Vulnerability Database / CVE-2026-0891 CVE-2026-0891: Firefox & Thunderbird Memory RCE Vulnerability CVE-2026-0891 is a memory corruption RCE flaw in Firefox and Thunderbird that enables attackers to execute arbitrary code. This article covers technical details, affected versions, security impact, and mitigation. Updated : January 22, 2026 CVE-2026-0891 Overview CVE-2026-0891 is a memory safety vulnerability affecting Mozilla Firefox and Thunderbird products. Memory safety bugs were identified in Firefox ESR 140.6, Thunderbird ESR 140.6, Firefox 146, and Thunderbird 146. Some of these bugs showed evidence of memory corruption, and Mozilla presumes that with enough effort, some of these could have been exploited to run arbitrary code. This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). Critical Impact Successful exploitation could allow attackers to execute arbitrary code through memory corruption, potentially leading to complete system compromise via malicious web content or email attachments. Affected Products Firefox versions prior to 147 Firefox ESR versions prior to 140.7 Thunderbird ESR 140.6 and Thunderbird 146 (related products) Discovery Timeline 2026-01-13 - CVE-2026-0891 published to NVD 2026-01-13 - Last updated in NVD database Technical Details for CVE-2026-0891 Vulnerability Analysis This vulnerability stems from multiple memory safety bugs that collectively represent improper restriction of operations within the bounds of a memory buffer. The flaws manifest as memory corruption issues that could potentially be leveraged for arbitrary code execution. The network-based attack vector means exploitation can occur remotely, though the high attack complexity indicates that successful exploitation requires specific conditions to be met. Mozilla has identified multiple related bugs (tracked as bug IDs 1964722, 2000981, 2003100, and 2003278) that contribute to this vulnerability. Evidence of memory corruption in these bugs suggests that heap or stack memory regions may be improperly handled, creating opportunities for attackers to manipulate program execution flow. Root Cause The root cause is improper memory management within Firefox and Thunderbird's rendering and processing components. CWE-119 vulnerabilities typically arise from buffer operations that fail to properly validate boundaries, leading to out-of-bounds memory access. These memory safety issues can result in data corruption, crashes, or in worst-case scenarios, allow attackers to inject and execute malicious code by carefully crafting inputs that overwrite critical memory structures. Attack Vector The vulnerability is exploitable over the network, requiring no authentication or user privileges. An attacker could craft malicious web content or email attachments that trigger the memory corruption when processed by vulnerable Firefox or Thunderbird versions. While no user interaction is explicitly required in the CVSS metrics, typical browser exploitation scenarios involve luring users to malicious websites or sending crafted emails. The high attack complexity indicates that exploitation is not straightforward and may require: Precise memory layout manipulation Bypassing existing memory protection mechanisms (ASLR, DEP) Specific timing or environmental conditions Successful exploitation could result in complete confidentiality, integrity, and availability compromise of the affected system. Detection Methods for CVE-2026-0891 Indicators of Compromise Unexpected Firefox or Thunderbird crashes, particularly when rendering specific web content or processing emails Anomalous memory consumption patterns in browser processes Detection of known malicious URLs or domains serving exploit payloads targeting Firefox/Thunderbird Unusual child process spawning from browser or email client processes Detection Strategies Monitor for browser crash reports that indicate memory corruption patterns such as access violations or heap corruption Deploy network-based detection for known exploit patterns targeting Mozilla products Implement endpoint detection rules to identify suspicious process behavior originating from Firefox or Thunderbird Review security logs for evidence of exploitation attempts through web or email vectors Monitoring Recommendations Enable enhanced crash reporting and analyze crash dumps for signs of exploitation attempts Monitor network traffic for connections to suspicious domains known for hosting browser exploits Track software versions across endpoints to identify systems running vulnerable Firefox or Thunderbird versions Implement browser isolation technologies to contain potential exploitation attempts How to Mitigate CVE-2026-0891 Immediate Actions Required Update Firefox to version 147 or later immediately Update Firefox ESR to version 140.7 or later Update Thunderbird to patched versions as specified in Mozilla security advisories Prioritize patching for systems w