CVE-2026-112921 in Robots::Validate
Summary
by MITRE • 10/21/2026
Robots::Validate versions from 0.4.3 before 0.4.10 for Perl report clients as imposter robots when User-Agent matcher state carries over from a previous request.
Each Robots::Validate object holds one pure-Perl Aho-Corasick search machine and feeds every User-Agent into it without resetting its state. A User-Agent that ends part-way through a robot name leaves the machine mid-match, so the next User-Agent checked by the same object can complete that name. That client's address is then checked against the robot's DNS rule, which it fails.
A remote client can use this to make a site that blocks bad robots block other visitors where the prefix User-Agent string is a suffix of a robot being validated.
Note that deployments with Algorithm::AhoCorasick::XS are also affected, since it was unused due to a different bug.
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Analysis
by VulDB Data Team • 10/22/2026
The Robots::Validate Perl module, specifically in versions prior to 0.4.10, contains a critical state management flaw within its core pattern matching logic that leads to incorrect classification of legitimate web clients as malicious bots or imposters. This vulnerability stems from the improper handling of the internal Aho-Corasick search machine used for User-Agent string analysis. The module is designed to maintain persistent objects that process incoming HTTP requests, but it fails to reset the state of this finite automaton between different client interactions. Consequently, if a previous request ends with a User-Agent string that partially matches one of the known robot signatures defined in the validation ruleset, the internal machine retains its partial match state rather than returning to an initial neutral position.
When a subsequent request arrives from a different user agent, the module feeds this new string into the same Aho-Corasick instance without clearing the residual state from the prior operation. If the beginning of this new User-Agent string happens to be a suffix that completes the partial match left over from the previous request, the machine incorrectly identifies the current client as one of the blocked robots. This false positive triggers the module's blocking mechanism based on DNS rules associated with the misidentified robot type. The flaw is particularly insidious because it relies on specific sequence dependencies between requests rather than a direct exploitation of input data in isolation, making it difficult to detect through standard static analysis or single-request testing scenarios.
The operational impact of this vulnerability allows for a denial-of-service condition against legitimate users without requiring authentication or complex payload construction beyond manipulating the order and content of User-Agent strings. An attacker can craft a series of requests where the first request leaves the matcher in a specific partial state, followed by subsequent requests from innocent clients whose User-Agent headers happen to complete that pattern. This effectively creates a targeted blocking mechanism against any visitor whose browser identification string contains characters that follow those left over from an initial malicious probe. The vulnerability affects deployments using both pure-Perl implementations and those attempting to use the Algorithm::AhoCorasick::XS extension, as the latter was rendered ineffective due to a separate bug preventing its proper initialization or usage in this context.
From a classification perspective, this issue aligns with CWE-20 Improper Input Validation, specifically regarding the failure to properly validate and reset stateful processing contexts when handling sequential inputs. It also relates to CWE-471 Modification of Assumed-Immutable Data if one considers the internal machine state as mutable data that should be isolated per request context. In terms of attack vectors, this behavior can be mapped to ATT&CK technique T1568.002 Dynamic Resolution Techniques, where an attacker leverages environmental or contextual factors (in this case, server-side state leakage) to achieve their objective of blocking access. The lack of proper isolation between request contexts represents a fundamental design flaw in how the module manages persistent resources across multiple invocations.
Mitigation strategies must prioritize immediate upgrading to version 0.4.10 or later, where the developers have addressed the state reset issue and corrected the underlying bugs affecting both Perl and XS implementations. For organizations unable to upgrade immediately due to dependency constraints, a temporary workaround involves ensuring that Robots::Validate objects are instantiated fresh for each request rather than being reused across multiple connections if possible within their application architecture. Additionally, implementing reverse proxy rules or WAF policies can help filter out suspicious User-Agent patterns before they reach the Perl application layer, although this is less precise than fixing the root cause in the module itself. Regular auditing of dependency versions and patching schedules are essential to prevent exploitation of such state management vulnerabilities that degrade service availability for legitimate users.