Mozilla Thunderbird: Denial of service via crafted Exchange SOAP response
Summary
Mozilla Thunderbird's Exchange email component can be made to hang. A malicious, compromised or man-in-the-middle Exchange server can send a single well-formed SOAP response that keeps Thunderbird's main thread CPU-bound for seconds to hours. The attacker cannot run code or steal data through this flaw according to the available sources, but the Thunderbird client becomes unusable until it is stopped.
Affected
- Mozilla Thunderbird before 153.5 on the 153 branch
- Mozilla Thunderbird before 158 on the 158 branch
- Based on the unaffected ranges in the CVE record (153.5 to 153.* and 158 onward). Exact affected builds were not verified against Mozilla's own text.
Fixed in
- Thunderbird 158 (MFSA 2026-109)
- Thunderbird 153.5 (MFSA 2026-110)
- Newer builds such as 153.5.1, 158.0.1 and 153.6 also contain the fix.
Workaround
Unknown. No workaround is documented in the sources. The fix is the update.
Exploitation
- CISA SSVC: exploitation none, not automatable, technical impact total.
- No known exploitation and no public proof of concept found at the time of research.
- EPSS is 0.00171, a low predicted exploitation probability.
Our recommendation
- Update Thunderbird to 158.0 or later, or to 153.5 or later on the 153 branch, preferably the newest build of the branch in use.
- Prioritise users who access Exchange accounts from untrusted networks such as public Wi-Fi or while travelling.
- Check the vendor's advisories for additional Thunderbird fixes released in the same update cycle, such as MFSA 2026-115 covering AutoDiscover over plain HTTP.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Mozilla MFSA 2026-110 (Thunderbird 153.5) www.mozilla.org
- Mozilla MFSA 2026-109 (Thunderbird 158) www.mozilla.org
- Mozilla Thunderbird known vulnerabilities www.mozilla.org
- Mozilla Bugzilla bug 2077826 bugzilla.mozilla.org
- Ubuntu security CVE-2026-110249 ubuntu.com
- NVD/Vulners CVE-2026-110249 vulners.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 1 Nov 2026, 08:26.
Original description (NVD)
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.
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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