CVE-2026-73570
CVE-2026-73570 is a security flaw in Synacor Zimbra Collaboration Suite (ZCS), email and calendar software companies run on their own servers. Attackers can run their own code on Synacor Zimbra Collaboration Suite (ZCS) from anywhere, without logging in. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 21 Aug 2026.[1]
Synacor Zimbra Collaboration Suite (ZCS) is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed in version 10.1.20 of Collaboration.
Description
CISA calls it an OS command injection in Synacor Zimbra Collaboration Suite (ZCS). In its words: “Zimbra Collaboration Suite (ZCS) contains an OS command injection vulnerability which could allow an unauthenticated attacker to send specially crafted SMTP requests that may result in execution of arbitrary operating system commands as the Zimbra user.”[1]
The CVE record adds: “A remote code execution vulnerability exists in Zimbra Collaboration (ZCS) before 10.1.20 when the optional zimbra-snmp package is installed and SNMP notifications are enabled. Due to improper sanitization of untrusted input during SNMP notification processing, an unauthenticated attacker can send specially crafted SMTP requests that may result in execution of arbitrary operating system commands as the Zimbra user.”[2]
The underlying weakness is OS command injection (CWE-78), in MITRE’s list of common software weaknesses.
It scores 8.9 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by the organisation that published the CVE using CVSS 3.1.[2]
Affected versions
NVD’s record lists one fixed version: everything before it is affected.[2]
| Product | Fixed in |
|---|---|
| Collaboration | 10.1.20 |
Most families won’t have Synacor Zimbra Collaboration Suite (ZCS); it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 21 Aug 2026, 8 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 24 Aug 2026 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 72%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
Synacor has published a fix or advice.[3] CISA’s required action: “Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes).”[1]
- Check whether you use it.Synacor Zimbra Collaboration Suite (ZCS) is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed (fixed in version 10.1.20 of Collaboration), or not in use. CISA gave US government agencies until 24 Aug 2026 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 8.9.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 72%.[4] |
See also
Other Synacor Zimbra Collaboration Suite (ZCS) flaws
- CVE-2025-48700: cross-site scripting ·
- CVE-2025-66376: cross-site scripting ·
- CVE-2025-68645: PHP remote file inclusion ·
- CVE-2025-27915: cross-site scripting ·
- CVE-2019-9621: server-side request forgery (SSRF) ·
- CVE-2024-27443: cross-site scripting (XSS) ·
Other Synacor flaws
- CVE-2020-7796, Synacor Zimbra Collaboration Suite: (ZCS) server-side request forgery ·
- CVE-2022-24682, Synacor Zimbra Collaborate Suite (ZCS): cross-site scripting ·
Other OS command injection flaws
- CVE-2026-110447, Collabora Online – Built-in CODE Server: online – built-in CODE server OS command injection ·
- CVE-2026-106931, EG Secure Solutions SiteGuard Server Edition: OS command injection ·
- CVE-2026-83549, SonicWall SMA1000 Appliances: OS command injection ·
- CVE-2026-49869, Kestra OSS: OSS OS command injection ·
- CVE-2026-16812, Arista VeloCloud Orchestrator: on-prem OS command injection ·
Every Synacor flaw on CISA’s list · Everything added in August 2026
References
- ^ Cybersecurity and Infrastructure Security Agency. “Zimbra Collaboration Suite (ZCS) OS Command Injection Vulnerability”. Known Exploited Vulnerabilities Catalog, added 21 Aug 2026.
- ^ National Institute of Standards and Technology. “CVE-2026-73570 Detail”. National Vulnerability Database, published 13 Aug 2026.
- ^ Synacor. Advisories and fixes: wiki.zimbra.com, blog.zimbra.com, wiki.zimbra.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.717, percentile 99.4, 31 Oct 2026.
- CVE Program. “CVE-2026-73570”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.