Forum Discussion
Nishal_Rai
Cirrocumulus
CirrocumulusMitigating Stored XSS Attacks with F5 Big-IP ASM: Insights Needed
Hello Everyone,
Could someone provide insights into how F5 Big-IP ASM handles stored XSS attacks?
My understanding is that ASM primarily focuses on inspecting and enforcing XSS signature sets on incoming requests. However, stored XSS attacks involve legitimate requests but malicious scripts embedded in server responses.
While attempting to enforce ASM's XSS signature set on server responses seems impractical, I'm curious if ASM has the capability to analyze and mitigate XSS vulnerabilities within server responses.
Can anyone shed light on this aspect of F5 ASM's functionality?"
- Daniel_Wolf
MVP
Hi Nishal_Rai,
ASM compares the request or response against the attack signatures associated with your security policy. If a matching pattern is detected, ASM triggers an Attack signature detected violation, and either alarms or blocks based on the enforcement mode of your security policy.
See here: https://techdocs.f5.com/en-us/bigip-14-1-0/big-ip-asm-attack-and-bot-signatures-14-1-0/assigning-attack-signatures-to-security-policies.htmlI don't have DVWA running atm to test your case, but I'm confident this will work.
KR
Daniel 
Amine_KadimiHi,
If it is stored XSS, then it probably got stored previously following a bad request from an attacker, this request should have been blocked. If an attacker can bypass ASM to send the malicious payload, then it might be a bad signatures configuration. Can you provide more details on the scenario if I'm wrong?
- Nishal_Rai
Hi Daniel & Amine,
Actually, I've been testing stored XSS payloads on DVWA, bypassing F5 ASM, and requesting via F5 AWAF. Like trying to create a scenario when the client is unaware that there application has been affected by the stored xss payload.
Wondering if F5 ASM can detect and block responses containing these payloads from affected servers. Any insights?- Amine_Kadimi
I suggest you enable signature enforcement for server responses. In a lab environment it should be OK but in production it might impact the performance of the app
- Daniel_Wolf
Yes, that's what I said. Signatures can be enforced also on responses.
- Daniel_Wolf
Just had the time to look into this. Only found one signature that is tagged XSS and is applied to responses.
You might miss a stored XSS.
- zamroni777
Nacreous
you use dvwa as example so i presume that it was not production environment.
if the situation happens in production, i suggest fix the root causes with the help of ASM log:
a. enable response filtering in ASM and enable log of it.
b. use the ASM log to help app developer pinpoint the root source in application DB and clean it.- Nishal_Rai
Hi zamroni777
Thanks for the idea and yes it is not in production environment. I am just trying to create hypothetical environment, where we somehow missed to address the stored XSS attack on the existing service and latter implemented f5 waf.
And regarding the response logging filtering, if the f5 asm cannot detect the payload in the response via the available signature sets then those logs will not be (blocked/illegal) flagged depending on the security enforcement mode and we would end up in the same position of being unknown about the existing stored XSS attack in the service.
