CVE-2021-27561
CVE-2021-27561 is a security flaw in Yealink Device Management. Attackers can run their own code on Yealink Device Management 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 3 Nov 2021.[1]
Yealink Device Management 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.
Description
CISA calls it a server-side request forgery (SSRF) in Yealink Device Management. In its words: “Yealink Device Management contains a server-side request forgery (SSRF) vulnerability that allows for unauthenticated remote code execution.”[1]
The underlying weakness is OS command injection (CWE-78), in MITRE’s list of common software weaknesses.[2]
It scores 9.8 out of 10 on the Common Vulnerability Scoring System, which rates it critical, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record doesn’t list version ranges for it. CISA names the product as Yealink Device Management.
Most families won’t have Yealink Device Management; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 3 Nov 2021, 19 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 17 Nov 2021 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 83%. That’s in the top 1% of the flaws it scores.[3]
CISA assesses it as automatable: attackers can run it against many targets at once.
CISA doesn’t say who was attacked, or how.
Fix
CISA’s required action: “Apply updates per vendor instructions.”[1]
- Check whether you use it.Yealink Device Management 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 17 Nov 2021 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 9.8.[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 83%.[3] |
See also
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-2026-73570, Synacor Zimbra Collaboration Suite (ZCS): OS command injection ·
Added the same day
- CVE-2021-42258, BQE BillQuick Web Suite: SQL injection ·
- CVE-2021-42013, Apache HTTP Server: path traversal ·
- CVE-2021-41773, Apache HTTP Server: path traversal ·
- CVE-2021-40539, Zoho ManageEngine: aDSelfService plus authentication bypass ·
- CVE-2021-40444, Microsoft MSHTML: remote code execution ·
References
- ^ Cybersecurity and Infrastructure Security Agency. “Yealink Device Management Server-Side Request Forgery (SSRF) Vulnerability”. Known Exploited Vulnerabilities Catalog, added 3 Nov 2021.
- ^ National Institute of Standards and Technology. “CVE-2021-27561 Detail”. National Vulnerability Database, published 15 Oct 2021.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.829, percentile 99.7, 31 Oct 2026.
- CVE Program. “CVE-2021-27561”. 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.