CVE-2026-34826: 
Ruby vulnerability analysis and mitigation

CVE-2026-34826 is a denial-of-service vulnerability in Rack, the modular Ruby web server interface, caused by uncontrolled parsing of the HTTP Range header. The Rack::Utils.get_byte_ranges function does not limit the number of individual byte ranges an attacker can supply, allowing a crafted request with thousands of overlapping ranges (e.g., 0-0,0-0,0-0,...) to trigger disproportionate CPU, memory, I/O, and bandwidth consumption. Affected versions include all Rack releases prior to 2.2.23, 3.0.0.beta1 through 3.1.20, and 3.2.0 through 3.2.5. The vulnerability was published on April 2, 2026, with a CVSS v3.1 base score of 5.3 (Moderate) (GitHub Advisory, Rack Advisory).

The root cause is classified as CWE-400 (Uncontrolled Resource Consumption) and CWE-770 (Allocation of Resources Without Limits or Throttling). While a prior fix for CVE-2024-26141 rejects Range headers whose total byte coverage exceeds the file size, it does not cap the count of individual ranges. An attacker can craft a Range header such as bytes=0-0,0-0,0-0,... containing thousands of overlapping one-byte ranges that collectively satisfy the total-size check. Each range triggers additional work in Rack's file-serving code: multipart response generation, per-range iteration, file seek/read operations, and temporary string allocation — allowing a small HTTP request to cause amplified server-side processing and a much larger multipart response (GitHub Advisory, Rack Advisory).

Successful exploitation results in a denial-of-service condition affecting availability only — there is no confidentiality or integrity impact. Unauthenticated attackers can exhaust CPU time, memory, file I/O, and bandwidth on affected servers by repeatedly sending crafted Range headers to file-serving endpoints. Sustained attacks may degrade or fully disable application availability, increase garbage collection pressure on Ruby workers, and affect downstream services that depend on the Rack-based application (GitHub Advisory). IBM products including Aspera Faspex, Aspera Enterprise WebApps, Aspera Shares, API Connect, CloudPak for AIOps, and License Metric Tool are also affected as downstream consumers of Rack (Feedly).

No public proof-of-concept exploit code has been identified, and there is no evidence of in-the-wild exploitation at this time (GitHub Advisory). The EPSS score is approximately 0.021% (6th percentile), indicating a low near-term exploitation probability. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. No threat actor attribution has been reported. The attack requires no authentication, no user interaction, and low complexity, making it straightforward to attempt once the technique is known.

1. Reconnaissance: Identify internet-facing Ruby web applications built on Rack (e.g., Rails, Sinatra) that serve static files or expose file-download endpoints with byte range support. Tools like Shodan or Censys can help identify targets.
2. Identify file-serving endpoints: Locate endpoints that respond to HTTP Range requests with 206 Partial Content and multipart/byteranges content type, indicating Rack's file-serving code is active.
3. Craft malicious Range header: Construct an HTTP GET request with a Range header containing thousands of overlapping one-byte ranges, e.g., Range: bytes=0-0,0-0,0-0,... (repeated thousands of times), ensuring the total byte coverage stays within the file size to bypass the CVE-2024-26141 check.
4. Send repeated requests: Dispatch the crafted requests at high frequency to amplify resource exhaustion — each request triggers multipart response generation, file seek/read operations, and string allocations proportional to the number of ranges.
5. Observe impact: Monitor for degraded response times, worker process crashes, or full service unavailability as CPU, memory, and I/O are exhausted on the target server (Rack Advisory).

• Network: High volume of HTTP GET requests to file-serving endpoints containing abnormally long Range headers with many comma-separated byte ranges (e.g., bytes=0-0,0-0,... repeated hundreds or thousands of times); unusually large multipart HTTP responses from the server relative to the requested file size.
• Logs: Web server or Rack access logs showing repeated 206 Partial Content responses to the same endpoint from one or few source IPs; requests with Range header values exceeding typical length (e.g., several kilobytes).
• Process/System: Elevated CPU utilization on Ruby worker processes; increased memory consumption and garbage collection frequency; degraded response times or worker timeouts correlating with Range header requests.

Upgrade Rack to a patched version: 2.2.23, 3.1.21, or 3.2.6 depending on the branch in use (GitHub Advisory, Rack Advisory). If immediate patching is not feasible, consider the following interim mitigations:

• Configure a reverse proxy (e.g., nginx, Apache) to reject or truncate Range headers exceeding a reasonable length or range count.
• Disable multipart byte range support at the application or proxy layer where it is not required.
• Apply rate limiting on requests containing Range headers to reduce the impact of repeated attacks.

IBM product users should apply the relevant IBM security bulletins for Aspera Faspex, Aspera Enterprise WebApps, Aspera Shares, API Connect, CloudPak for AIOps, and License Metric Tool.

The vulnerability was coordinated and published by Rack maintainer ioquatix, with orenyomtov credited as the reporter and jeremyevans as the remediation developer (Rack Advisory). Ubuntu issued security notice USN-8182-1 and SUSE released updates (SUSE-SU-2026:1745-1 and SUSE-SU-2026:1964-1) addressing this vulnerability. IBM published multiple security bulletins covering affected downstream products. No significant social media controversy or broad community debate has been observed beyond standard patch notification channels.