CVE-2026-69184 in c-ares
Summary
by MITRE • 09/18/2026
c-ares is an asynchronous resolver library. Prior to 1.34.7, ares_dns_name_parse() enforces backward DNS compression pointers but does not bound the total pointer hops or assembled name length. A malicious DNS server can send a response containing a long descending pointer chain and many resource records whose NAME or RDATA fields refer to the chain, causing repeated decompression work that grows quadratically with message size. A single crafted response can stall the single-threaded c-ares event loop and deny DNS resolution, without causing memory corruption or information disclosure. This issue is fixed in version 1.34.7.
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Analysis
by VulDB Data Team • 09/18/2026
The vulnerability identified in the c-ares asynchronous resolver library prior to version 1.34.7 represents a significant availability risk stemming from insufficient input validation during DNS name parsing operations. As an essential component for network applications requiring domain name resolution, c-ares processes incoming DNS responses by decompressing names that utilize pointer compression mechanisms defined in RFC 1035. The core technical flaw lies within the implementation of the ares_dns_name_parse function, which enforces backward compatibility with legacy DNS compression pointers but fails to impose strict bounds on either the total number of pointer hops or the final assembled name length. This oversight allows an attacker who controls a malicious DNS server to craft responses containing deeply nested chains of compression pointers that exceed reasonable processing limits without triggering immediate termination errors.
When such a crafted response is received, the resolver library enters a state of excessive computational load due to repeated decompression work. The complexity of this operation grows quadratically relative to the size of the message because each pointer in the chain may trigger further lookups and expansions within the same packet structure. Since c-ares typically operates using a single-threaded event loop architecture, this disproportionate processing demand effectively stalls the entire thread. Consequently, the application relying on c-ares becomes unresponsive as it is unable to process other network events or complete pending DNS queries during the prolonged parsing phase of the malicious response.
The operational impact of this vulnerability is primarily centered around denial of service rather than data integrity or confidentiality breaches. The exploitation does not result in memory corruption, buffer overflows, or information disclosure vulnerabilities that might compromise system stability or leak sensitive data. Instead, it creates a resource exhaustion scenario where legitimate DNS resolution services are blocked until the malicious parsing operation completes or times out. This can lead to application timeouts, degraded user experience, and potential cascading failures in systems dependent on timely network connectivity for critical functions.
To mitigate this risk, organizations utilizing c-ares must ensure that their software dependencies are updated to version 1.34.7 or later, where the parsing logic has been hardened with appropriate limits on pointer hops and name length. In environments where immediate patching is not feasible, implementing network-level filtering rules can help block suspicious DNS traffic patterns indicative of such attacks. Additionally, configuring application timeouts for DNS resolution requests can limit the duration of potential stalls caused by malicious inputs. This vulnerability aligns with CWE-787 Out-of-bounds Write and CWE-20 Improper Input Validation concepts, while its exploitation method corresponds to ATT&CK technique T1499 Endpoint Denial of Service through resource exhaustion mechanisms targeting application-layer services.