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🔬 How CVE-2026-28318 Works
CVE-2026-28318 is an uncontrolled resource consumption vulnerability (CWE-400) affecting SolarWinds Serv-U, a multi‑protocol file server. The flaw resides in how the service handles HTTP POST requests that include the `Content-Encoding: deflate` header. This header indicates that the request body is compressed using the DEFLATE algorithm.
By sending a specially crafted, highly compressed payload in the body of a POST request, an attacker can trigger a decompression bomb (also known as a “zip bomb” effect). When Serv-U attempts to decompress this payload, it consumes an excessive amount of system resources (CPU, memory, or disk I/O) without any bounds or limits. This uncontrolled resource exhaustion causes the Serv‑U service to crash, leading to a denial‑of‑service (DoS) condition.
Notably, the attack requires no authentication; the vulnerable endpoint is exposed to any unauthenticated network user. The only precondition is that the targeted Serv‑U instance is reachable over the network (attack vector: NETWORK) and that it accepts POST requests.
The vulnerability has a CVSSv3.1 base score of 7.5 (HIGH), with the vector string AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. The high availability impact (A:H) reflects the complete crash of the service, rendering the file server unavailable to legitimate users.
CISA added this flaw to its Known Exploited Vulnerabilities (KEV) catalog on June 5, 2026, citing evidence of active exploitation in the wild. The vulnerability was fixed in Serv‑U version 15.5.4 Hotfix 1.
🧾 DailyCVE Form
Platform: SolarWinds Serv-U
Version: 15.5.4 & older
Vulnerability: Uncontrolled resource consumption (CWE-400)
Severity: 7.5 HIGH
Date: 2026-06-04
Prediction: Patch by 2026-06-19
📊 What Undercode Say
1. Detect vulnerable version
curl -s -k -I https://<target>:<port>/ | grep -i "Server: Serv-U"
2. Simulate a malformed POST request with Content-Encoding: deflate
curl -X POST https://<target>:<port>/vulnerable/endpoint \
-H "Content-Encoding: deflate" \
--data-binary @deflate_bomb.bin
3. Check service status after attack (Windows)
sc query "Serv-U" | findstr "STATE"
4. Restart the service after attack (requires admin)
net stop "Serv-U" && net start "Serv-U"
5. Monitor for repeated crashes via Event Log (Windows)
Get-WinEvent -LogName System | Where-Object { $_.Message -like "Serv-Ucrash" }
6. Block malformed requests using a WAF rule (example for ModSecurity)
SecRule REQUEST_HEADERS:Content-Encoding "deflate" \
"id:10001,phase:1,deny,status:403,msg:'Blocked Content-Encoding deflate'"
💥 Exploit
An unauthenticated attacker sends a POST request to any listening Serv-U port (e.g., 80, 443) with the header `Content-Encoding: deflate` and a compressed payload designed to amplify resource consumption. The decompression process exhausts available memory or CPU, causing the Serv-U service to crash immediately. No special privileges or prior access are required; the attack is trivial to execute once the vulnerable endpoint is identified.
🛡️ Protection
- Patch immediately – Upgrade to SolarWinds Serv-U 15.5.4 Hotfix 1 (the only complete fix).
- Block the `Content-Encoding` header in WAF, load balancer, or firewall, as the Serv-U service does not legitimately need this functionality.
- Restrict network access to the Serv-U instance to trusted IP addresses only.
- Monitor for repeated crashes and alert on suspicious patterns of POST requests with compression headers.
- Implement rate limiting on the affected endpoints to reduce the impact of repeated exploit attempts.
⚠️ Impact
- Service availability – The Serv-U service crashes, denying access to all file transfer functions (FTP, SFTP, HTTP/S).
- Business disruption – Dependent applications and workflows that rely on file transfers are blocked, potentially halting critical operations.
- Repetitive exploitation – Since no authentication is required, an attacker can re‑trigger the crash as many times as desired, keeping the service persistently unavailable.
- No confidentiality or integrity loss – The vulnerability does not allow data theft or modification; it is purely a denial‑of‑service flaw.
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Sources:
Reported By: www.cve.org
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