CVE-2026-110249 in Thunderbird
Summary
by MITRE • 10/13/2026
A malicious, compromised, or MITM-controlled Exchange server can return a single well-formed SOAP response containing an element with enough attributes to keep Thunderbird's main thread CPU-bound for seconds to hours. This vulnerability was fixed in Thunderbird 158 and Thunderbird 153.5.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 10/13/2026
The identified vulnerability represents a significant denial of service risk within Mozilla Thunderbird, specifically targeting the email client's parsing engine when processing Simple Object Access Protocol responses from Microsoft Exchange servers. This flaw exploits the resource consumption characteristics of the main application thread during XML and SOAP envelope processing. The core technical issue lies in how Thunderbird handles elements with an excessive number of attributes within a single, well-formed SOAP response. While the input data is syntactically correct and does not trigger traditional parsing errors or crashes, it triggers a computational bottleneck that forces the main UI thread to remain busy for extended periods ranging from seconds to hours. This behavior effectively freezes the user interface, preventing any interaction with the application until the processing completes or times out, thereby rendering the email client unusable during this window.
The attack vector relies on an adversary controlling or compromising a Microsoft Exchange server that communicates with Thunderbird via protocols such as Web Services (EWS) or potentially other SOAP-based interfaces. By injecting a specially crafted response containing elements with a disproportionately large number of attributes, the attacker can induce high CPU utilization within the client application. This is not a memory exhaustion attack but rather a computational complexity exploit designed to tie up processing resources. The impact is severe for end-users who rely on Thunderbird for critical communication workflows, as they are left unable to send or receive emails, access contacts, or perform any other actions until the malicious payload finishes parsing. In enterprise environments where automated scripts might poll Exchange servers frequently, this could lead to widespread service degradation across multiple client machines simultaneously if a compromised server is used to broadcast such payloads.
From a vulnerability classification perspective, this issue aligns with CWE-400, which covers Uncontrolled Resource Consumption, specifically manifesting as a Denial of Service through CPU exhaustion. The exploitation technique mirrors patterns found in the MITRE ATT&CK framework under T1499, Endpoint Denial of Service, where an attacker uses resource-intensive inputs to degrade system performance rather than compromising confidentiality or integrity directly. Although the vulnerability requires network access and often implies some level of trust or compromise within the email infrastructure (such as a Man-in-the-Middle position or a compromised mail server), it highlights critical weaknesses in input validation strategies regarding attribute count limits in XML parsing libraries used by Thunderbird.
Mitigation for this vulnerability involves applying the patches released in Thunderbird versions 158 and 153.5, which implement stricter limits on the number of attributes allowed per element during SOAP response processing. Administrators should ensure that all client endpoints are updated to these secure versions immediately to prevent exploitation. Additionally, network-level controls such as deep packet inspection or web application firewalls can be configured to detect and block unusually large XML payloads with excessive attribute counts before they reach the email clients. Organizations relying on Exchange integration should also monitor for anomalous traffic patterns from mail servers that might indicate an attempt to exploit this resource exhaustion flaw, ensuring both client-side patching and server-side monitoring are in place to maintain operational continuity.