CVE-2026-82309: Robots::Validate Perl Module DOS Vulnerability

CVE-2026-82309 Overview

CVE-2026-82309 affects Robots::Validate for Perl, versions 0.3.2 through 0.3.10. The module performs forward-confirmed reverse DNS validation on client addresses when User-Agent strings match a configured rule. The _check_dns routine issues a PTR query for the client address, then issues a forward DNS query for every returned name that matches the rule's domain. Because the module places no bound on the number of names processed, a client controlling its own reverse DNS zone can force the validator to issue arbitrary numbers of synchronous DNS queries [CWE-405].

Critical Impact
An authenticated client can trigger unbounded outbound DNS queries per validation call, holding the calling process synchronously and generating query amplification against authoritative servers.

Affected Products

Robots::Validate for Perl, versions 0.3.2 through 0.3.10
Fixed in Robots::Validatev0.4.0
Applications using Robots::Validate for User-Agent verification via DNS

Discovery Timeline

2026-09-04 - CVE-2026-82309 published to NVD
2026-09-04 - Public discussion posted to OpenWall oss-security list
2026-09-08 - Last updated in NVD database

Technical Details for CVE-2026-82309

Vulnerability Analysis

The flaw resides in the _check_dns function of Robots::Validate. The function performs a PTR lookup against the client IP address, filters the returned names to those matching the rule's configured domain, and issues an A/AAAA query for each retained name until one resolves back to the client address. The module does not cap the size of this list, and the client controls the reverse DNS zone for its own address. That control allows the client to determine how many PTR names the answer contains.

The Net::DNS resolver used by the module defaults to retrying truncated responses over TCP. That behavior prevents the standard 512-byte UDP payload from acting as an implicit cap on PTR answer size. Each forward name generated by the client is distinct, so cache lookups miss and every forward query reaches the authoritative servers for the matched domain. The queries execute synchronously, blocking the caller until every lookup either resolves or times out.

Root Cause

The root cause is uncontrolled resource consumption [CWE-405]. _check_dns iterates over an attacker-controlled list of hostnames without applying a maximum-count constraint. The fix in v0.4.0 introduces a max_forward_lookups attribute that bounds the number of reverse-DNS-driven forward lookups per validation.

Attack Vector

An attacker sends a request with a User-Agent that matches a rule containing a domain constraint. The attacker's IP address resolves via PTR to a large list of hostnames within that domain, each constructed to be unique and non-cacheable. Robots::Validate issues a forward DNS query for every name, blocking the calling worker and generating outbound query volume proportional to the size of the attacker's PTR answer.

text

# Security patch entry in Changes for Robots-Validate v0.4.0
  [Security]
  - Add a max_matches attribute to limit the number of matches checked when using greedy matching.

+  - Add a max_forward_lookups attribute to limit the number of reverse DNS lookups. (CVE-2026-82309)
+
  [Documentation]
  - Added missing minimum version for greedy attribute.

Source: GitHub Patch Update

Detection Methods for CVE-2026-82309

Indicators of Compromise

- Bursts of outbound A/AAAA queries from application hosts to a single authoritative domain, with each query name distinct and cache-missing.
- PTR responses arriving from client-controlled reverse zones containing large numbers of hostnames.
- Worker processes stalled in synchronous DNS resolution while handling requests that match rules using Robots::Validate.

Detection Strategies

- Instrument DNS resolver metrics on application hosts to alert on sustained high query rates per originating process or worker.
- Correlate inbound HTTP requests with matching User-Agent rules against outbound DNS query volume from the handling process.
- Inspect Robots::Validate version in deployed Perl environments; flag any release from 0.3.2 through 0.3.10.

Monitoring Recommendations

- Log the size of PTR answers returned by upstream resolvers and alert when any single response exceeds a small threshold.
- Track per-request latency for endpoints that invoke Robots::Validate and investigate outliers driven by DNS wait time.
- Monitor authoritative DNS server load for query patterns consistent with amplification against a single domain.

How to Mitigate CVE-2026-82309

Immediate Actions Required

- Upgrade Robots::Validate to v0.4.0 or later and set an explicit max_forward_lookups value appropriate for the deployment.
- Audit application code for calls into Robots::Validate and confirm rules that invoke DNS validation are still required.
- Enforce recursive resolver query rate limits per client for application worker hosts.

Patch Information

The fix is delivered in Robots::Validatev0.4.0 via commit 6426178c49ff6c440922f31a92a6feb3c661b143. The patch adds a max_forward_lookups attribute that bounds the number of reverse-DNS-driven forward lookups performed per validation. See the GitHub Security Advisory GHSA-6399-5qhh-48h5 and the MetaCPAN Release Changes for release details.

Workarounds

- Remove domain-based rules from Robots::Validate configurations so that _check_dns is not invoked.
- Front the application with a short resolver timeout and a per-worker DNS query cap to bound the blocking window.
- Restrict access to endpoints that trigger validation using upstream authentication or IP allow-lists until the upgrade is deployed.

bash

# Upgrade to the fixed release using cpanm
cpanm RRWO/Robots-Validate-v0.4.0.tar.gz

# Verify installed version
perl -MRobots::Validate -e 'print $Robots::Validate::VERSION, "\n"'