harvest information, but the researcher warned in his initial disclosure that subsequent stages in the exploit chain may include remote code execution and a sandbox escape.  Adobe has confirmed that exploitation of CVE-2026-34621 can lead to code execution, rather than just information disclosure. Based on the analysis of an exploit sample uploaded to VirusTotal, researchers determined that exploitation of CVE-2026-34621 started as early as November 2025. Li indicated that an APT is likely behind the attacks, and a threat intelligence analyst who uses the online moniker Gi7w0rm noted that the malicious PDFs used Russian-language lures and referenced current events in Russia’s oil and gas sector.   More information on who is behind the attacks will likely surface in the coming days as more members of the cybersecurity community analyze the exploits. Li has made technical details available, and others have released indicators of compromise (IoCs) to help defenders detect potential exploitation of CVE-2026-34621.   Related: Juniper Networks Patches Dozens of Junos OS Vulnerabilities Related: Orthanc DICOM Vulnerabilities Lead to Crashes, RCE Related: Critical Marimo Flaw Exploited Hours After Public Disclosure WRITTEN BY Eduard Kovacs Eduard Kovacs (@EduardKovacs) is senior managing editor at SecurityWeek. He worked as a high school IT teacher before starting a career in journalism in 2011. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering. 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