CitrixBleed 2 beckons sweeping alarm as exploits spread across the globe

The number of Citrix customers impacted by CVE-2025-5777 remains unknown, but researchers have already observed more than 11.5 million attack attempts, targeting thousands of sites.

Authorities and researchers are intensifying warnings about active exploitation and pervasive scanning of a critical vulnerability affecting multiple versions of Citrix NetScaler products.

There is now widespread agreement among security professionals that the critical vulnerability, CVE-2025-5777, which Citrix disclosed June 17, is serious and harkens back to a 2023 defect in the same products: “CitrixBleed,” or CVE-2023-4966. Naturally, threat hunters are scrambling to assess and stop the strikingly similar challenges summoned by exploits of the newest CVE. 

For some Citrix customers, the warnings are too late. Vulnerability scans confirm active exploits occurred within a week of disclosure, and attackers have been swarming, hunting for exposed instances of the impacted devices since exploit details were publicly released earlier this month. 

“This vulnerability in Citrix NetScaler ADC and Gateway systems, also referred to as CitrixBleed 2, poses a significant, unacceptable risk to the security of the federal civilian enterprise,” Chris Butera, acting executive assistant director for cybersecurity at the Cybersecurity and Infrastructure Security Agency, said in a statement. CISA added the exploit to its known exploited vulnerabilities catalog on July 10.

“As America’s cyber defense agency and the operational lead for federal civilian cybersecurity, CISA is taking urgent action by directing agencies to patch within 24 hours and we encourage all organizations to patch right away,” Butera added. The agency typically requires agencies to resolve “high risk” vulnerabilities within 30 days and “critical risk” vulnerabilities within 15 days.

The pre-authentication remote memory disclosure vulnerability, which has a CVSS score of 9.3, has been increasingly targeted for attacks globally. Imperva researchers on Friday said they’ve observed more than 11.5 million attack attempts targeting thousands of sites since the exploit was disclosed. 

“Attackers appear to be scanning extensively for exposed instances and attempting to exploit the memory-leak vulnerability to harvest sensitive data,” Imperva researchers said in a blog post.

Nearly 2 in 5 attack attempts have targeted sites in the financial services industry and 3 in 5 of those targeted sites are based in the United States, according to Imperva.

GreyNoise scans have observed 22 unique malicious IPs attempting to exploit CVE-2025-5777 thus far. The first malicious IP was observed June 23 and a spike of 11 unique malicious IPs was observed Friday. 

“I haven’t seen any attrition yet. This could be as bad or even worse than CitrixBleed,” Dustin Childs, head of threat awareness at Trend Micro’s Zero Day Initiative, told CyberScoop. “The attack is very repeatable and those systems rarely have network monitoring. They also aren’t regularly updated, so patching them may be an issue.”

The number of Citrix customers already impacted remains unknown and victims have yet to come forward. 

“A lot of the attacks seem opportunistic, so there are likely multiple threat actors using the bug,” Childs said.

Citrix maintains there was no evidence of active exploitation when it disclosed the vulnerability. The vendor hasn’t shared much publicly in almost three weeks, other than an update in a June 26 blog post noting that CISA was aware of evidence of active exploitation. The company did not respond to a request for comment.

In the June blog post, Anil Shetty, senior vice president of engineering at NetScaler, disputed comparisons between CVE-2025-5777 and CVE-2023-4966. “While the vulnerabilities share some characteristics, Cloud Software Group has found no evidence to indicate that they are related,” Shetty wrote. Cloud Software Group is the parent company of Citrix.

Researchers are also leveling criticism at Citrix for the relative ease by which an attacker can compromise a vulnerable instance of Citrix NetScaler with just a few requests. 

‘“The term “CitrixBleed’ is used because the memory leak can be triggered repeatedly by sending the same payload, with each attempt leaking a new chunk of stack memory — effectively bleeding sensitive information,” Akamai Security Intelligence Group said in a blog post.

Akamai researchers described the root cause of the vulnerability as “an uninitialized login variable, combined with improper memory handling, lack of input validation and missing error handling in Citrix NetScaler’s authentication logic.”

Zach Edwards, an independent cybersecurity researcher, told CyberScoop that CVE-2025-5777 and CVE-2023-4966 are “extremely similar,” aside from subtle differences in the versions of NetScaler impacted.

“The fact that these pre-authentication vulnerabilities keep coming up, which can facilitate complete compromises, is disappointing to see,” Edwards said. “It’s unclear how these significant vulnerabilities keep making their way through development processes, but Citrix clients, especially in the government and enterprise sectors, should be demanding more and requiring additional public context about the steps Citrix takes to test its software prior to a release.”

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