CriticalCVE-2021-35247

    Improper Input Validation Vulnerability in Serv-U (CVE-2021-35247)

    Published In CISA KEV since Sources: CVE.org

    What is affected, what to do

    Serv-U web login screen to LDAP authentication was allowing characters that were not sufficiently sanitized. SolarWinds has updated the input mechanism to perform additional validation and sanitization.

    The vulnerability is being actively exploited; treat it as a priority. Update SolarWinds Serv-U to version 15.3. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • SolarWinds Serv-U lt 15.2.5 and previous versions..15.3
    Fixed in
    • SolarWinds Serv-U 15.3

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    5.3

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    4.3

    CVSS v3

    Severity Medium on the scale from 0 to 10.

    Source: CVE.org

    3.5 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 89.0

    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
    185.13 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 exploited
    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
    5.3 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
    4.3
    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
    3.5 %
    Moderate — model estimates 1-10% 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
    89.0
    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 · not automatable · Technical impact: partial
    Source: CISA Vulnrichment, as of 29 January 2025
    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-20

    Response and mitigation

    Why act now?

    Prioritisation rationale

    The flaw is reachable over the network without authentication, but its direct impact is limited. The vendor states that LDAP servers ignored the malformed characters, and there is no confidentiality or availability impact. CISA lists it because Serv-U is a favoured target in attacks on file-transfer platforms. Its login path has been actively probed with injection payloads, which serve as reconnaissance or as a stepping stone when chained with more severe Serv-U vulnerabilities. Exploitation likelihood is elevated, and no ransomware use is flagged. Treat internet-facing Serv-U with AD integration (common in partner-facing MFT portals in critical infrastructure and manufacturing) as high priority and patch within days. Internal-only instances without LDAP can follow the regular patch cycle.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory: Find every Serv-U instance (Windows and Linux) and check its version in the Management Console. All releases up to and including 15.2.5 are affected.
      • Patch: Upgrade to Serv-U 15.3 or later. 15.3 adds input validation and sanitisation to the web login path used for LDAP authentication. Start with instances whose web client (usually 80/443) is reachable from the internet.
      • Reduce exposure: If you cannot upgrade immediately, restrict the web client at the perimeter firewall to known partner IP ranges or VPN-only access. The Management Console should never be internet-facing.
      • Review LDAP integration: Check whether Serv-U authenticates against Active Directory or another LDAP directory (389/636).
        • If it does: identify the bind account and rotate its password if anything looks suspicious.
        • If there is no business need, disable LDAP authentication on the web login for now.
      • Preserve logs: Snapshot Serv-U domain logs and reverse-proxy/WAF logs for the last 90 days before rotation overwrites them.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Place Serv-U in a DMZ. Allow egress only to designated domain controllers or LDAP servers over LDAP/LDAPS. Deny all other outbound traffic from the host, especially LDAP, RMI or HTTP to external destinations.
      • WAF/IPS: In front of the web client, block or log login POST username and password parameters that contain LDAP metacharacters or filter constructs:
        • Metacharacters such as *, (, ), |, &, \00
        • Filter constructs such as )(, *)(, ${jndi:
        • ModSecurity example: SecRule ARGS_POST "@rx [\(\)\*\|&\\\x00]|\$\{jndi:" "id:1035247,phase:2,deny,log,msg:'Serv-U LDAP injection attempt CVE-2021-35247'"
      • IAM/least privilege: Scope the Serv-U LDAP bind account to read-only access on the required OU. Give it no privileged group memberships, and where possible restrict its logon to the Serv-U host.
      • Configuration hardening:
        • Enable Serv-U brute-force protection and IP block lists.
        • Disable unused protocols (cleartext FTP/21, HTTP without TLS).
        • Prefer MFA or key-based SFTP for external users.
      • Directory side: Enforce LDAP signing and channel binding on domain controllers. Move the Serv-U integration to LDAPS/636.

    3. Runbook step 3

      Detection rules

      • Serv-U/web logs: Flag login requests with LDAP special characters in the username.
        • Sigma shape: logsource: webserver; selection: cs-uri-stem|contains: 'Login' AND cs-username|re: '[\(\)\*\|&]|\$\{jndi:'
        • SPL: index=web host=servu* (uri="*Login*") | regex user="[\(\)\*\|&]|\$\{jndi:"
      • Domain controllers: Enable Event ID 1644 via Field Engineering diagnostics (15 Field Engineering = 5). Then hunt for LDAP filters issued by the Serv-U bind account that contain wildcards or unusual nested (|(…)) constructs.
      • Network telemetry (Zeek/Suricata): Alert on outbound LDAP (389/636/1389) or RMI connections from the Serv-U host to destinations not on the allow list.
        • KQL: DeviceNetworkEvents | where DeviceName has "servu" and RemotePort in (389,636,1389,1099) and RemoteIP !in (dc_list)
      • EDR process ancestry: Alert when Serv-U.exe or the Linux Serv-U daemon spawns cmd.exe, powershell.exe, sh or bash (Sysmon EID 1). This is abnormal and suggests follow-on compromise through other Serv-U flaws.
      • Authentication anomalies: Watch for bursts of failed web logins from one source using varying usernames with special characters, followed by a success. Sources are the Serv-U log and Windows 4625/4624 events for the bind account.

    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

    Serv-U web login screen to LDAP authentication was allowing characters that were not sufficiently sanitized. SolarWinds has updated the input mechanism to perform additional validation and sanitization. Please Note: No downstream affect has been detected as the LDAP servers ignored improper characters. To insure proper input validation is completed in all environments. SolarWinds recommends scheduling an update to the latest version of Serv-U.

    Source: NVD (NIST), CISA KEV Catalog, CVE.org (MITRE), cvelistv5

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    SolarWindsServ-U
    • 15.2.5 and previous versions – < 15.3
    • 15.3
    solarwindsserv-u
    • < 15.3
    • 15.3

    Vendors and products

    Weakness classes (CWE)

    • CWE-20Improper Input Validation

      Class

      The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.

      Entry at MITRE CWE-20

    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.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited

    References & sources

    • solarwinds.comtrust-center/security-advisories/cve-2021-35247x_refsource_MISC, Broken link, Vendor advisory
    • documentation.solarwinds.comen/success_center/servu/content/release_notes/servu_15-3_releas…x_refsource_MISC, Release notes, Vendor advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-35247government-resource, US government resource
    Embed badge

    Show the severity and KEV status of this vulnerability on your site – with a link back here. The image is updated hourly.

    Badge preview for CVE-2021-35247
    <a href="https://intel.neosec.eu/en/cve/CVE-2021-35247"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-35247.svg?lang=en" alt="CVE-2021-35247 – NEOSEC Intel" height="20"></a>