CVE-2026-89090

Executive Summary

CVE-2026-89090 is an unrecovered panic in the event stream header decoder of the Amazon AWS SDK for Go v2, affecting releases prior to release-2026-03-23. An unauthenticated remote actor can send a crafted event stream response frame containing a header value type outside the valid range, causing the consuming application process to terminate. The vulnerability is a denial-of-service condition (CWE-248) with high availability impact and no confidentiality or integrity impact, scored CVSS 5.9.

Technical Analysis

The flaw resides in the event stream header decoder. Amazon services such as those that use the AWS event stream protocol (a binary framing format used for streaming responses) deliver frames consisting of headers and payload. Each header carries a value type field that must fall within a known, bounded set of valid types. The decoder in affected SDK versions does not validate that the incoming header value type is within the valid range before acting on it. When a frame supplies a value type outside that range, the decoder encounters an unrecovered (unhandled) panic condition rather than returning a structured error, which propagates up and terminates the Go process. This maps to CWE-248 (Uncaught Exception): the root cause is missing input validation on an attacker-influenced field combined with no top-level panic recovery in the decoding path.

Attack surface: any application built with the affected SDK that consumes event stream responses from a remote endpoint and processes untrusted frames. The remote actor does not require authentication to trigger the condition because the crafted frame is processed as part of the response-decoding path rather than behind an authorization check. The CVSS vector reflects network attack vector (AV:N), high attack complexity (AC:H — the attacker must be positioned to deliver a crafted event stream frame and the malformed value type must reach the vulnerable decoder path), no privileges required (PR:N), no user interaction (UI:N), scope unchanged (S:U), and high availability impact only (C:N/I:N/A:H). EPSS is not yet scored by FIRST.org, and no public PoC, Nuclei template, or exploit code is available. No patch evidence is present in tracked sources (patch confidence 0/4), though the CNA prescribes upgrading to release-2026-03-23 or later and patching forked/derivative code.

Exploitation Scenarios

No public PoC is available; the following chain is inferred from the vulnerability class (unvalidated header field → unhandled exception) and the limited details in the NVD description.

1. Reconnaissance / target identification. The attacker identifies a target application that uses an affected AWS SDK for Go v2 release and that consumes event stream responses over the network (for example, a service or client processing streaming responses proxied through or terminating at the target).

2. Triggering the flaw class. The attacker delivers an event stream response frame in which a header's value type is set to a value outside the valid range. Because the decoder does not validate the type before use, this malformed field reaches the decoding logic in an unexpected state.

3. Payload/execution method. At the vulnerability-class level, the attacker supplies a syntactically valid frame with an out-of-range type indicator. No function names, endpoints, or parameter names are disclosed in the available data, so the precise byte layout and delivery channel cannot be specified here beyond "crafted event stream response frame." The attacker does not need credentials.

4. Post-exploitation impact. The decoder raises an unrecovered panic that terminates the consuming application process — a denial-of-service condition. There is no evidence of code execution, data exfiltration, or persistence; impact is limited to availability (A:H).

Impact Assessment

The primary impact is availability loss: successful triggering crashes the application process, disrupting any service dependent on the SDK's event stream handling and potentially cascading to upstream consumers, load balancers, or dependent workloads. Because the attack is remote (AV:N) and requires no privileges (PR:N) or user interaction (UI:N), a reachable vulnerable endpoint broadens the blast radius to any exposed consumer of streamed AWS responses. Integrity and confidentiality are unaffected (C:N/I:N), and scope is unchanged (S:U). Exploitability is reduced by high attack complexity (AC:H), meaning the attacker must satisfy preconditions to place a crafted frame in the vulnerable path. Business impact includes service outages, failed streaming operations, restart loops, and operational cost from incident response. The CVSS base score is 5.9 (Medium). EPSS: not yet scored by FIRST.org. CISA KEV: not listed.

Detection Guidance

Sigma Rule

title: AWS SDK for Go v2 Event Stream Decoder Panic (CVE-2026-89090)
id: 8f2c1a44-0b7e-4d9a-9c3e-awsgosdkstream
status: experimental
description: Detects crashes/panics in Go applications referencing the AWS event stream header decoder, indicative of CVE-2026-89090 exploitation.
logsource:
  product: linux
  service: application
detection:
  selection_panic:
    message|contains:
      - 'panic'
      - 'runtime error'
  selection_decoder:
    message|contains:
      - 'event stream'
      - 'eventstream'
      - 'header decoder'
      - 'aws-sdk-go-v2'
  condition: selection_panic and selection_decoder
falsepositives:
  - Unrelated Go panics in code that coincidentally logs similar strings
level: medium
tags:
  - attack.t1499
  - cve.2026-89090

YARA Rule

rule CVE_2026_89090_AWS_Go_SDK_EventStream_Header
{
    meta:
        description = "Detects AWS SDK for Go v2 event stream header decoding references tied to CVE-2026-89090"
        author = "valtersit.com"
        date = "2026-03-23"
        cve = "CVE-2026-89090"
    strings:
        $s1 = "aws-sdk-go-v2" ascii wide
        $s2 = "eventstream" ascii nocase
        $s3 = "event stream header" ascii nocase
        $s4 = "HeaderValueType" ascii nocase
        $s5 = "release-2026-03-23" ascii
    condition:
        uint16(0) == 0x5A4D or uint32(0) == 0x464C457F
        and filesize < 200MB
        and any of ($s1, $s2, $s3, $s4)
        and not $s5
}

Suricata IDS Rule

alert tcp any any -> any any (msg:"CVE-2026-89090 Potential AWS Event Stream Header Type Abuse"; flow:established,to_client; content:"|00 00|"; depth:2; byte_test:1,>,127,0,relative; threshold:type limit, track by_src, count 1, seconds 60; classtype:attempted-dos; sid:90089090; rev:1; reference:cve,2026-89090;)

Elastic Detection

process where event.action == "process-terminated" or event.action == "crashed"
  and process.name != null
  and (
    process.args like~ "*event*stream*" or
    process.command_line like~ "*aws-sdk-go-v2*"
  )
  and message like~ "*panic*"

Splunk SPL Query

index=app_logs (message="*panic*" OR message="*runtime error*")
  (message="*event stream*" OR message="*eventstream*" OR message="*aws-sdk-go-v2*")
| stats count min(_time) as first_seen max(_time) as last_seen by host, process, message
| where count > 0
| convert ctime(first_seen) ctime(last_seen)

Wazuh OSSEC Rule

<group name="aws_sdk_dos,cve_2026_89090,">
  <rule id="100890" level="10">
    <decoded_as>json</decoded_as>
    <match>panic|runtime error</match>
    <field name="message">event stream|eventstream|aws-sdk-go-v2</field>
    <description>CVE-2026-89090: Go application panic in AWS SDK event stream decoder (possible DoS)</description>
    <mitre>
      <id>T1499</id>
    </mitre>
    <group>denial_of_service,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source).

1. Immediate mitigations:

• Upgrade the AWS SDK for Go v2 to release-2026-03-23 or later, and patch any forked or derivative code that vendors the affected decoder logic — this is the CNA-prescribed remediation.
• Where upgrade cannot be performed immediately, isolate or restrict network paths that deliver event stream responses to the consuming application, since exploitation requires the attacker to place a crafted frame in the decoding path.
• Add process supervision (restart policies, health checks) to reduce downtime from process termination, and apply top-level panic recovery/sandboxing around stream-decoding workloads as a temporary compensating control.

2. Patch/upgrade steps:

• Pin the dependency to release-2026-03-23 or later, rebuild, and redeploy all services and consumers of event stream responses.
• Audit vendor forks and copied code; reapply the fix or replace the affected decoder.
• Verify the deployed artifact no longer resolves the vulnerable SDK version (e.g., go.mod / vendor tree inspection).

3. Long-term hardening:

• Enforce strict input validation and bounds checking for all externally influenced framing/protocol fields, treating value-type fields as untrusted.
• Wrap network-facing decoders with recover-based guards so malformed input yields an error rather than a process crash.
• Track SDK/dependency advisories and automate dependency upgrades; monitor OSV/NVD for CVE-2026-89090 updates and any future EPSS scoring.
• Maintain forked code governance so upstream fixes are propagated.

References

• NVD — CVE-2026-89090: https://nvd.nist.gov/vuln/detail/CVE-2026-89090
• CWE-248 — Uncaught Exception: https://cwe.mitre.org/data/definitions/248.html
• CISA Known Exploited Vulnerabilities Catalog (CVE-2026-89090 not listed at time of writing): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• FIRST EPSS (score not yet published): https://www.first.org/epss/
• AWS SDK for Go v2 project repository (verify release-2026-03-23 tag): https://github.com/aws/aws-sdk-go-v2

No threat actor campaigns or exploit-in-the-wild evidence are known; no public PoC, Nuclei template, or exploit code is available.