CVE-2025-6543
CVE-2025-6543 is a security flaw in Citrix NetScaler ADC and Gateway, the gateway companies use to let staff reach internal apps from outside. Attackers can corrupt memory in Citrix NetScaler ADC and Gateway, which they use to take it over. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 30 Jun 2025.[1]
Citrix NetScaler ADC and Gateway 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 47.46, 59.19 and 37.236.
Description
CISA calls it a buffer overflow in Citrix NetScaler ADC and Gateway. In its words: “Citrix NetScaler ADC and Gateway contain a buffer overflow vulnerability leading to unintended control flow and Denial of Service. NetScaler must be configured as Gateway (VPN virtual server, ICA Proxy, CVPN, RDP Proxy) OR AAA virtual server.”[1]
The underlying weakness is memory buffer error (CWE-119), 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 lists these fixed versions: in each line, everything before the version shown is affected.[2]
| Product | Fixed in |
|---|---|
| ADC | 47.46 |
| ADC | 59.19 |
| ADC | 37.236 |
| Gateway | 47.46 |
| Gateway | 59.19 |
| Gateway | 37.236 |
Most families won’t have Citrix NetScaler ADC and Gateway; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 30 Jun 2025, 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 21 Jul 2025 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 10%. That’s in the top 5% 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
Citrix 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.Citrix NetScaler ADC and Gateway 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 47.46, 59.19 and 37.236), or not in use. CISA gave US government agencies until 21 Jul 2025 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 10%.[4] |
See also
Other Citrix NetScaler ADC and Gateway flaws
- CVE-2025-5777: out-of-bounds read ·
Other Citrix flaws
- CVE-2026-88779, Citrix NetScaler: improper restriction of operations within the bounds of a memory buffer ·
- CVE-2026-88772, Citrix NetScaler: improper restriction of operations within the bounds of a memory buffer ·
- CVE-2026-88771, Citrix NetScaler: improper input validation ·
- CVE-2026-19490, Citrix NetScaler: authentication bypass using an alternate path or channel ·
- CVE-2026-8452, Citrix NetScaler ADC and NetScaler Gateway: improper restriction of operations within the bounds of a memory buffer ·
Other memory buffer error flaws
- CVE-2009-3459, Adobe Acrobat and Reader: heap-based buffer overflow ·
- CVE-2025-31277, Apple devices: buffer overflow ·
- CVE-2026-3910, Chrome and other Chromium browsers: improper restriction of operations within the bounds of a memory buffer ·
- CVE-2008-0015, Microsoft Windows: video activeX control remote code execution ·
- CVE-2026-20700, Apple devices: multiple buffer overflow ·
Every Citrix flaw on CISA’s list · Everything added in June 2025
References
- ^ Cybersecurity and Infrastructure Security Agency. “Citrix NetScaler ADC and Gateway Buffer Overflow Vulnerability”. Known Exploited Vulnerabilities Catalog, added 30 Jun 2025.
- ^ National Institute of Standards and Technology. “CVE-2025-6543 Detail”. National Vulnerability Database, published 25 Jun 2025.
- ^ Citrix. Advisories and fixes: support.citrix.com, netscaler.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.104, percentile 95.7, 31 Oct 2026.
- CVE Program. “CVE-2025-6543”. 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.