CVE-2026-118993 in CallBackery
Summary
by MITRE • 10/30/2026
CallBackery versions before 0.62.0 for Perl allow denial of service via the request path interpolated as a regular expression under CGI, FastCGI and PSGI.
The before_dispatch hook installed by startup() removes PATH_INFO from the end of the request URI with a substitution that interpolates PATH_INFO unquoted. Under CGI, FastCGI and PSGI both values come from the request, so the client chooses both the pattern and the string it is matched against.
A short unauthenticated request with a backreference in the path backtracks exponentially and can keep a worker process busy for minutes.
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Analysis
by VulDB Data Team • 10/30/2026
The vulnerability identified in CallBackery versions prior to 0.62.0 represents a critical server-side denial of service condition rooted in improper input validation and unsafe regular expression handling within Perl-based web application frameworks. This flaw specifically affects environments utilizing CGI, FastCGI, or PSGI protocols, where the request path is directly interpolated into a regular expression without proper sanitization. The core technical issue resides in the before_dispatch hook installed by the startup() function, which attempts to remove PATH_INFO from the end of the request URI using a substitution operation. Crucially, this substitution interpolates the PATH_INFO variable unquoted, meaning that any special characters or regex metacharacters present in the user-supplied path are interpreted as part of the regular expression pattern rather than literal string data.
In standard web server architectures under CGI, FastCGI, and PSGI implementations, both the search pattern and the target string for such operations can be influenced by external inputs if not strictly controlled. In this specific case, because PATH_INFO is derived directly from the incoming HTTP request, an attacker gains full control over both components of the regex match operation. This creates a scenario where the client effectively chooses the regular expression pattern that will be executed against the server's internal state or other variables during the dispatch process. The lack of escaping allows for the injection of complex regex constructs that exploit known performance weaknesses in Perl's regex engine, particularly regarding backtracking behavior when dealing with nested quantifiers and backreferences.
The operational impact of this vulnerability is severe, manifesting as a resource exhaustion attack capable of rendering application services unavailable to legitimate users. By crafting a short but syntactically malicious request path containing specific backreference patterns, an attacker can trigger exponential time complexity in the regex engine's matching algorithm. This phenomenon, often referred to as ReDoS or Regular Expression Denial of Service, causes the worker process handling the request to become stuck in an infinite or near-infinite loop while attempting to resolve the pattern match. Consequently, a single unauthenticated request can keep a server worker busy for several minutes, effectively blocking that thread from processing any other requests and degrading overall system performance. In high-traffic environments or when multiple such requests are sent concurrently, this leads to complete service outage due to resource starvation.
From a classification perspective, this vulnerability aligns with CWE-400, which covers Uncontrolled Resource Consumption, as the attacker forces the server to consume excessive computational resources through inefficient algorithmic complexity. It also relates closely to CWE-16, Configuration Error, specifically regarding the failure to properly sanitize input before use in dynamic code execution or pattern matching contexts. Furthermore, this attack vector is consistent with MITRE ATT&CK technique T1498, Network Denial of Service, where an adversary leverages specific application logic flaws rather than brute-force volume attacks to disrupt availability. The exploitation does not require authentication, making it particularly dangerous as any anonymous user on the internet can trigger the condition.
Mitigation strategies must focus on immediate patching and defensive coding practices. Organizations running CallBackery should upgrade immediately to version 0.62.0 or later, where this interpolation flaw has been addressed by properly escaping or quoting the PATH_INFO variable before its use in regex operations. For systems that cannot be patched instantly, temporary mitigations include implementing a Web Application Firewall rule set designed to detect and block requests containing suspicious regular expression metacharacters within URI paths, such as backslashes followed by digits indicative of backreferences. Additionally, configuring server timeouts to aggressively terminate long-running processes can limit the duration of impact from successful exploitation attempts, although this is only a partial countermeasure that may affect legitimate slow connections. Long-term remediation requires enforcing strict input validation policies where all user-supplied data used in dynamic pattern construction is rigorously sanitized using language-specific escaping functions like quotemeta in Perl to ensure literal interpretation rather than regex execution.