CVE-2026-82309 in Robots::Validate
Summary
by MITRE • 09/04/2026
Robots::Validate versions from 0.3.2 before 0.3.11 for Perl allow unbounded outbound DNS queries per validation via a forward-confirmation loop that does not bound the names it queries.
_check_dns issues one PTR query for the client address, keeps the returned names matching the rule's domain, and issues a forward query for each until one resolves back to that address. Nothing bounds that list, and a client controls the reverse zone for its own address, so it chooses how many names the PTR answer holds. Net::DNS refetches a truncated answer over TCP by default, so the 512-byte UDP payload does not cap it either.
Any client whose User-Agent matches a rule with a domain reaches _check_dns. Each forward name is distinct and client-chosen, so every query misses the local cache and is resolved against the authoritative servers for that domain. The queries are synchronous, so the caller is held until all of them answer or time out.
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Analysis
by VulDB Data Team • 09/04/2026
The vulnerability identified in Perl module Robots::Validate prior to version 0.3.11 represents a significant resource exhaustion flaw rooted in unbounded outbound DNS query generation during validation processes. The core technical deficiency lies within the _check_dns subroutine, which is designed to verify client addresses by performing reverse and forward DNS lookups. Specifically, when validating an IP address, the module issues one PTR (pointer) query for the client's address. Upon receiving a response containing multiple domain names associated with that address, the system filters these names against specific rule domains and subsequently initiates individual forward A or AAAA queries for each matching name until it finds one that resolves back to the original IP address. This logic fails to impose any limit on the number of forward queries executed based on the volume of PTR records returned.
This architectural oversight allows a malicious client to exploit the validation mechanism by controlling their own reverse DNS zone. By configuring numerous domain names within their reverse zone, an attacker can cause the initial PTR query to return a large list of hostnames. Since each hostname is distinct and likely not present in local caches, every subsequent forward lookup must be resolved against authoritative name servers for those respective domains. The synchronous nature of these queries means that the calling process remains blocked until all outstanding DNS requests complete or time out. Consequently, this results in a denial-of-service condition where system resources are consumed by excessive network I/O and CPU cycles spent on resolving an artificially inflated number of domain names, effectively stalling legitimate operations dependent on Robots::Validate.
The severity of this issue is compounded by the behavior of the underlying Net::DNS library used for resolution. By default, Net::DNS attempts to fetch truncated responses over TCP if they exceed the standard 512-byte UDP payload limit. This mechanism prevents the natural cap that might otherwise restrict the volume of data returned in a single transaction from mitigating the attack vector. Because there is no explicit bound on the list of names processed after the PTR lookup, an attacker can generate hundreds or thousands of outbound queries per validation request. This unbounded growth directly impacts server availability and network bandwidth, creating a scalable amplification effect that disproportionately affects systems relying on this module for access control or logging verification.
From a classification perspective, this vulnerability aligns with CWE-787: Out-of-bounds Write in terms of resource consumption logic, though more accurately it falls under CWE-400: Uncontrolled Resource Consumption and CWE-611: Improper Restriction of XML External Entity Reference if viewed through the lens of untrusted data processing leading to excessive load. In the context of the MITRE ATT&CK framework, this behavior is indicative of T1498: Network Denial of Service, specifically where an attacker leverages application logic flaws rather than raw bandwidth saturation to disrupt service availability. The attack vector relies on social engineering or automated probing to trigger validation routines with crafted DNS responses that maximize query volume.
Mitigation strategies must focus on implementing strict limits within the Robots::Validate module itself. Developers should enforce a maximum threshold for the number of forward queries allowed per single PTR response, ensuring that only a reasonable subset of domain names is tested against the original IP address. Additionally, introducing caching mechanisms for DNS lookups can reduce redundant resolution attempts and mitigate some performance impact during normal operation. Upgrading to version 0.3.11 or later resolves this issue by addressing the unbounded query loop. Until an upgrade is feasible, applying network-level rate limiting on outbound DNS traffic from affected servers may provide a partial defense-in-depth measure against excessive query generation.