CVE-2021-27137
CVE-2021-27137 is a security flaw in DD-WRT. Attackers can run their own code on DD-WRT 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 Jul 2026.[1]
DD-WRT 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 45724 of DD-WRT.
Description
CISA calls it a stack-based buffer overflow in DD-WRT DD-WRT. In its words: “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.”[1]
The CVE record adds: “An issue was discovered in router/upnp/src/ssdp.c in DD-WRT before 45724. An unsafe strcpy in the UPnP handling functionality allows an unauthenticated remote attacker to send a request that would overflow an internal fixed buffer. Exploitation requires the DD-WRT user to enable UPnP (which is off by default, and only listens on internal interfaces by default). This occurs in ssdp_msearch (reachable by an M-SEARCH request).”[2]
The underlying weakness is stack buffer overflow (CWE-121), in MITRE’s list of common software weaknesses.
It scores 8.1 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 |
|---|---|
| DD-WRT | 45724 |
Most families won’t have DD-WRT; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 21 Jul 2026, 5 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 Jul 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 4%. That’s in the top 10% 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
DD-WRT 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.DD-WRT 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 45724 of DD-WRT), or not in use. CISA gave US government agencies until 24 Jul 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 8.1.[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 4%.[4] |
See also
Other stack buffer overflow flaws
- CVE-2026-7273, Zyxel GS1900 Series Switches: stack-based buffer overflow ·
- CVE-2025-53521, F5 BIG-IP: stack-based buffer overflow ·
- CVE-2025-20352, Cisco IOS and IOS XE: software SNMP denial of service and remote code execution ·
- CVE-2025-42599, Qualitia Active! Mail: stack-based buffer overflow ·
- CVE-2025-22457, Ivanti Connect Secure, Policy Secure, and ZTA Gateways: stack-based buffer overflow ·
Added the same day
- CVE-2026-63030, WordPress Core: interpretation conflict ·
- CVE-2026-60137, WordPress Core: SQL injection ·
- CVE-2026-0770, Langflow: inclusion of functionality from untrusted control sphere ·
References
- ^ Cybersecurity and Infrastructure Security Agency. “DD-WRT Stack-Based Buffer Overflow Vulnerability”. Known Exploited Vulnerabilities Catalog, added 21 Jul 2026.
- ^ National Institute of Standards and Technology. “CVE-2021-27137 Detail”. National Vulnerability Database, published 16 Jul 2026.
- ^ DD-WRT. Advisories and fixes: svn.dd-wrt.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.040, percentile 90.5, 31 Oct 2026.
- CVE Program. “CVE-2021-27137”. 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.