CVE-2020-35730
CVE-2020-35730 is a security flaw in Roundcube Webmail, webmail software many hosting companies and universities run. Attackers can run code in the browser of someone using Roundcube Webmail and act as them. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 22 Jun 2023.[1]
Roundcube Webmail 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. Roundcube has published a fix or advice.
Description
CISA calls it a webmail cross-site scripting (XSS) in Roundcube Roundcube Webmail. In its words: “Roundcube Webmail contains a cross-site scripting (XSS) vulnerability that allows an attacker to send a plain text e-mail message with Javascript in a link reference element that is mishandled by linkref_addinindex in rcube_string_replacer.php.”[1]
The underlying weakness is cross-site scripting (CWE-79), in MITRE’s list of common software weaknesses.[2]
It scores 6.1 out of 10 on the Common Vulnerability Scoring System, which rates it medium, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record doesn’t list version ranges for it; Roundcube’s advisory has them[3]. CISA names the product as Roundcube Roundcube Webmail.
Most families won’t have Roundcube Webmail; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 22 Jun 2023, 906 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 13 Jul 2023 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 33%. That’s in the top 2% 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
Roundcube has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Check whether you use it.Roundcube Webmail 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, or not in use. CISA gave US government agencies until 13 Jul 2023 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 6.1.[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 33%.[4] |
See also
Other Roundcube Webmail flaws
- CVE-2021-44026: webmail SQL injection ·
- CVE-2020-12641: webmail remote code execution ·
- CVE-2017-16651: webmail file disclosure ·
Other Roundcube flaws
- CVE-2025-68461, Roundcube Webmail: cross-site scripting ·
- CVE-2025-49113, Roundcube Webmail: deserialization of untrusted data ·
- CVE-2024-42009, Roundcube Webmail: cross-site scripting ·
- CVE-2024-37383, Roundcube Webmail: cross-site scripting (XSS) ·
- CVE-2020-13965, Roundcube Webmail: cross-site scripting (XSS) ·
Other cross-site scripting flaws
- CVE-2026-42897, Microsoft: exchange server cross-site scripting ·
- CVE-2025-48700, Synacor Zimbra Collaboration Suite (ZCS): cross-site scripting ·
- CVE-2025-66376, Synacor Zimbra Collaboration Suite (ZCS): cross-site scripting ·
- CVE-2021-26829, OpenPLC ScadaBR: cross-site scripting ·
- CVE-2025-27915, Synacor Zimbra Collaboration Suite (ZCS): cross-site scripting ·
Added the same day
- CVE-2023-20887, VMware Aria Operations for Networks: command injection ·
- CVE-2021-44026, Roundcube Webmail: webmail SQL injection ·
- CVE-2020-12641, Roundcube Webmail: webmail remote code execution ·
- CVE-2016-9079, Mozilla Firefox, Firefox ESR, and Thunderbird: use-after-free ·
- CVE-2016-0165, Microsoft Win32k: privilege escalation ·
Every Roundcube flaw on CISA’s list · Everything added in June 2023
References
- ^ Cybersecurity and Infrastructure Security Agency. “Roundcube Webmail Cross-Site Scripting (XSS) Vulnerability”. Known Exploited Vulnerabilities Catalog, added 22 Jun 2023.
- ^ National Institute of Standards and Technology. “CVE-2020-35730 Detail”. National Vulnerability Database, published 28 Dec 2020.
- ^ Roundcube. Advisories and fixes: roundcube.net, lists.fedoraproject.org, lists.fedoraproject.org.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.329, percentile 98.4, 31 Oct 2026.
- CVE Program. “CVE-2020-35730”. 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.