CVE-2026-113867 in Thunderbird
Summary
by MITRE • 10/20/2026
Thunderbird did not escape attribute values when serializing outgoing XMPP stanzas. Because values from incoming requests were echoed back in automatic replies, a remote XMPP user could send a crafted request that caused Thunderbird to inject arbitrary stanzas into the user's connection, which the server would then process as if sent by the user. This could be used to send messages or change contact and subscription settings on behalf of the user, without any interaction. This bug only affects Thunderbird's chat feature with XMPP accounts. This vulnerability was fixed in Thunderbird 158.0.1.
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Analysis
by VulDB Data Team • 10/20/2026
The identified vulnerability represents a critical failure in input validation and output encoding within Mozilla Thunderbird’s implementation of the Extensible Messaging and Presence Protocol, commonly known as XMPP. Specifically, the application failed to properly escape attribute values when serializing outgoing XML stanzas for chat communications. In secure software development, particularly when handling structured data formats like XML or HTML, it is imperative that all user-supplied input be sanitized before being rendered in a context where it could be interpreted as code or structural markup. The absence of this escaping mechanism creates an environment ripe for injection attacks, allowing malicious actors to manipulate the structure and content of network traffic generated by the client application.
The technical flaw operates through a reflection-based attack vector wherein values received from incoming XMPP requests are directly echoed back into automatic replies without sanitization. When a remote attacker sends a crafted request containing specially constructed XML attributes or elements, Thunderbird incorporates these unescaped inputs into its outgoing responses. Because the server processes these outgoing stanzas as legitimate traffic originating from the authenticated user account, the injected content is executed within the context of that session. This mechanism effectively bypasses standard authentication checks for specific actions, as the malicious commands appear to originate from a trusted source—the victim’s own client software communicating with its designated XMPP server.
The operational impact of this vulnerability is severe and multifaceted, primarily affecting the confidentiality, integrity, and availability of user communications via Thunderbird's chat feature. An attacker can exploit this flaw to send arbitrary messages on behalf of the victim, potentially engaging in social engineering or spreading malware through trusted channels. Furthermore, the ability to modify contact lists and subscription settings allows for significant disruption of service and privacy violations. By altering subscriptions, an adversary could subscribe a user to malicious mailing lists or remove them from critical communication groups without any interaction required from the target. This lack of user consent undermines the fundamental trust model of instant messaging platforms, where users expect their identity to remain distinct from automated system responses unless explicitly authorized.
From a classification perspective, this vulnerability aligns with CWE-79, which describes Improper Neutralization of Input During Web Page Generation known as Cross-site Scripting, although in this context it manifests as XML Injection or Server-Side Request Forgery variants depending on the specific payload and server behavior. It also maps to MITRE ATT&CK techniques related to Command and Control via Application Layer Protocol, specifically leveraging legitimate services for malicious communication. The attack vector is classified as Remote with Low Complexity, requiring no user interaction beyond receiving a message from an attacker who has crafted the initial exploit string. This ease of exploitation significantly elevates the risk profile associated with this flaw.
Mitigation strategies must prioritize immediate patching and defensive configuration adjustments. Users running Thunderbird should upgrade to version 158.0.1 or later, where the serialization logic has been corrected to properly escape attribute values before inclusion in outgoing stanzas. For organizations managing large deployments of Thunderbird clients, automated software update mechanisms should be employed to ensure rapid dissemination of this security fix across all endpoints. Additionally, administrators may consider implementing network-level filtering rules that inspect XMPP traffic for anomalous XML structures or unexpected character sequences indicative of injection attempts, although such measures serve as a compensating control rather than a primary defense. Long-term remediation involves enforcing strict input validation policies in future development cycles and conducting regular code audits focused on data serialization processes to prevent similar vulnerabilities from arising in other protocol implementations within the application suite.