CVE-2025-24294 DoS in Ruby resolv DNS name decompression - patch now

A deceptively small bug in Ruby’s bundled DNS resolver library, resolv, can be weaponized to grind application threads to a halt: CVE-2025-24294 is a name‑decompression weakness that allows an attacker to feed a crafted DNS packet with an aggressively compressed domain name and force excessive CPU and memory work during decompression, producing a reliable Denial‑of‑Service (DoS) against processes that use the vulnerable resolv versions. (ruby-lang.org)

Background / Overview​

The vulnerability was publicly disclosed in early July 2025 and assigned CVE‑2025‑24294. Ruby’s security bulletin describing the issue was posted on July 8, 2025 and contains the canonical technical summary: an insufficient check on the length of a decompressed domain name within a DNS packet allows a maliciously crafted packet to expand into an extremely large or pathological name during decompression, consuming CPU and blocking the application thread that invoked resolv. (ruby-lang.org)
NVD’s entry for CVE‑2025‑24294 mirrors that description and records the CVE’s publication window around mid‑July 2025. Several vendor advisories and distro security notes (Amazon Linux, Ubuntu, and others) classified the weakness as availability‑first and assigned medium‑to‑high operational priority to patching. (nvd.nist.gov)
Why this matters: DNS name compression is a core optimization in DNS messages; implementations that decompress pointer‑based names must limit resulting lengths to avoid unbounded expansion. When they don’t, a network‑facing attacker can craft packets that force the parser into extreme work or into copying far more data than intended — in practice this becomes a denial of service on the host process that performed the decompression. The underlying protocol behavior is defined in RFC 1035 (Section 4.1.4), which explains the pointer and label scheme responsible for the risk. (rfc-editor.org)

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The technical root cause — what actually goes wrong​

DNS compression in one paragraph​

DNS compresses repeated label sequences in a message using two‑byte pointers that reference earlier offsets in the same message. That pointer mechanism reduces message sizes but requires resolvers to follow pointers safely, detect loops, and bound the final decompressed length. RFC 1035 documents the pointer format and the general rules for compression and decompression. (rfc-editor.org)

Where resolv fails​

The resolv implementation in Ruby failed to enforce a robust upper bound on the length of the decompressed domain name produced after following pointer chains. An attacker-controlled packet can be crafted so that pointer sequences cause name expansion behavior that is either:

• computationally expensive to process (repeated pointer traversal, large expansions), or
• effectively unbounded in logic, creating pathological loops or extreme label growth if the implementation does not both detect pointers cycles and cap decompressed size.

Because resolv’s decompression did not limit the final name length early enough, the code can consume excessive CPU and block the application thread that invoked the lookup. In short: malformed compression pointers + missing length checks = application DoS. (ruby-lang.org)

Why name compression is tricky in parsers​

The RFC’s pointer mechanism was designed for compactness and predates many modern safety practices. Implementations must handle:

• pointers pointing to any prior offset in the message,
• chains of pointers that may reference other pointers, and
• the possibility of deliberately malformed messages that create very long expansions or pointer loops.

If code naively follows pointers without tracking visited offsets or imposing strict decompressed size limits, it becomes straightforward to force pathological workloads or crashes. Several modern analyses and RFC commentary warn exactly about this class of problem and recommended defensive limits in decompression routines. (dn.org)

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Affected versions and the patch story — what to upgrade​

Ruby’s advisory lists the affected resolv gem versions and the Ruby series that bundle them:

• Ruby 3.2 series: resolv version 0.2.2 and earlier
• Ruby 3.3 series: resolv version 0.3.0
• Ruby 3.4 series: resolv version 0.6.1 and earlier. (ruby-lang.org)

GitHub’s advisory database records both the affected version ranges and the patched releases. The published patched gem versions are: 0.2.3, 0.3.1, and 0.6.2 (and corresponding Ruby point releases that include these updated gems). In addition, Ruby language releases that include the fix were rolled out (for example, the Ruby 3.2.9 release explicitly lists CVE‑2025‑24294 as addressed). (github.com)
Vendor and distribution trackers followed quickly. Amazon Linux 2 and other distro advisories published updates in August 2025 referencing the CVE and their fixed package mappings; Ubuntu, Debian, and other maintainers also added the CVE to their security queues and released packages for affected branches. Operators should always prefer distro packages (or the patched Ruby release) rather than ad‑hoc, unreviewed backports. (alas.aws.amazon.com)

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Attack surface, exploitability, and real‑world risk​

Attack vector​

This is a network‑facing attack against software that performs DNS parsing using the vulnerable resolv library. An attacker needs only to deliver a crafted DNS response (or other packet payloads that resolv will attempt to parse) to trigger the problem; the vulnerability does not require code execution or elevated local privileges to cause the denial of service. Typical exploitation scenarios include:

• a malicious DNS server contacted by a vulnerable client,
• a man‑in‑the‑middle DNS responder inserting crafted responses, or
• an attacker able to send spoofed or on‑path DNS replies to an application using resolv. (ruby-lang.org)

Exploit availability and public proof‑of‑concepts​

As of the most recent vendor trackers and public feeds consulted during reporting, there is no authoritative evidence of a widely adopted public proof‑of‑concept exploit being weaponized in the wild for CVE‑2025‑24294. Several aggregated feeds explicitly note no evidence of public PoC; however, this class of parsing bug is relatively straightforward to exploit for DoS by a moderately skilled attacker, so lack of a published PoC is not a guarantee of safety. Treat the absence of published PoCs as temporary and maintain a patch‑first posture. (feedly.com)

Severity and scoring — what the numbers say​

Different trackers assigned varying scores: some maintainers labeled the CVE as Medium while others produced higher CVSS derivations (Feed aggregators reported CVSS equivalents in the 5–7+ range). The disparity reflects differing scoring choices (network vector vs. scope and whether the application context is considered). Practically speaking, the vulnerability is an availability‑first issue that can produce a sustained or persistent DoS against any application thread that invokes resolv on untrusted input. For production systems, the operational impact (application unresponsiveness, service outages) is the key metric, not a raw CVSS number. (explore.alas.aws.amazon.com)

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Detection, indicators, and hunting​

What to look for in logs and telemetry​

• Repeated or transient spikes in CPU usage attributable to PID(s) running Ruby processes that perform DNS lookups.
• Application threads blocked or unresponsive during DNS resolution calls; seen as hung worker threads in web servers or job processors.
• Network traces showing abnormal DNS responses that contain many compression pointers or suspiciously formatted DNS name fields — look for unusual packet size vs. decompressed name length.
• Exception, timeout, or restart cycles for Ruby processes that correlate to inbound DNS responses from specific resolvers or upstream network hops.

Hunting steps​

• Identify hosts and services that use the system or bundled resolv implementation (Ruby apps, background jobs, cron tasks, container images).
• Instrument DNS requests and responses (pcap or Netflow) in a staging environment and sample DNS replies to inspect compression pointer patterns.
• Add application‑level timeouts around DNS resolution calls or centralize DNS resolution to a hardened stub resolver (see mitigations below).
• Alert on CPU usage patterns or stuck Ruby threads that coincide with DNS activity. Use APM traces to map DNS latency/hangs to specific code paths.

Note: reproducing the attack in production is unsafe — always use isolated testbeds or controlled lab environments for any reproduction. (ruby-lang.org)

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Mitigation and remediation — priority actions for operators​

Immediate (within hours)​

• Patch first. Upgrade the resolv gem or the Ruby runtime to versions that include the patched resolv release (patched gem versions include 0.2.3, 0.3.1, 0.6.2; or upgrade to Ruby releases that bundle those fixes). Use your distribution’s security packages where available. (github.com)
• Isolate DNS inputs. Where possible, restrict DNS server endpoints that your apps contact to trusted resolvers (local caching resolvers, enterprise DNS servers that enforce filtering) and disallow direct queries to arbitrary/remote nameservers.
• Apply timeouts. Ensure Ruby threads performing DNS lookups have reasonable timeouts and that worker pools are protected from indefinite blocking (circuit breakers, pool timeouts).

Short to medium term​

• Centralize DNS resolution. Replace ad‑hoc in‑app DNS queries with a hardened, vetted caching resolver (e.g., a local DNS cache with rate limiting and sanity checks) so that application code delegates parsing to a single trusted component.
• Harden resolvers. Configure DNS forwarders and authoritative servers to validate or reject malformed compression pointers where feasible, and to limit the use of aggressive compression in generated responses.
• Network controls. Use network egress controls to force all DNS traffic through trusted resolvers, and employ ingress filtering / DNS RPZ or firewall rules to block suspicious replies.

Long term / architectural​

• Dependency hygiene. Include third‑party library and bundled gem inventory in SBOMs and stitch them to your CVE management system so dev teams can rapidly know which artifacts are vulnerable. Vendor advisories and machine‑readable VEX/CSAF data help automate this mapping; prefer distro security packages where possible. (alas.aws.amazon.com)
• Defense‑in‑depth. Combine application timeouts, network controls, and central resolvers with monitoring that ties thread‑hangs to DNS behavior to reduce the chance of a single failure mode disabling a service.

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Operational playbook — step‑by‑step​

• Inventory: list all Ruby runtimes and gems in use across hosts, containers, and images. Query installed gem versions and Ruby point releases.
• Prioritize: put externally reachable services and high‑value backends first (APIs, web servers, daemons performing resolution) for immediate patching.
• Patch: apply the resolv gem patches or upgrade Ruby to fixed point releases; coordinate maintenance windows for restarts where required. (github.com)
• Validate: run smoke tests that exercise DNS resolution paths and confirm no thread hangs or CPU spikes after patching.
• Mitigate interim: if you cannot patch immediately, route DNS through a trusted caching resolver and enforce per‑process timeouts to reduce blast radius.
• Monitor: deploy alerts for CPU spikes, thread unresponsiveness, and anomalous DNS reply patterns. Correlate with packet captures if you suspect ongoing exploitation.
• Document: record the remediation steps, impacted images, and timeline for audit and incident readiness.

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Why this class of bug is important in supply‑chain context​

DNS compression edge cases are often passive and buried in innocuous code paths (DNS lookup libraries are widely reused). A parsing bug like CVE‑2025‑24294 highlights several supply‑chain realities:

• Bundled libraries: resolv is bundled with the Ruby runtime in many distributions, so vulnerable code can propagate through packaged images, containers, and downstream vendor artifacts. Distro and vendor advisories therefore become the source of truth for production owners. (alas.aws.amazon.com)
• Attack surface diffusion: any application that issues DNS lookups — web apps, background jobs, CI runners — potentially inherits exposure, making inventory difficult.
• Low complexity, high impact: remote attackers with the ability to send or manipulate DNS replies can cause outsized operational disruption without needing privileged access. The exploit complexity is low for DoS outcomes, raising urgency even if confidentiality or code‑execution impacts are absent.

These characteristics make patch prioritization straightforward: if your service does DNS resolution and uses an affected resolv version, treat this as a patch‑first availability issue. (ruby-lang.org)

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What we know — and what we don’t (areas that require caution)​

• Confirmed: the vulnerability is a decompression length check problem in resolv that enables DoS through CPU/memory resource exhaustion. Multiple authoritative sources (Ruby advisory, NVD summary, GitHub Advisory) confirm the root cause and the patched releases. (ruby-lang.org)
• Confirmed: patched gem releases and Ruby point releases were published (patched gem versions include 0.2.3, 0.3.1, 0.6.2; Ruby 3.2.9 is an example of a release that bundles fixes). (github.com)
• Uncertain / evolving: public exploit code availability appeared to be absent or limited at the time of advisories; however, lack of PoC is no safety guarantee. The underlying attack technique — crafting compressed DNS names — is straightforward, so threat actors could produce weaponized code quickly. Monitor public exploit repositories and threat feeds for changes. (feedly.com)
• Deployment nuance: not all Ruby installations are equally exposed. Applications that centralize DNS resolution, run behind hardened resolvers, or run on distros where the resolv gem is not installed are less likely to be impacted. Always verify via inventory rather than assumptions. (alas.aws.amazon.com)

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Final assessment and recommendations​

CVE‑2025‑24294 is a pragmatic, availability‑focused vulnerability with straightforward mitigations: upgrade the resolv gem or Ruby runtime to the patched versions, and apply conservative DNS‑handling controls where patching cannot occur immediately. The technical root cause — missing length checks when decompressing pointer‑based domain names — is classic but still effective when overlooked.
For defenders and operators:

• Treat any Ruby application that performs DNS resolution as potentially vulnerable until proven otherwise. Inventory is the first priority.
• Patch distributions or Ruby installations to the listed patched gem or Ruby point releases as soon as practicable. Use vendor packages where available. (github.com)
• If immediate patching is impossible, centralize DNS via hardened resolvers, enforce tight timeouts on resolution calls, and add monitoring for DNS‑related process hangs.
• Keep an eye on threat intelligence for PoC or exploitation reports; the absence of published exploit code today does not guarantee it will remain so.

This is a textbook example of how protocol parsing choices and longstanding compression mechanisms can create modern operational hazards. The practical answer is straightforward: patch, control DNS inputs, and instrument your stacks. If you do those three things, you remove the window of opportunity for this denial‑of‑service vector to interrupt your services. (ruby-lang.org)

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Concluding note: the resolv issue reminds engineers and security teams that even small libraries carry large responsibilities. DNS is everywhere and defensive limits — explicit decompression bounds, pointer loop detection, and centralization of parsing — are essential to keep network‑level tricks from becoming application outages. Apply the vendor fixes, validate behavior in a staged environment, and harden DNS flows where possible to close this attack path. (ruby-lang.org)

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Source: MSRC Security Update Guide - Microsoft Security Response Center