CVE-2025-31498 in c-ares
Summary
by MITRE • 04/08/2025
c-ares is an asynchronous resolver library. From 1.32.3 through 1.34.4, there is a use-after-free in read_answers() when process_answer() may re-enqueue a query either due to a DNS Cookie Failure or when the upstream server does not properly support EDNS, or possibly on TCP queries if the remote closed the connection immediately after a response. If there was an issue trying to put that new transaction on the wire, it would close the connection handle, but read_answers() was still expecting the connection handle to be available to possibly dequeue other responses. In theory a remote attacker might be able to trigger this by flooding the target with ICMP UNREACHABLE packets if they also control the upstream nameserver and can return a result with one of those conditions, this has been untested. Otherwise only a local attacker might be able to change system behavior to make send()/write() return a failure condition. This vulnerability is fixed in 1.34.5.
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Analysis
by VulDB Data Team • 08/16/2025
The CVE-2025-31498 vulnerability represents a critical use-after-free condition within the c-ares asynchronous resolver library, affecting versions 1.32.3 through 1.34.4. This flaw resides in the read_answers() function where process_answer() may re-enqueue a query under specific DNS error conditions including DNS Cookie Failures, EDNS support issues from upstream servers, or TCP connection closure scenarios. The vulnerability manifests when the library attempts to handle query re-enqueuing while the connection handle has already been closed due to transmission failures, creating a scenario where freed memory is accessed during subsequent operations.
The technical implementation of this vulnerability stems from improper resource management within the DNS resolution process. When process_answer() encounters conditions requiring query re-enqueuing, it attempts to maintain connection state that has already been terminated by send() or write() operations returning failure conditions. The read_answers() function continues to reference the closed connection handle, leading to memory access violations that can result in undefined behavior, potential crashes, or memory corruption. This represents a classic use-after-free vulnerability classified under CWE-416, where memory is accessed after it has been freed, creating opportunities for exploitation through memory corruption attacks.
The operational impact of this vulnerability extends beyond simple application instability to potentially enable remote code execution or denial of service conditions. While the attack vector requires specific conditions including control over upstream nameservers and the ability to trigger ICMP UNREACHABLE packets, the local attack scenario presents a more accessible threat vector where attackers can manipulate system behavior to force send() operations to fail. The vulnerability's exploitation potential aligns with ATT&CK technique T1210 for exploitation of remote services and T1059 for command execution through compromised systems. Network administrators should consider this vulnerability as a potential entry point for attackers seeking to establish persistent access through DNS-based attack vectors.
Mitigation strategies should focus on immediate patching to version 1.34.5 which contains the necessary fixes for proper connection handle management during query re-enqueuing operations. System administrators should also implement network monitoring to detect unusual ICMP traffic patterns that might indicate attempted exploitation, particularly around DNS query processing. Additional defensive measures include implementing proper resource validation before memory access operations, establishing connection timeout mechanisms, and deploying intrusion detection systems that can identify abnormal DNS resolution patterns. The fix addresses the core issue by ensuring that connection handles are properly validated before access and that query re-enqueuing operations do not reference freed memory structures, thereby preventing the use-after-free condition that enabled potential exploitation.