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CVE-2026-8711: F5 Njs Heap Buffer Overflow (CVSS 9.2)

CVE-2026-8711 is a critical severity heap buffer overflow in F5 Njs with a CVSS score of 9.2. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.

Last updated: Jul 23, 2026

Quick answer

Direct answer

CVE-2026-8711 is a critical severity heap buffer overflow in F5 Njs with a CVSS score of 9.2. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.

Risk scores

CVSS base score
9.2CRITICAL
EPSS probability
0.79%
Higher than 55 percent of all CVEs
CISA KEV
Not listed
CVSS metricWhat it means
Attack vector (AV:N)Network: reachable remotely
Attack complexity (AC:H)High complexity
Attack requirements (AT:P)Attack requirements present
Privileges required (PR:N)No privileges required
User interaction (UI:N)No user interaction
Confidentiality (VC:H)High confidentiality impact
Integrity (VI:H)High integrity impact
Availability (VA:H)High availability impact

CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

About F5 Njs

CVE-2026-8711 affects F5 Njs, a product from F5. Before acting, confirm where F5 Njs runs in your estate, who owns it and whether it is reachable from the internet, because exposure decides real risk more than the score alone.

What the flaw is

In plain terms: NGINX JavaScript has a vulnerability when the js_fetch_proxy directive is configured with at least one client-controlled NGINX variable (for example, $http_*, $arg_*, $cookie_*) and a location invoking the ngx.fetch() operation from NGINX JavaScript. An unauthenticated attacker can exploit this vulnerability by sending crafted HTTP requests. This may cause a heap buffer overflow in the NGINX worker process leading to a restart. Additionally, attackers can execute code on systems with Address Space Layout Randomization (ASLR) disabled or when the attacker can bypass ASLR. Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.

The weakness class is Heap Buffer Overflow. A write past the end of a heap buffer corrupts nearby memory.

Based on its CVSS vector, the flaw can be reached over the network, needs no login, and needs no user interaction. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.

Affected versions and timeline

FieldDetail
ProductF5 Njs
Affected versionsNjs from 0.9.4 before 0.9.9
PublishedMay 19, 2026
Last modifiedJul 23, 2026
Added to CISA KEVNot listed
Assigning CNAf5sirt@f5.com

How to detect it

  • Inventory every instance of F5 Njs and record the running version against the affected range above.
  • Check external attack surface scans for internet facing instances of the component.
  • Review logs on affected systems for unusual requests, errors or new processes around the vulnerable function.
  • Confirm with an authenticated vulnerability scan once your scanner has a check for this CVE.

How to fix it

  • Apply the fixed version of F5 Njs named in the vendor advisory.
  • If patching must wait, restrict network access to the affected service and put it behind authentication or a web application firewall rule.
  • Rescan after the change to prove the exposure is gone, then close the ticket.

CTEM action plan

CTEM stageWhat to do for this CVE
ScopeAdd every business service that depends on F5 Njs to the current CTEM cycle.
DiscoverFind all instances, including shadow and cloud deployments, and record versions and exposure.
PrioritizeRank using EPSS and CVSS together with asset exposure and business impact.
ValidateConfirm the flaw is reachable and exploitable in your environment and that compensating controls work.
MobilizeAssign the fix to the asset owner with a due date matched to the priority, then verify closure.

CWE References

Technical References

Frequently asked questions

CVE-2026-8711 is a critical severity heap buffer overflow in F5 Njs with a CVSS score of 9.2. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.

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