SolarWinds Serv-U Unauthenticated Denial of Service Vulnerability (CVE-2026-28318)
Published Last modified In CISA KEV since Sources: BSI, pwn.watch, CVE.org, SingCERT
- Actively exploited
- Source: VulnCheck KEV (opens in a new tab), CSA/SingCERT How we classify
- Public exploit
What is affected, what to do
SolarWinds Serv-U is susceptible to specially crafted POST requests that crash the Serv-U service without authentication using Content-Encoding: deflate. Mitigation steps are provided to secure customer environments in the SolarWinds Trust Center if you are unable to deploy the update
The vulnerability is being actively exploited; treat it as a priority. Update serv-u to version 15.5.4. Immediate actions, mitigation and detection rules are in the runbook below.
- Affected
- SolarWinds Serv-U
lte *..15.5.4
- SolarWinds Serv-U
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Metrics
7.5
Priority
XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.
Source: NVD, CVSS v3
7.5
CVSS v3
Severity High on the scale from 0 to 10.
Source: BSI, pwn.watch, CVE.org, SingCERT
43.7 %
EPSS score
Elevated — model estimates 10-50% exploitation likelihood.
Source: FIRST EPSS, percentile 98.7
Show all metrics
- Severity
- Critical
Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 233.90 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- actively exploitedSource: VulnCheck KEV (opens in a new tab), CSA/SingCERT How we classify
XIEM Effective CVSS
XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.
Read in glossary- 7.5 Methodology
CVSS v3
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 7.5
EPSS score
The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.
Read in glossary- 43.7 %
- Elevated — model estimates 10-50% exploitation likelihood.
EPSS percentile
The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.
Read in glossary- 98.7
- Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- Actively exploited
- Published
In CISA KEV since
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- CISA assessment (SSVC)
- Exploitation: active · automatable · Technical impact: partial
- Source: CISA Vulnrichment, as of 5 June 2026
CWE
CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.
Read in glossary- CWE-400
Response and mitigation
Why act now?
Prioritisation rationale
The flaw is reachable from the network without authentication and purely degrades availability — a single crafted POST crashes the service, which makes exploitation trivially scriptable and repeatable; that profile is exactly why CISA added it to KEV despite the absence of confidentiality or integrity impact. For organisations that run Serv-U as a managed file-transfer component in supply chains, EDI, or regulatory reporting (common in manufacturing, finance, and energy, so firmly in NIS2/KRITIS scope), a sustained DoS has real business consequences. CISA does not flag ransomware use, and exploitation likelihood sits in the upper mid-range — treat internet-exposed instances as P1 and internally reachable systems with partner access as P2.
Runbook step 1
Immediate response (0-24 h)
- Inventory all Serv-U instances (FTP/FTPS/SFTP/HTTP/HTTPS on standard ports 21/22/80/443/990 plus the management console on TCP/443) and upgrade to version 15.5.4 HF1 or later. The vendor hotfix fixes the deflate-parser flaw; download via the SolarWinds Customer Portal and schedule the service restart in a maintenance window.
- If the hotfix cannot be deployed immediately, remove the Serv-U HTTP/HTTPS listener from the public internet and front it with a filtering reverse proxy or require VPN access. File-transfer protocols (FTPS/SFTP) can keep running as long as the HTTP management surface is no longer directly reachable.
- Set Serv-U process-availability monitoring to a check interval below 60 seconds (Windows service "Serv-U File Server" or Linux serv-u daemon). Any unexpected crash must raise a high-priority ticket because the attack is unauthenticated and trivially repeatable.
- Impose a change freeze on Serv-U configuration and take a backup of the config database and TLS certificates before patching, enabling a clean rollback if the hotfix regresses.
- Rehearse DoS incident response: review business-continuity for file-exchange workflows (EDI, supplier portals) and stage a fallback channel such as an alternative SFTP endpoint or Managed File Transfer.
Runbook step 2
Mitigation layers
- Network: place a WAF or reverse proxy (nginx, HAProxy, F5) in front of the Serv-U HTTP(S) listener that either blocks requests carrying "Content-Encoding: deflate" or decompresses them upstream and validates the ratio against Content-Length. Enforce a hard cap on request-body size.
- Segmentation: move Serv-U hosts into a dedicated DMZ VLAN with a tight egress policy that only permits documented backend destinations (AD, logging, backup). Expose the management interface exclusively through a jump host plus admin VPN.
- IPS/WAF signature: deploy a Suricata/Snort rule for POST requests against Serv-U URIs that carry "Content-Encoding: deflate" with a suspicious ratio of raw Content-Length to decompressed payload (zip-bomb pattern). Add per-source-IP rate limiting to catch variants that evade the signature.
- Host hardening: run the Serv-U service under a dedicated low-privilege account and configure a watchdog (Windows Service Recovery "Restart on Failure" for first and second failure, reset after 5 minutes; systemd unit with Restart=on-failure and StartLimitBurst) to dampen availability impact — this reduces damage but does not replace the patch.
- IAM: enforce MFA and source-IP allow-listing on Serv-U management accounts; rotate service-account credentials if the host was recently reachable without controls.
Runbook step 3
Detection rules
- Web/proxy logs: hunt for POST requests against Serv-U endpoints carrying "Content-Encoding: deflate" followed within seconds by a TCP RST or HTTP 5xx. KQL shape:
WebLogs | where RequestMethod == "POST" and RequestHeaders has "deflate" | join kind=inner (ServiceEvents | where EventType == "ServiceCrash") on Host. - Windows Event Log: Event ID 7031/7034 (Service Control Manager) for "Serv-U File Server" plus Event ID 1000 (Application Error) with the Serv-U process as faulting module, correlated with external source IPs in the preceding proxy logs.
- Linux: auditd/journald on repeated restarts of the serv-u.service unit with non-zero exit code. Sigma shape:
service_name: 'serv-u' and (status: 'failed' or exit_code: '!=0')with more than two hits in a 10-minute window. - Network: Zeek http.log for POST requests whose request_body_len is significantly smaller than the expected decompressed payload against the Serv-U listener port, with the response missing or the matching conn.log entry showing conn_state "RSTO"/"RSTR".
- SIEM correlation: wire a high-priority alert for more than three Serv-U service restarts per hour or any availability gap over 60 seconds, even in the absence of request logs — attackers may suppress or starve logging.
- Web/proxy logs: hunt for POST requests against Serv-U endpoints carrying "Content-Encoding: deflate" followed within seconds by a TCP RST or HTTP 5xx. KQL shape:
Detection rules
No NEOSEC detection rule is available for this vulnerability yet.
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
SolarWinds Serv-U is susceptible to specially crafted POST requests that crash the Serv-U service without authentication using Content-Encoding: deflate. Mitigation steps are provided to secure customer environments in the SolarWinds Trust Center if you are unable to deploy the update
Source: BSI CSAF, BSI WID Portal, pwn.watch, cvelistv5, CSA/SingCERT, NVD (NIST), CISA KEV Catalog, CVE.org (MITRE)
Affected products
The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.
| Affected versions | Fixed in | ||
|---|---|---|---|
| SolarWinds | Serv-U |
|
|
| solarwinds | serv-u |
|
|
Vendors and products
Weakness classes (CWE)
CWE-400Uncontrolled Resource Consumption
Class
The product does not properly control the allocation and maintenance of a limited resource.
Entry at MITRE CWE-400
Public exploit references
Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.
| CISA KEV Catalog | CISA Known Exploited Vulnerability (opens in a new tab)cisa.gov | actively exploited | |
| CSA/SingCERT | High-Severity Vulnerability in SolarWinds Serv-U (opens in a new tab)CSA/SingCERT | actively exploited | |
| PoC-in-GitHub (nomi-sec) | CVE-2026-28318-check (opens in a new tab)github.com | PoC (publicly reported)reported, not verified | |
| PoC-in-GitHub (nomi-sec) | servu-cve-2026-28318-poc (opens in a new tab)github.com | PoC (publicly reported)reported, not verified |
References & sources
- solarwinds.comSolarWinds Serv-U — CVE-2026-28318Vendor advisory, pwn-watch
- documentation.solarwinds.comen/success_center/servu/content/release_notes/servu_15-5-4-hotf…Release notes
- cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2026-28318government-resource, US government resource
- pwn.watchpwn.watch: SolarWinds Serv-U: SolarWinds Serv-U Uncontrolled Resource Consumption Vulnerabilitypwn-watch
- NVD (NIST)NVD: CVE-2026-28318pwn-watch
- cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch
Changes to this CVE
Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).
Record modifiedsolarwinds.com
Show 7 original changes
Individual changes of this event as published by NVD Type Action Before After Affected versions (publisher) added —Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/28xxx/CVE-2026-28318.json">CVE-2026-28318</a>Reference added —https://documentation.solarwinds.com/en/success_center/servu/content/release_notes/servu_15-5-4-hotfix-1_release_notes.htmReference added —https://www.solarwinds.com/trust-center/security-advisories/CVE-2026-28318Reference removed https://documentation.solarwinds.com/en/success_center/servu/content/release_notes/servu_15-5-4-hotfix-1_release_notes.htm—Reference removed https://www.solarwinds.com/trust-center/security-advisories/CVE-2026-28318—Reference type removed https://documentation.solarwinds.com/en/success_center/servu/content/release_notes/servu_15-5-4-hotfix-1_release_notes.htm Types: Release Notes—Reference type removed https://www.solarwinds.com/trust-center/security-advisories/CVE-2026-28318 Types: Vendor Advisory—Source identifier: psirt@solarwinds.com
Record modifiedCISA ADP
Show 3 original changes
Individual changes of this event as published by NVD Type Action Before After Reference added —https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-28318Reference removed https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-28318—Reference type removed https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-28318 Types: US Government Resource—Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0
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<a href="https://intel.neosec.eu/en/cve/CVE-2026-28318"><img src="https://intel.neosec.eu/badge/cve/CVE-2026-28318.svg?lang=en" alt="CVE-2026-28318 – NEOSEC Intel" height="20"></a>