Pre-auth code execution via uninitialized cipher context
Published Jul 14, 2021 · Updated Oct 21, 2025
Remote code execution in SolarWinds Serv-U before 15.2.3 HF2 allows remote attackers to run privileged code through the SSH service. The SSH KEXINIT and NEWKEYS handlers activate an AES128-CTR cipher context before key material initializes it, so later packet decryption calls an attacker-controlled function pointer. An unauthenticated malformed SSH handshake reaches the flaw when an SSH listener is enabled; Windows hosts can be fully compromised.
Summary
What happened
Remote code execution in SolarWinds Serv-U before 15.2.3 HF2 allows remote attackers to run privileged code through the SSH service. The SSH KEXINIT and NEWKEYS handlers activate an AES128-CTR cipher context before key material initializes it, so later packet decryption calls an attacker-controlled function pointer. An unauthenticated malformed SSH handshake reaches the flaw when an SSH listener is enabled; Windows hosts can be fully compromised.
The record
- CVE
- CVE-2021-35211
- Published
- Jul 14, 2021
- Updated
- Oct 21, 2025
- Vendor
- SolarWinds
- Product
- Serv-U
- Classifications
- CWE-787, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jul 14, 2021CVE publishedPublication date reported by the CVE source.
- Nov 3, 2021Exploitation reportedCISA Known Exploited Vulnerabilities entry
- Oct 21, 2025Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
Compare your product and version with the public record. A matching version still requires validation against your environment.
Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=SolarWinds Serv-U <15.2.3 HF1
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Public exploit references
- Microsoft Serv-U SSH control-flow proof of conceptproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Reported exploitation
- DEV-0322 targeted Serv-U campaignthreat actor
- Ransomware campaign use recorded by CISAransomware
- CISA Known Exploited Vulnerabilities entryknown exploited catalog
Technologies
Your stack
See the directory against your own environment.
Your stack
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