A community report on the tor-relays mailing list presents custom metrics from approximately 300 relays, identifying 30 distinct flood windows over 48 hours. The data indicates that descriptor fetching and HTTP-wait closures dominated the traffic, with one relay recording over 14 million fetches. The report notes that Tor 0.4.9.15 is expected to rate limit some of these requests, while a separate incident report describes a relay experiencing memory exhaustion and OOM termination during a similar period.
Why it matters The detailed metrics provide specific evidence of the traffic composition during recent network-wide resource exhaustion events, helping to distinguish between different types of flood attacks and validate the effectiveness of upcoming rate-limiting measures.
The Tor Project released version 0.4.9.15, a security update introducing new defenses against large volumes of directory requests. The release makes the HSDir flag sticky across restarts and updates directory authorities to require specific protocol capabilities, rendering Tor versions 0.4.8 and earlier incompatible with the network. Operators are advised to upgrade relays as soon as possible.
Why it matters The release includes critical DoS mitigations for relays and enforces protocol changes that will disconnect older clients from the network.
A security report identified an incorrect format string in Tor's consensus generation that the report says allowed three colluding directory authorities to prevent consensus publication. The project labeled the severity as low because exploitation requires control of multiple authorities. A comment by @arma states the issue is listed as fixed in 0.4.9.15 as of commit 407ccd28a.
Why it matters The report states that three colluding directory authorities can wedge consensus on all honest authorities, resulting in a network-wide DoS.
A security report filed via HackerOne describes a vulnerability in Arti's onion-service implementation where invalid proof-of-work requests cause unbounded growth of persistent replay logs. The report states that an unauthenticated attacker can append approximately 16 bytes of data per request without solving the proof of work, potentially leading to filesystem exhaustion and service unavailability. The issue remains open in the Arti development tracker.
Why it matters The report indicates that this vulnerability could allow an attacker to cause a denial of service against onion services by exhausting disk space, requiring operator intervention for recovery.
A security report filed via HackerOne and published on GitLab describes a defect in Arti's rendezvous queue where a client sending no proof-of-work can evict a queued client that did. The report states this defeats the priority queue's purpose but notes it does not demonstrate a sustained denial of service. A maintainer commented that the issue is a bug but does not seem like a DoS issue, and a fix issue was created.
Why it matters The report states that the defect defeats the purpose of the priority queue, allowing clients to bypass proof-of-work requirements for queue admission.
A relay operator reports a sustained circuit-building DoS against their guard relay family starting June 25, 2026, characterized by an 8x sustained increase in open circuits and a 20x peak. The attack involved millions of near-idle circuits and onionskin processing spikes across multiple regions, with bandwidth increasing only 20%. While DoS defenses engaged and prevented TCP port exhaustion, EU relays shed 10-20% of real traffic due to capacity constraints.
Why it matters The report suggests the pattern is network-wide or family-targeted, and notes that EU relays shed 10-20% of real traffic.
