CVE-2026-93894 in Varnish Cache
Summary
by MITRE • 09/18/2026
In Vinyl Cache before 9.0,2, workspace buffer overflow vulnerability was found in the .upper() and .lower() string type methods of VCL. This can be used as a remote denial of service (DoS) vector to make the child process segfault or assert, and then restart. Effectively exploiting this vulnerability requires prior knowledge about the VCL in use and the ability to craft a request that contains a string that is long enough to fill the remaining workspace at the call site while staying under the different request size limits (http_req_size, http_req_hdr_len, etc.).
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Analysis
by VulDB Data Team • 09/18/2026
The Vinyl Cache software version 9.02 and earlier versions contain a critical buffer overflow vulnerability located within its Virtual Configuration Language interpreter, specifically affecting the string manipulation methods .upper() and .lower(). This flaw arises from improper handling of memory allocation when processing input strings that exceed the capacity of the pre-allocated workspace buffers associated with these operations. The vulnerability is rooted in CWE-120, which classifies buffer overflow without bounds checking, allowing an attacker to write data beyond the intended boundaries of the allocated memory space. This type of error typically occurs when a program copies more data into a fixed-size buffer than it can hold, leading to corruption of adjacent memory structures and unpredictable system behavior.
Exploitation of this vulnerability requires specific conditions that distinguish it from trivial remote attacks. An attacker must possess prior knowledge of the active VCL configuration on the target server to craft a payload that effectively targets the vulnerable workspace allocation logic. The attack vector involves sending HTTP requests containing string arguments for the .upper() or .lower() methods that are sufficiently long to overflow the buffer but remain within global request size limits such as http_req_size and http_req_hdr_len. This constraint means that while the vulnerability is remotely exploitable, it demands a high degree of precision in payload construction to bypass standard ingress filtering mechanisms designed to limit overall request volume or header length.
The operational impact of successfully exploiting this buffer overflow is primarily focused on service availability rather than data confidentiality or integrity. When the malformed string triggers the memory corruption, it causes the Varnish child process to segfault or trigger an assertion failure within its error handling routines. This results in a denial of service condition where the specific worker process crashes and subsequently restarts automatically by the parent management process. While this self-healing mechanism prevents total system collapse, it introduces significant latency and potential packet loss during the restart cycle, degrading performance for all users served through that instance. In high-traffic environments or if exploited in a sustained manner, these repeated crashes can lead to resource exhaustion and prolonged service degradation.
From a threat intelligence perspective, this vulnerability aligns with ATT&CK technique T1498, Network Denial of Service, specifically the sub-category of Resource Exhaustion via Flooding or Crash Induction. The attacker leverages the application's own restart mechanism against it, creating a cycle of instability that disrupts normal operations without necessarily requiring privilege escalation or code execution capabilities on the underlying operating system. This makes detection challenging as standard intrusion prevention systems may not flag the traffic if it adheres to size limits and appears syntactically valid from a network perspective.
Mitigation strategies should prioritize immediate patching to version 9.02 or later, where this buffer overflow has been addressed through improved bounds checking in the VCL string methods. In environments where upgrading is not immediately feasible, administrators can implement strict input validation at the reverse proxy level before requests reach the cache engine. This includes enforcing stricter limits on header lengths and body sizes that are lower than the thresholds required to trigger the vulnerability, effectively creating a buffer against overflow attempts. Additionally, enabling detailed logging of VCL execution errors can help in identifying attempted exploits early, allowing for rapid response through IP blocking or request filtering rules tailored to detect anomalous string patterns associated with this specific memory corruption flaw.