CVE-2021-27137 DD-WRT DD-WRT: memory-safety flaw
In plain words: The program mishandles the memory it uses to keep track of data. A specially made input can scramble that memory. Usually lets an attacker crash the program or run their own code with its permissions.
Confirmed exploited by CISA on · US federal patch deadline Jul 24, 2026
What to do
- Install DD-WRT's security update for DD-WRT. If no update exists, follow the vendor's workaround or stop using the product until one ships.
- Keep a list of every internet-facing server you run so nothing is forgotten.
- Patch anything in this catalog first, ahead of normal patch cycles.
- Put admin and file-transfer apps behind a VPN or single sign-on instead of the open internet.
Who is affected
Work IT. Aimed mostly at company systems — you are affected through the companies you use, not your own phone.
What's at risk: Mass data theft and ransomware: a single flaw in a file-transfer or email server has exposed thousands of organisations at once.
The details
- Official name
- DD-WRT Stack-Based Buffer Overflow Vulnerability
- Product
- DD-WRT DD-WRT
- CISA description
- DD-WRT contains a stack-based buffer overflow vulnerability that could allow an unauthenticated attacker to overflow an internal buffer used by UPnP and trigger a code execution vulnerability.
- Type of flaw
- Memory-safety flaw (CWE-121)
- Confirmed exploited
- Jul 21, 2026
- US federal patch deadline
- Jul 24, 2026
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Answering about CVE-2021-27137 ·
Questions about CVE-2021-27137
Is CVE-2021-27137 being exploited?
Yes. CISA added CVE-2021-27137 to its Known Exploited Vulnerabilities catalog on Jul 21, 2026, which means there is reliable evidence of real-world attacks.
How do I fix CVE-2021-27137?
Install DD-WRT's security update for DD-WRT. If no update exists, follow the vendor's workaround or stop using the product until one ships. Keep a list of every internet-facing server you run so nothing is forgotten.
Am I affected by CVE-2021-27137?
Only if you use DD-WRT DD-WRT. Aimed mostly at company systems — you are affected through the companies you use, not your own phone.