CVE-2026-57962 Overview
CVE-2026-57962 is a memory exhaustion vulnerability in Mozilla Thunderbird's Lightweight Directory Access Protocol (LDAP) client. A malicious LDAP server, queried by Thunderbird for address-book autocomplete, can push arbitrarily large volumes of attacker-supplied data into the client. The client accumulates this data without bound until the process crashes. The flaw maps to Uncontrolled Resource Consumption [CWE-400] and requires the user to interact with an address-book field that triggers the LDAP query. Mozilla fixed the issue in Thunderbird 152.0.1 and Thunderbird 140.12.1.
Critical Impact
A hostile or compromised LDAP directory can force Thunderbird clients to consume all available memory, terminating the mail client and disrupting user communications.
Affected Products
- Mozilla Thunderbird versions prior to 140.12.1 (ESR branch)
- Mozilla Thunderbird versions prior to 152.0.1 (release branch)
- Any Thunderbird deployment configured to query an untrusted LDAP server for address-book autocomplete
Discovery Timeline
- 2026-07-01 - CVE-2026-57962 published to the National Vulnerability Database
- 2026-07-01 - Last updated in NVD database
Technical Details for CVE-2026-57962
Vulnerability Analysis
Thunderbird supports address-book autocomplete backed by remote LDAP directories. When a user types in a recipient field, the client issues LDAP search queries and buffers responses to populate suggestions. The client does not enforce upper bounds on the size or count of attribute values it accepts from the server. A server that returns arbitrarily large responses can force the client to allocate memory until the process crashes. The condition is categorized under [CWE-400] Uncontrolled Resource Consumption and results in a denial-of-service outcome against the mail client.
Root Cause
The root cause is missing size and rate limits in the Thunderbird LDAP client's handling of server responses. The client trusts the directory server to return well-formed, bounded results. There is no ceiling on aggregate buffer growth, no cap on per-attribute value size, and no timeout that discards oversized replies before allocation succeeds.
Attack Vector
Exploitation requires that a Thunderbird user has configured an LDAP directory controlled or reachable by the attacker. User interaction is required — the victim must trigger an autocomplete lookup, typically by composing a message and typing into the To, Cc, or Bcc field. The malicious server then responds with attacker-controlled data sized to exhaust available memory. No authentication to the client is required and the network attack vector applies because the LDAP session traverses the network.
No public proof-of-concept exploit is available. See the Mozilla Bug Report #2042872 for technical discussion of the response-handling path.
Detection Methods for CVE-2026-57962
Indicators of Compromise
- Thunderbird processes terminating with out-of-memory conditions shortly after opening a compose window or performing address lookups
- Unusually large inbound data volumes on TCP port 389 or 636 from an LDAP server to Thunderbird endpoints
- Repeated LDAP search responses from a single directory host that dwarf historical response sizes for the same query patterns
- Endpoint memory pressure alerts correlated with thunderbird.exe or thunderbird process ancestry
Detection Strategies
- Monitor process crash telemetry for Thunderbird binaries and correlate with active LDAP sessions on the host
- Inspect network flow data for asymmetric LDAP conversations where server-to-client bytes exceed expected directory response sizes
- Alert on repeated Thunderbird process restarts on the same endpoint within short windows, which indicate a persistent malicious upstream
Monitoring Recommendations
- Log outbound LDAP connections from user endpoints and baseline the destination directory servers permitted by policy
- Track Thunderbird version inventory across the fleet and flag hosts running builds older than 140.12.1 or 152.0.1
- Forward endpoint process termination and memory-exhaustion events to a central analytics platform for correlation with network telemetry
How to Mitigate CVE-2026-57962
Immediate Actions Required
- Upgrade Thunderbird to version 152.0.1 or, for the ESR channel, to 140.12.1
- Audit all configured LDAP address-book servers under Preferences → Composition → Addressing and remove entries pointing to untrusted or unmanaged directories
- Restrict outbound LDAP (TCP 389/636) from user workstations to approved corporate directory servers only
Patch Information
Mozilla released fixes in Thunderbird 152.0.1 and Thunderbird 140.12.1. Details are documented in Mozilla Security Advisory MFSA-2026-63 and Mozilla Security Advisory MFSA-2026-64. Deploy the updates through managed software distribution channels and verify the running version via the Help → About Thunderbird dialog.
Workarounds
- Disable LDAP address-book autocomplete in Thunderbird preferences until patches are applied
- Remove or disable LDAP directory entries in Address Book → Tools → Options for any server not under organizational control
- Enforce network egress rules that permit LDAP traffic only to vetted internal directory servers
- Require TLS-protected LDAP (LDAPS) with certificate validation to reduce the risk of on-path server substitution
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
