CVE-2026-6068

Executive Summary

CVE-2026-6068 is a heap use-after-free vulnerability in NASM (Netwide Assembler) located in the response-file (-@) processing logic. The flaw arises when a dangling pointer to freed memory is stored in the global depend_file variable and later dereferenced after the response-file buffer has already been freed, which can lead to data corruption or unexpected program behavior. The vulnerability is present in the NASM development tree (confirmed at commit b1ee0338a3f8027cf6886ca21ee31d444c615373) and is currently unpatched across all tracked sources. Successful exploitation requires an attacker to supply a malicious response file to a victim running NASM, with a CVSS v3.1 base score of 6.5 reflecting network-based delivery, no privileges, and no user interaction beyond opening the file.

Technical Analysis

Vulnerability Mechanism

CVE-2026-6068 is classified under CWE-416: Use After Free, a memory-corruption class in which a program continues to dereference a pointer after the memory it references has been freed. In this specific instance, the flaw resides in NASM's response-file processing path, activated via the -@ command-line option.

Response files are text files that contain additional command-line arguments, allowing users to pass large or complex argument sets to NASM without exceeding OS command-line length limits. When NASM processes a response file, it reads the file contents into a heap-allocated buffer, parses the arguments, and subsequently uses certain parsed values to populate internal global state.

The NVD description identifies two specific artifacts of the flaw:

1. A dangling pointer is stored in the global variable depend_file — this global is used by NASM's dependency-generation feature (-MD, -MF, etc.) to track the output dependency file path.
2. The response-file buffer is freed before the pointer stored in depend_file is later dereferenced.

The lifetime mismatch is the core issue: NASM allocates the response-file buffer, parses arguments from it, and stores a pointer into that buffer (or a pointer derived from it) inside depend_file. The buffer is then freed — likely at the end of the response-file parsing routine — but depend_file still holds the orphaned address. Later in the execution pipeline (e.g., during dependency-file generation or at program teardown), NASM dereferences depend_file, touching freed heap memory.

Technical Consequences

The consequences of dereferencing freed heap memory are two-fold:

1. Data corruption: Between the free and the dereference, the heap allocator may reuse the freed chunk for a subsequent allocation. If the depend_file pointer is used as a string (e.g., in a file-path context), the assembler may read attacker-influenced bytes from a reused heap chunk and interpret them as a file path, potentially writing output dependency data to an attacker-controlled location.

2. Unexpected behavior / crash: If the freed memory has been returned to the OS or consolidated into a large free chunk, dereferencing the dangling pointer may trigger a segmentation fault, causing a denial-of-service condition for the assembling process.

The exact code-level sequence (function names, call sites, allocation sizes) is not disclosed in the NVD description beyond the identifiers depend_file and the -@ option. No further code-level detail is available from the provided data.

Attack Surface

The attack surface is the NASM command-line interface. The vulnerability is triggered when NASM processes a specially crafted response file. Because response files may be nested (a response file may reference another response file), and because the content is fully attacker-controlled, the input to the vulnerable path is entirely up to the attacker.

Important context: the CVSS vector AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N implies network delivery with no user interaction. This maps to a scenario where a user downloads and opens a malicious project archive or receives a crafted response file. In automated build pipelines that fetch untrusted source archives and invoke NASM with embedded response files, the exploitation potential is materially higher.

Exploitation Scenarios

Prerequisite: An attacker must be able to supply a malicious response file (or a project archive containing one) that a victim processes with NASM using the -@ flag.

Step 1: Reconnaissance / Target Identification

An attacker identifies targets that invoke NASM with response files. Common victim profiles include:

• Developers building projects that vendor NASM or use it as a build tool.
• CI/CD pipelines that compile assembly code from untrusted upstream archives.
• Security researchers or reverse engineers assembling modified source trees.

Attacker research focuses on discovering projects that (a) accept untrusted input archives, (b) invoke NASM programmatically, and (c) use response-file functionality for argument passing.

Step 2: Triggering the Flaw Class

The attacker crafts a response file whose content is designed to place attacker-controlled data in the heap region that will later be reused. Because the vulnerability is a use-after-free, successful exploitation generally requires heap grooming — the attacker must influence subsequent allocations so that the freed memory region is reallocated and filled with attacker-chosen bytes prior to the dereference of the dangling depend_file pointer.

At a vulnerability-class level, the attacker controls two inputs:

• The contents of the response file (primary trigger).
• Additional assembler inputs (source files, include paths) that cause NASM to perform intermediate heap allocations between the free and the dereference.

Step 3: Payload / Execution Method

The NVD description indicates the impact is "data corruption or unexpected behavior" — it does not state that arbitrary code execution is achievable. Based strictly on the description, the most plausible attacker objectives are:

• File write redirection: By corrupting the depend_file path string, an attacker may cause NASM to write dependency output (-MD output) to an arbitrary file path the attacker chooses, potentially overwriting files writable by the NASM process.
• Denial of service: Triggering an immediate crash by dereferencing freed memory that has been unmapped.

No public PoC demonstrating code execution is available; the PoC/Exploit status is listed as Public, but exploit code is not available. The public PoC likely consists of a crafted response file that reliably triggers a crash or visible memory corruption.

Step 4: Post-Exploitation Impact

The CVSS sub-scores (Confidentiality Impact: Low, Integrity Impact: Low, Availability Impact: None) bound the post-exploitation impact to:

• Confidentiality (Low): If heap-layout knowledge permits reading freed/reused memory contents, limited information disclosure of other heap-stored data is conceivable.
• Integrity (Low): Corruption of dependency output or influence over depend_file-driven file writes.
• Availability (None per CVSS): Although a crash is possible, the CVSS score indicates the attacker generally does not achieve persistent availability impact in the scored scenario.

The use-after-free does not immediately yield code execution without a reliable heap-exploitation primitive, which has not been publicly demonstrated.

Impact Assessment

Business Impact:

• Build-Pipeline Integrity: Organizations running NASM in automated build environments that ingest third-party assembly source are at risk. A malicious source archive can corrupt dependency files or crash builds, poisoning the software supply chain at the build stage.
• Data Integrity: The depend_file corruption primitive could allow an attacker to redirect dependency-file output to arbitrary writable paths, potentially overwriting build artifacts or configuration files.
• Availability: No CVSS availability impact is scored, but in practice a crafted response file can crash NASM, disrupting builds that depend on it.

Affected Systems:

• All NASM builds from the development tree at or after commit b1ee0338a3f8027cf6886ca21ee31d444c615373 that include the vulnerable response-file handling code.
• No stable release version has been explicitly listed as affected; the OSV package entry identifies the development commit as the earliest confirmed vulnerable point. Downstream distributions that snapshot the development tree may be affected.

Blast Radius:

• Local: Any user who assembles untrusted code with the -@ option.
• Network-Indirect: The CVSS vector AV:N reflects remote delivery of the malicious file, not a remotely reachable service. The blast radius is limited to systems where a user processes attacker-supplied response files.

CVSS v3.1 Breakdown (from the stated vector):
Base Score: 6.5 (Medium)
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

• Attack Vector: Network (file delivered remotely)
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None (file processing is automated/scripted)
• Scope: Unchanged
• Confidentiality: Low
• Integrity: Low
• Availability: None

EPSS Context: EPSS score 0.00426 (35.7th percentile) indicates a low current probability of exploitation in the wild, consistent with no known exploit campaigns.

Detection Guidance

Sigma Rule

title: NASM Response File Use-After-Free Exploitation Attempt
id: 9f1c3b2a-4d5e-4f6a-8b7c-1d2e3f4a5b6c
status: experimental
description: Detects NASM invocation with response file (-@) processing followed by abnormal crash or dependency file manipulation, indicative of CVE-2026-6068 exploitation activity.
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-6068
author: Valtersit Threat Intelligence
date: 2026/05/15
logsource:
    category: process_creation
    product: linux
detection:
    selection_nasm:
        Image|endswith: '/nasm'
        CommandLine|contains|all:
            - '-@'
    selection_crash:
        # Process exit code indicating segfault (139) or abnormal termination
        ExitCode:
            - 139
            - 134
    condition: selection_nasm and selection_crash
falsepositives:
    - Legitimate NASM crashes unrelated to this vulnerability
    - NASM response files processed with intentional malformed content in testing
level: medium
tags:
    - attack.t1203
    - cve.2026-6068
    - cwe.416

YARA Rule

rule CVE_2026_6068_NASM_ResponseFile_HeapSpray
{
    meta:
        author = "Valtersit Threat Intelligence"
        description = "Detects potential heap-grooming payloads embedded in NASM response files targeting CVE-2026-6068 use-after-free"
        cve = "CVE-2026-6068"
        cwe = "CWE-416"
        date = "2026-05-15"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-6068"
    strings:
        // Common NASM command-line tokens found in response files
        $nasm_flag = "-f" ascii
        $nasm_object = "-o" ascii
        $nasm_dep = "-MD" ascii
        $nasm_depfile = "-MF" ascii
        $nasm_at = "-@" ascii

        // Heap groom pattern: repetitive long argument sequences without spaces
        // often used to control heap layout across allocations
        $heap_groom_1 = { 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 } // 16x 'A'
        $heap_groom_2 = { 42 42 42 42 42 42 42 42 42 42 42 42 42 42 42 42 } // 16x 'B'
        $heap_groom_3 = { 43 43 43 43 43 43 43 43 43 43 43 43 43 43 43 43 } // 16x 'C'
    condition:
        uint32be(0) == 0x2D40 // file starts with "-@" or contains it early
        and 2 of ($nasm_*)
        and 1 of ($heap_groom_*)
        and filesize < 1MB // response files are typically small
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-6068 NASM Response File Use-After-Free - Malicious Download Attempt"; flow:established,to_client; content:"nasm"; nocase; http.header_names; content:".zip"; http.uri; content:".tar"; http.uri; metadata:created_at 2026_05_15, updated_at 2026_05_15; reference:url,https://nvd.nist.gov/vuln/detail/CVE-2026-6068; sid:20266068; rev:1;)

Note: NASM is not a network service; the Suricata rule above detects the delivery channel (download of a project archive potentially containing a malicious response file). Since no network protocol carries NASM response files natively, a more precise network signature is not feasible without knowing the transport (HTTP, FTP, SMB). A template rule for HTTP archive download is provided as a generic delivery-detection heuristic; adjust the file extensions and host patterns to match the organization's ingestion channels.

Elastic Detection

// Detect NASM process execution with response file then abnormal exit
sequence by host.name, process.entity_id with maxspan=30s
  [process where event.type == "start" and
   process.name == "nasm" and
   process.args : ("-@", "*@*")]
  [process where event.type == "end" and
   process.name == "nasm" and
   process.exit_code in (139, 134)]

// Detect concurrent file write to dependency target followed by NASM crash
sequence by host.name with maxspan=60s
  [file where event.action == "write" and
   file.path : "*/*.d" and
   process.name == "nasm"]
  [process where event.type == "end" and
   process.name == "nasm" and
   process.exit_code == 139]

Splunk SPL Query

index=windows OR index=linux sourcetype=WinEventLog:Security OR sourcetype=linux_audit
| search (CommandLine="*-@*" AND Image="*nasm*") OR (process="*nasm*" AND cmdline="*-@*")
| eval crash_indicators=if(match(exit_code, "139|134"), "true", "false")
| eval dep_write=if(match(file_path, "\.d$"), "true", "false")
| where crash_indicators="true" OR dep_write="true"
| stats count by host, user, Image, CommandLine, exit_code, file_path
| sort - count
| table _time, host, user, Image, CommandLine, exit_code, file_path, count

Wazuh OSSEC Rule

<group name="nasm,memory_corruption,cve_2026_6068,">
  <!-- Rule 1: NASM crash (SIGSEGV=139 or SIGABRT=134) after response file processing -->
  <rule id="100501" level="12">
    <if_sid>571</if_sid>
    <field name="audit.data.syscall">execve</field>
    <match>nasm</match>
    <regex>-\@</regex>
    <description>CVE-2026-6068: NASM terminated abnormally after response file (-@) processing - possible use-after-free</description>
    <group>memory_corruption,cve_2026_6068,</group>
    <mitre>
      <id>T1203</id>
    </mitre>
  </rule>

  <!-- Rule 2: Auditd - nasm process crash exit -->
  <rule id="100502" level="10">
    <if_sid>571</if_sid>
    <field name="audit.data.syscall">execve</field>
    <match>nasm</match>
    <description>NASM process exit code suggests crash (SIGSEGV/SIGABRT) - investigate for CVE-2026-6068</description>
    <group>memory_corruption,cve_2026_6068,</group>
  </rule>

  <!-- Rule 3: Syslog kernel segfault message referencing nasm -->
  <rule id="100503" level="12">
    <if_sid>1002</if_sid>
    <match>nasm</match>
    <regex>segfault at</regex>
    <description>CVE-2026-6068: Kernel reported segfault in NASM process - use-after-free exploitation suspected</description>
    <group>memory_corruption,linux,kernel,cve_2026_6068,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Status: UNPATCHED — Patch Confidence 0/4. No patch evidence from any tracked source. The OSV package entry shows the vulnerable commit b1ee0338a3f8027cf6886ca21ee31d444c615373 with status "unfixed." No vendor advisory, no CVE-mitigating release, no backport has been identified.

1) Immediate Mitigations (until a patch exists)

• Do not process untrusted response files: Treat any NASM -@ response file originating from external sources (downloaded projects, third-party SDKs, email attachments) as potentially malicious. Manually review response file contents before invocation.
• Avoid -@ in automated pipelines: Refactor build scripts to pass arguments directly on the command line where feasible, eliminating the vulnerable code path entirely.
• Substitute depend_file usage: If dependency generation is not strictly required, omit -MD/-MF flags to avoid dereferencing the corrupted global state, reducing the tangible impact of the flaw.
• Run NASM in a sandbox: Execute NASM inside a container or virtual machine with restricted filesystem write access when processing untrusted source, limiting the blast radius of a potential arbitrary-file-write primitive.
• Monitor for crashes: Deploy the detection rules above to alert on NASM segfaults/aborts, which may indicate an exploitation attempt against CVE-2026-6068.

2) Patch / Upgrade Steps

• No official patch available. Monitor the following for updates:
  • The official NASM repository and mailing list (https://www.nasm.us/).
  • Downstream Linux distributions (Debian, Ubuntu, Fedora, RedHat) that package NASM — check for backported patches in security trackers.
  • OSV database entry (https://osv.dev/vulnerability/) for the referenced commit b1ee0338a3f8027cf6886ca21ee31d444c615373.
• If patched in the future: Upgrade to the fixed NASM release and verify the depend_file pointer lifetime has been corrected (the fix should ensure the response-file buffer is not freed before all references to its contents are cleared or that depend_file is deep-copied).

3) Long-Term Hardening

• Source provenance verification: In CI/CD pipelines, verify checksums/signatures of any downloaded source archives before build steps that invoke assemblers.
• Compiler hardening: When building NASM from source, enable ASLR, stack canaries, and FORTIFY_SOURCE. Note: these do not mitigate heap use-after-free but reduce exploit reliability.
• Regular security reviews: Include NASM and other command-line build tools in the software inventory and vulnerability scanning process (OSV-Scanner, Trivy, Grype).
• Fallback alternatives: Evaluate whether the assembler workflow can migrate to a maintained alternative (e.g., YASM, LLVM's integrated assembler) for automated builds where response-file processing is common.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-6068 — Official CVE record with CVSS v3.1 vector, CWE-416 classification, and NVD description.
2. OSV Database Entry: https://osv.dev/vulnerability/CVE-2026-6068 — Package-level tracking including vulnerable commit b1ee0338a3f8027cf6886ca21ee31d444c615373 and current "unfixed" status.
3. NASM Official Site: https://www.nasm.us/ — Monitor for advisory and future patched releases.
4. CWE-416 Reference: https://cwe.mitre.org/data/definitions/416.html — MITRE classification of Use After Free.

Threat Actor Campaigns / Exploit-in-the-Wild Evidence: None identified. CISA KEV status is NO (not listed in Known Exploited Vulnerabilities catalog). EPSS score of 0.00426 (35.7th percentile) reflects a low but non-trivial probability of exploitation, consistent with no observed in-the-wild activity to date. A public PoC described as causing heap corruption/crash exists, but no weaponized exploit code has been released.