CVE-2025-53521
CVE-2025-53521 is a security flaw in F5 BIG-IP, equipment big companies use to route traffic to their websites and apps. Attackers can run their own code on F5 BIG-IP. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 27 Mar 2026.[1]
F5 BIG-IP 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 17.5.1.3, 17.1.3, 16.1.6.1 and 15.1.10.8 of BIG-IP.
Description
CISA calls it a stack-based buffer overflow in F5 BIG-IP. In its words: “F5 BIG-IP APM contains a stack-based buffer overflow vulnerability that could allow a threat actor to achieve remote code execution.”[1]
The CVE record adds: “When a BIG-IP APM access policy is configured on a virtual server, specific malicious traffic can lead to Remote Code Execution (RCE). Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.”[2]
The underlying weakness is stack buffer overflow (CWE-121), in MITRE’s list of common software weaknesses.
It scores 9.8 out of 10 on the Common Vulnerability Scoring System, which rates it critical, scored by the organisation that published the CVE using CVSS 3.1.[2]
Affected versions
NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]
| Product | Fixed in |
|---|---|
| BIG-IP | 17.5.1.3 |
| BIG-IP | 17.1.3 |
| BIG-IP | 16.1.6.1 |
| BIG-IP | 15.1.10.8 |
Most families won’t have F5 BIG-IP; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 27 Mar 2026, 163 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 30 Mar 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 2%. That’s in the top 17% of the flaws it scores.[4]
CISA assesses it as automatable: attackers can run it against many targets at once.
CISA doesn’t say who was attacked, or how.
Fix
F5 has published a fix or advice.[3] CISA’s required action: “Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.”[1]
- Check whether you use it.F5 BIG-IP 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 17.5.1.3, 17.1.3, 16.1.6.1 and 15.1.10.8 of BIG-IP), or not in use. CISA gave US government agencies until 30 Mar 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 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 2%.[4] |
See also
Other F5 BIG-IP flaws
- CVE-2022-1388: missing authentication ·
- CVE-2020-5902: traffic management user interface (TMUI) remote code execution ·
Other F5 flaws
- CVE-2026-94127, F5 BIG-IP APM: heap-based buffer overflow ·
- CVE-2023-46748, F5 BIG-IP Configuration Utility: SQL injection ·
- CVE-2023-46747, F5 BIG-IP Configuration Utility: authentication bypass ·
- CVE-2021-22991, F5 BIG-IP Traffic Management Microkernel: buffer overflow ·
- CVE-2021-22986, F5 BIG-IP and BIG-IQ Centralized Management: icontrol REST remote code execution ·
Other stack buffer overflow flaws
- CVE-2026-7273, Zyxel GS1900 Series Switches: stack-based buffer overflow ·
- CVE-2021-27137, DD-WRT: 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 ·
Every F5 flaw on CISA’s list · Everything added in March 2026
References
- ^ Cybersecurity and Infrastructure Security Agency. “F5 BIG-IP Stack-Based Buffer Overflow Vulnerability”. Known Exploited Vulnerabilities Catalog, added 27 Mar 2026.
- ^ National Institute of Standards and Technology. “CVE-2025-53521 Detail”. National Vulnerability Database, published 15 Oct 2025.
- ^ F5. Advisories and fixes: my.f5.com, my.f5.com, my.f5.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.023, percentile 83.3, 31 Oct 2026.
- CVE Program. “CVE-2025-53521”. 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.