CVE-2026-82259

Executive Summary

CVE-2026-82259 is a high-severity deserialization expansion vulnerability (CVSS 7.5, AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H) affecting SvelteKit versions 2.49.0 through 2.53.2. The flaw resides in the experimental form remote function, where unvalidated files array processing allows an attacker to submit small payloads that expand into massive file arrays, triggering expensive server-side processing and resulting in a denial of service (DoS). No authentication or user interaction is required, making it remotely exploitable by any unauthenticated network attacker. The vendor has remediated the issue in version 2.53.3, with fixes confirmed by two independent sources. A public PoC exists, though no in-the-wild exploitation has been reported.

Technical Analysis

The vulnerability is classified under CWE-502 (Deserialization of Untrusted Data) and arises from an expansion issue within the experimental remoteFunctions feature of SvelteKit. Specifically, the form function — when enabled via experimental.remoteFunctions — processes an incoming files array. The NVD description indicates that the application does not validate files.length or individual file sizes prior to processing.

Attack Mechanism:

• Deserialization Expansion: The flaw leverages a classic "zip bomb" or "billion laughs" style amplification pattern. The attacker serializes a compact payload representing a file array structure. When deserialized server-side, this payload expands into a disproportionately large in-memory representation, exhausting CPU and/or memory resources.
• Lack of Input Validation: The code path trusts the files array contents without enforcing upper bounds on the number of elements (files.length) or the byte size of each element. This permits nesting/compression tricks where a few bytes of input become gigabytes of allocated structures.
• Experimental Surface: Because this feature is labeled experimental and is not enabled by default, affected deployments are those that intentionally set experimental.remoteFunctions and rely on the form handler to process file uploads or file-like data.

Attack Surface:

• Network-facing application endpoints that invoke the form function remotely. Any public route that accepts serialized data and routes it to form is exposed.
• The vulnerability requires the target application to have opted into the experimental feature — it is not exploitable on default SvelteKit installations.

Resource Exhaustion: The core impact is availability. Successful exploitation forces the server to perform expensive allocations and processing (e.g., repeatedly instantiating file objects), degrading response times or crashing the process. CVSS subscores confirm this: Availability Impact is High (A:H), while Confidentiality and Integrity are None (C:N/I:N).

Insufficient technical detail is available regarding the exact serialization format or the internal form implementation; the above mechanism is inferred from the CWE-502 classification and the "expansion" language in the NVD description.

Exploitation Scenarios

The attack chain is derived purely from the NVD description; no specific exploit code was provided, but a public PoC exists.

1. Reconnaissance / Target Identification: An attacker locates a SvelteKit application running version 2.49.0–2.53.2 that has experimental.remoteFunctions enabled. Fingerprinting may involve HTTP response headers (e.g., x-powered-by: SvelteKit), route probing for form-action endpoints, or observing application behavior under malformed input.

2. Flaw Trigger: The attacker targets an HTTP endpoint that invokes the form remote function. Because the endpoint processes a files array, the attacker crafts a serialized request destined for that function. The request is small — possibly a few hundred bytes — containing a deeply nested or compressed structure that, when deserialized, expands exponentially in size.

3. Payload / Execution Method: At the vulnerability-class level (CWE-502), the attacker delivers a malicious serialized object as the files parameter. No authentication is required (PR:N). The payload exploits the lack of validation of files.length and per-file size: the deserializer recursively constructs thousands or millions of file entries, each triggering allocation and possibly type conversions. This is analogous to a decompression bomb — the input is small, but the expanded in-memory representation is large.

4. Post-Exploitation Impact: The server's process consumes excessive memory and CPU, leading to significant latency, unresponsiveness, or full service crash (Denial of Service). With S:U, the attack is single-system; however, in containerized or shared-infrastructure deployments, resource exhaustion may propagate to co-tenants. The DoS is repeatable — the attacker can resend the payload indefinitely — and requires no cleanup.

Note: No public PoC code or exploit module is available for referencing specific injection syntax; the chain is based on the vulnerability class and the description's explicit reference to unvalidated files.length.

Impact Assessment

• Business Impact: High availability risk for any SvelteKit application that has adopted the experimental feature. A successful attack renders the application unusable, disrupting user-facing services, API consumers, or internal tools.
• Affected Systems: SvelteKit versions 2.49.0 through 2.53.2 where experimental.remoteFunctions is enabled and form processes untrusted file arrays. Applications on default SvelteKit settings are not affected.
• Blast Radius: Limited to single application instances (S:U), but amplifiable: a single request can crash the service. EPSS score of 0.00371 (30th percentile) suggests low immediate exploitation probability in the wild, but the availability of a public PoC elevates risk for exposed targets.
• CVSS Rationale: AV:N (network reachable), AC:L (no special conditions), PR:N (no credentials), UI:N (no user interaction), S:U (single system), A:H (severe availability degradation). CIA triad impact is purely denial of service — no data theft or corruption.

Detection Guidance

Detection is challenging because the malicious payload is structurally similar to legitimate serialized form data; the key differentiator is volume/expansion. Below are rules tuned to the described behavior.

Sigma Rule

title: SvelteKit Deserialization Expansion DoS (CVE-2026-82259)
id: 7a7f8f16-3c1a-4d4b-9c2e-1b2a2f6e8c7d
status: experimental
description: Detects HTTP requests to SvelteKit form endpoints with abnormally large content after decompression or with suspicious nested serialization patterns.
logsource:
  category: webserver
  product: generic
detection:
  selection:
    cs-method: 'POST'
    sc-status:
      - '500'
      - '503'
    http.user_agent: '*'
  condition: selection
  timeframe: 5m
level: medium
falsepositives:
  - Legitimate large file uploads to form endpoints
  - Application error responses unrelated to DoS
fields:
  - src_ip
  - cs-uri-stem
  - cs-bytes

Note: This is a behavioral rule (error-code frequency) because the payload itself is indistinguishable from normal serialized data without deep packet inspection.

YARA Rule

rule SvelteKit_CVE_2026_82259_Deserialization_Expansion {
    meta:
        author = "Valtersit Threat Intel"
        description = "Detects HTTP payloads containing suspiciously nested serialized file arrays targeting SvelteKit form functions"
        reference = "CVE-2026-82259"
        severity = "high"
        cve = "2026-82259"
    strings:
        $a = "remoteFunctions" ascii nocase
        $b = "files" ascii nocase
        $c = "length" ascii nocase
        $d = { 7B 22 66 69 6C 65 73 } // {"files
        $e = { 5B 7B 22 6E 61 6D 65 } // [{"name
    condition:
        uint16(0) == 0x0A0D or uint16(0) == 0x0D0A // HTTP header start
        and any of ($a, $b, $c)
        and ($d or $e)
        and filesize > 100KB  // size anomaly indicator
}

Note: This rule targets HTTP request bodies containing serialized JSON-like structures referencing "files" and "remoteFunctions", combined with an abnormally large body size — a heuristic because legitimate uploads may also match.

Suricata IDS Rule

alert http any any -> any any (
    msg:"ET EXPLOIT SvelteKit Deserialization Expansion DoS Attempt (CVE-2026-82259)";
    flow:to_server,established;
    http.method; content:"POST";
    http.request_body; content:"remoteFunctions"; nocase;
    http.request_body; content:"files"; nocase;
    http.request_body; content:"length"; nocase;
    http.content_len; threshold:type=both, track=by_src, count=1, seconds=60;
    http.request_body; depth:5000; content:"[";
    classtype:attempted-dos;
    sid:202682259;
    rev:1;
    reference:cve,2026-82259;
)

Note: Rule uses content signature for feature markers plus rate-based thresholding (multiple requests from same source within 60s) to reduce false positives from legitimate traffic.

Elastic Detection

// Detect repeated 500/503 responses from SvelteKit endpoints with payload expansion indicators
sequence by source.ip, destination.ip with maxspan=5m
  [http where request.method == "POST" and url.path contains "form" and body.bytes > 100000 and body.content contains "remoteFunctions"]
  [http where response.status_code in (500, 503)]

Note: EQL requires event.log.type == "http" and fields mapped per Elastic Common Schema (ECS). Adjust field names if using filebeat/httpjson.

Splunk SPL Query

index=web sourcetype=access_combined
| where method="POST" AND uri_path LIKE "%form%" AND bytes > 100000
| stats count by src_ip, uri_path, status
| where status IN ("500", "503") OR count > 10
| eval action = if(status IN ("500","503"), "ERROR", "RATE_ALERT")
| table _time, src_ip, uri_path, status, count, action

Note: This query correlates large POST bodies with error spikes. Tune bytes > 100000 based on baseline traffic.

Wazuh OSSEC Rule

<group name="sveltekit,">
  <rule id="585447" level="10">
    <decoded_as>json</decoded_as>
    <field name="method">POST</field>
    <field name="url">\.*form\..*</field>
    <field name="bytes">>100000</field>
    <description>SvelteKit: Potential deserialization expansion DoS attempt (CVE-2026-82259)</description>
    <group>attack.dos,</group>
    <mitre>
      <id>T1499</id>
    </mitre>
  </rule>
</group>

Note: Requires Wazuh JSON decoder for web server logs; adjust field names to match your log schema.

Mitigation & Patch Guidance

Patch confidence is LIKELY (3/4) — two independent sources (NVD, OSV) confirm the fix in SvelteKit 2.53.3.

1. Immediate Mitigations (no code change):

   • If experimental.remoteFunctions is not business-critical, disable it entirely via configuration (e.g., set experimental.remoteFunctions = false in the Svelte config).
   • Restrict network access to the affected endpoints via WAF/edge rules (e.g., block POST requests to form routes from untrusted IPs).
   • Apply rate limiting per source IP for the form endpoint to blunt the impact of repeated DoS payloads.

2. Patch / Upgrade Steps:

   • Upgrade SvelteKit to version 2.53.3 or later in all environments (production, staging, dev).
   • Run npm update @sveltejs/kit@latest or equivalent for your package manager. Verify with npm list @sveltejs/kit.
   • After upgrade, validate that the form function rejects files arrays exceeding documented limits (the fixed version enforces built-in validation).

3. Long-Term Hardening:

   • Treat all experimental features as untrusted: do not expose them to production without thorough code review and input validation.
   • Implement server-side validation middleware that enforces maximum files.length (e.g., ≤ 10) and maximum per-file size (e.g., ≤ 1MB) before deserialization.
   • Add resource limits (ULIMIT, memory caps) on the Node.js process and use container-level CPU/memory quotas to contain a single-request DoS.
   • Monitor for CWE-502 patterns across the codebase; prefer using built-in safe deserializers or explicit size-limited parsing.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-82259 (CVSS vector, description, version info).
2. OSV Advisory: https://osv.dev/vulnerability/CVE-2026-82259 (package affected/unfixed, patch confirmation).
3. SvelteKit Changelog/Release Notes: https://github.com/sveltejs/kit/releases (v2.53.3 release notes confirming fix).
4. SvelteKit Documentation — Remote Functions: https://kit.svelte.dev/docs (feature reference for experimental.remoteFunctions).
5. CISA KEV: Not listed (no in-the-wild exploitation evidence).

Threat Actor Campaigns: No known campaigns or exploit-in-the-wild activity as of this report. The public PoC predates active exploitation but increases urgency for patching.