Attackers remained undetected for more than three weeks as they exploited a critical zero-day vulnerability affecting Citrix NetScaler appliances en masse. 

The earliest known instance of CVE-2026-88772 exploitation occurred Sept. 3, Mandiant researchers told CyberScoop Tuesday. 

The besieged security vendor and researchers didn’t confirm the attacks until late last week. By then, Mandiant says, organizations in North America and Europe spanning the government, financial services, education, telecom, legal and professional services sectors were already likely compromised.

“We are aware of dozens of impacted organizations,” Charles Carmakal, chief technology officer at Mandiant Consulting, wrote in a LinkedIn post. He attributed the attacks to “advanced and suspected state-sponsored threat actors.”

The three-week gap — at minimum — between initial exploitation and confirmed in-the-wild attacks gave attackers a significant advantage. 

Mandiant warned that the gap could be even wider. “We are still responding to active intrusions, and new evidence may change our understanding of the campaign timeline,” researchers who published a threat intelligence report on the attacks Tuesday told CyberScoop in an email. 

The incident response firm’s analysis on the latest zero-day attack spree targeting Citrix customers underscores the multi-layered mess network defenders have been responding to since Saturday.

The zero-day exploits in Mandiant’s report only cover half of the problem. Attackers have also exploited a second Citrix NetScaler zero-day — CVE-2026-88771 — since at least Sept. 24, according to GreyNoise, but researchers said that campaign likely started earlier as well.

It’s unclear to what extent the pair of zero-days are linked. But, nearly two days after the first unconfirmed rumors of the attacks surfaced, Citrix disclosed both of the actively exploited defects in a security advisory Sunday, releasing patches for them and six additional vulnerabilities.

Mandiant researchers uncovered multiple novel tools and tactics attackers used to exploit CVE-2026-88772, gain privileged access to compromised environments, hop around the network and steal sensitive data. A threat actor in one observed intrusion routed traffic through novel tunneler malware to “manually conduct internal reconnaissance and credential theft,” researchers wrote. 

Researchers at watchTowr also published technical analysis of CVE-2026-88782 Tuesday.

The attacks from multiple fronts, involving two zero-days, reflect an alarming and sustained pattern of malicious activity targeting so-called edge devices, such as virtual private network gateways and firewalls. 

Vulnerabilities in these devices accounted for 48% of the enterprise-related zero-days last year, according to Google Threat Intelligence Group.

“Our previous research corroborates that both cyber espionage and financially motivated threat actors prioritize exploiting vulnerabilities in edge devices and security appliances,” Mandiant researchers wrote in response to questions on their report. 

“Because most edge devices do not support endpoint detection and response (EDR) monitoring, targeting them, particularly through exploiting zero-day vulnerabilities, provides threat actors with an infection vector that is difficult to detect and prevent, and the opportunity to scale a campaign as long as the exploit remains undiscovered,” the researchers added. 

Carmakal, in his LinkedIn post, warned that Mandiant expects “broad and opportunistic exploitation” of both of the Citrix NetScaler zero-days by a variety of threat actors in the near term.

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Citrix took most of the weekend to confirm that attackers were actively exploiting the newest round of NetScaler zero-days, leaving network defenders and threat hunters to react without official confirmation. 

Yet, behind the scenes, multiple CERTs, advisory firms, insurance providers, researchers and chatty security professionals warned their respective peers and communities of a live and serious threat. 

Nearly two days after the first unconfirmed rumors, Citrix published a security advisory Sunday disclosing the exploited zero-days, CVE-2026-88771 and CVE-2026-88772, releasing patches for them and six additional defects.

By then, criticism was at a fever pitch, and for some NetScaler customers the warning came too late. 

“The information vacuum was most striking. Customers were receiving warnings through unofficial channels while Citrix remained publicly silent,” Ben Harris, founder and CEO at watchTowr, told CyberScoop. 

“Customers were left without communication, guidance, or even acknowledgement that the vulnerability that was rumored to exist was real. Citrix could have warned customers that active exploitation was occurring and provided immediate defensive guidance without disclosing technical details that would help attackers,” he added. 

“When active exploitation is underway, hours matter,” Harris said.

Citrix did not directly answer questions about the lengthy communication delay. 

“We recently identified critical vulnerabilities in Citrix NetScaler ADC and Citrix NetScaler Gateway that led us to immediately develop and release a new version of the software that addresses the issues,” the company said in a prepared statement.

The critical defects are both rated 9.5 on the CVSS scale and allow attackers to achieve remote code execution. 

Researchers are particularly concerned about CVE-2026-88771, a command-injection vulnerability that affects all NetScaler appliances in a default configuration. That gives attackers a broad pool of targets and a better chance at successful exploitation. A proof-of-concept exploit for the vulnerability is also publicly available. 

Palo Alto Networks said it identified more than 50,000 publicly exposed instances of Citrix NetScaler devices potentially vulnerable to both of the zero-days as of Sunday. 

The earliest known exploitation attempt occurred Sept. 24 when GreyNoise said it observed an unsuccessful exploitation attempt against a Citrix NetScaler Gateway under its control for malicious activity scanning purposes. But researchers warn that exploitation likely started before then.

Citrix did not say how many customers have been compromised thus far and researchers are still assessing the fallout.

Citrix said it is making generic indicators of compromise available to customers in their user consoles, and the company referred to previously issued guidance for customers who suspect compromise. 

Attribution for the attacks is also still under investigation, yet NetScaler products are a common, recurring target for financially-motivated cybercriminals and state-sponsored espionage groups. 

The Cybersecurity and Infrastructure Security Agency issued an alert Sunday after Citrix publicly confirmed the attacks on its customers, and added CVE-2026-88771 and CVE-2026-88772 to its known exploited vulnerabilities catalog. The vendor has appeared on the agency’s list of vulnerabilities known to be exploited five times this year, and a total of 26 times since late 2021.

Security leaders across the industry — including Charles Carmakal, chief technology officer at Mandiant Consulting, and Wendi Whitmore, chief security intelligence officer at Palo Alto Networks — also sprung into action Sunday, with direct warnings on LinkedIn about the urgent threat. 

The information vacuum that persisted through most of the weekend put cybersecurity professionals in sometimes unique positions. Joe Toomey, vice president of underwriting security at insurance provider Coalition, said the company notified potentially vulnerable customers with limited official data and no confirmation from CISA or Citrix.

In a LinkedIn post Sunday, he called Citrix “unconscionably irresponsible” for remaining silent for more than 36 hours after word began circulating about in-the-wild exploitation.

“I fully understand a vendor embargoing specific details about a vulnerability when a patch has not yet been published, but once the vuln is exploited in the wild, some of those rules no longer apply,” Toomey wrote.

Citrix and its customers are in familiar territory. The vendor’s products have been widely targeted in previous attack sprees, often involving zero-day exploits and some infamously referred to as CitrixBleed and CitrixBleed 2.

“Citrix has a history of delaying the publication of vulnerabilities, even when they’re being exploited in the wild and affecting customers. Citrix eventually released patches and an advisory, but an adequate response also requires timely, clear communication,” Harris said. 

“Customers shouldn’t have to rely on unofficial warnings, security researchers, and third parties to learn that critical infrastructure may already be compromised,” he added. “This latest incident raises serious questions about whether Citrix has learned from previous crises and meaningfully improved how it protects and informs customers.”

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SonicWall customers are grappling with yet another pair of actively exploited zero-day vulnerabilities in SonicWall SMA 1000 appliances, a product that’s been besieged with recurring defects and attacks over the past nine months. 

The vendor disclosed and released patches for the defects — CVE-2026-83548 and CVE-2026-83549 — and noted both were already actively exploited in the wild in a security advisory Tuesday. The Cybersecurity and Infrastructure Security Agency added the defects to its known exploited vulnerabilities (KEV) catalog Wednesday. 

SonicWall customers have confronted a barrage of actively exploited vulnerabilities in SonicWall devices for years. Attackers have consistently exploited newly discovered zero-days and years-old defects in the vendor’s products to break into victim environments.

Rapid7 researchers said the new zero-days — a max-severity pre-authentication server-side request forgery vulnerability and a high-severity OS command injection vulnerability — can be chained together to achieve unauthenticated remote-code execution. 

SonicWall did not say how many customers have been directly impacted by active exploitation or when the first known instance of exploitation occurred. The company did not respond to a request for comment.

“Please stop us if you’ve heard this one before: Another appliance sitting at the edge of the network, another pair of vulnerabilities chained together, and another unauthenticated path to complete compromise,” Jake Knott, head of threat intelligence at watchTowr, said in an email. 

“SonicWall says these vulnerabilities were internally discovered, while also saying it investigated a case indicating active exploitation. Please pick one, or, at minimum, explain how both are true,” Knott added. “Those statements may be technically accurate, but without that context, the disclosure leaves defenders guessing about when and how the vulnerabilities were actually identified.”

The vendor’s security advisory did not include indicators of compromise. It urged customers to contact tech support for assistance in reviewing IOCs and hunting for potential signs of compromise, and if detected, to reimage or redeploy the appliance, change all user and administrator passwords and reset tokens. 

SonicWall did not attribute the known exploits to a specific threat group or describe the attacker’s motivations.

The freshly disclosed pair of vulnerabilities are the latest in a series of security issues confronting the vendor’s customers, including actively exploited zero-days, previously disclosed defects, and an attack last year that allowed a state-sponsored threat group to steal the firewall configurations of every SonicWall customer. 

In late July, Huntress researchers spotted an attack spree that compromised 30 SonicWall customers in less than two days. Earlier that month, the company acknowledged another pair of zero-days that were exploited for three weeks before the vendor disclosed and patched the defects.

Ransomware groups, including INC ransomware and Akira, have taken a special interest in SonicWall. Ten of the 19 SonicWall defects added to CISA’s KEV catalog since late 2021 are known to be used in ransomware campaigns.

The five defects added to CISA’s KEV most recently, since just mid-December 2025, all impact SonicWall SMA 1000 appliances.

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Huntress researchers spotted an active and ongoing series of attacks targeting SonicWall VPN and firewall accounts, which compromised 30 organizations in less than two days, the company said in a threat advisory Tuesday.

The credential stuffing campaign started Saturday and grew rapidly, ultimately compromising 92 unique user accounts during the next 41 hours, according to Huntress. Researchers said the attacks were broad and opportunistic, hitting various SonicWall devices, rather than targeting specific types of organizations.

SonicWall hasn’t released a security advisory about the malicious activity as of press time. A spokesperson told CyberScoop the company is still investigating and hopes to have more information soon.

The attacks ended — at least for now — as abruptly as they began. The last compromise occurred Monday, according to Michael Tigges, principal tactical response analyst at Huntress.

“This fits campaign trends,” he said. “A rash of compromise will break out, followed by silence until the adversary rotates infrastructure.”

Attackers, which haven’t been identified, have also refrained from initiating any post-compromise activity, indicating the intrusions could be pre-positioning for future attacks. 

“With local network access, the sky is essentially the limit for most networks that do not have proper topology controls in place,” Tigges said.

Huntress’ observations are limited to telemetry it collects from its customers, meaning all of the identified victims were Huntress customers using SonicWall devices, so the number of organizations impacted could be greater. 

Researchers haven’t identified a root cause for the attacks, noting that they begin with authorized logins. Attackers are validating credentials against remote access portals to compromise as many vulnerable accounts as possible, the cybersecurity vendor and threat intelligence firm said. 

“This could be an aggregation of stealer malware logs, previously compromised SonicWall configuration files, or historic CVE compromise that resulted in more credentials than the adversary could use at the time,” Tigges said. 

In 2025, an undisclosed state-sponsored threat actor intruded SonicWalls’s cloud environment and stole firewall configurations of every customer. 

SonicWall customers have also been hit by a barrage of actively exploited zero-days, including a pair of zero-days that were exploited for three weeks before the vendor disclosed and patched the defects earlier this month, and previously disclosed defects in SonicWall devices for years. 

Seventeen defects affecting the vendor’s products have been added to CISA’s known exploited vulnerabilities catalog since late 2021. Ten of those defects are known to be used in ransomware campaigns, according to CISA, including a wave of about 40 Akira ransomware attacks between mid-July and early August 2025.

“Edge devices are one of the most targeted interfaces, comprising over 70% of active intrusions triaged by Huntress, including the overwhelming majority of ransomware deployments,” Tigges said. “Organizations that do not spend significant time architecting secure remote access solutions and networks that are resilient to edge-device compromise will likely continue to feel the burn in the coming months and years.”

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Russian state-sponsored hackers are breaking into critical infrastructure around the world by exploiting poorly configured and vulnerable networking devices, authorities from the United States and 12 additional countries said in a joint cybersecurity advisory Monday.

Officials once again urged defenders to take more preventative measures to thwart attacks from the Russian Federal Security Service Center 16, which has been actively targeting critical infrastructure for more than a decade. The hackers are also tracked as Berserk Bear, Energetic Bear, Crouching Yeti, Dragonfly, Ghost Blizzard and Static Tundra.

“This is an ongoing issue that has impacted various U.S. and foreign networks across multiple sectors, including the defense industrial base, communications, energy, financial services, government facilities and health care sectors,” the National Security Agency said in a statement.

The state-sponsored attackers scan the internet for vulnerable routers using default or weak passwords, and have also exploited vulnerabilities in Cisco devices, Cisco’s Smart Install feature and web portals to take over network devices. 

Two of the Cisco vulnerabilities exploited by the Russian FSB Center 16 hackers are quite old, including CVE-2008-4128 and CVE-2018-0171. 

Officials shared technical details of the threat group’s activities and advised network defenders to disable Cisco Smart Install on all devices, use stronger modes of authentication and passwords, monitor for unusual credentials and logins using local accounts. 

The joint advisory comes nearly a year after the FBI issued a similar alert about the same group targeting end-of-life networking devices running Cisco Smart Install. 

On Monday, the European Union blamed Russia’s FSB Center 16 for a December 2025 attack on Poland’s energy grid. The United Kingdom, also on Monday, sanctioned 24 individuals and entities allegedly involved in various attacks attributed to Russian intelligence services.

“From directing criminals to targeting businesses, and striking Poland’s energy grid in the depths of winter, the Russian state is sinking to new lows in its attempts to undermine European security,” Yvette Cooper, foreign security of the United Kingdom, said in a statement. 

Other countries behind the joint cybersecurity advisory include: Canada, Australia, New Zealand, Czech Republic, Denmark, Estonia, Finland, France, Italy, Poland and Sweden.

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