CVE-2026-27821: Critical Stack Overflow in GPAC NHML Parser Opens Door to Remote Crashes and Possible Code Execution

GPAC NHML Parser Stack Buffer Overflow

CVE ID: CVE-2026-27821
Product: GPAC
Component: NHML demuxer (dmx_nhml.c)
Vulnerability Type: Stack-Based Buffer Overflow
CWE: CWE-121
CVSS Score: 7.7 (High)
Attack Vector: Network / File-based input
Privileges Required: None
User Interaction: None (if automatically processed)
Impact: Denial of Service / Potential Remote Code Execution
Exploit Availability: Public PoC

Overview

A stack-based buffer overflow vulnerability was identified in the NHML parser component of GPAC. The issue exists due to improper handling of the xmlHeaderEnd XML attribute inside the NHML demuxer code.

A fixed-size stack buffer of 1000 bytes is allocated to store this attribute. The value is copied using an unsafe function without validating length boundaries. When an excessively long value is supplied, memory beyond the stack buffer is overwritten.

This condition leads to stack corruption. Depending on compilation settings and runtime protections, exploitation may result in:

Immediate application crash (Denial of Service)
Stack smashing detection and forced termination
Instruction pointer overwrite
Arbitrary code execution in less protected environments

The flaw affects GPAC versions up to and including 26.02.0.

Technical Root Cause

Inside the NHML parsing routine:

A local stack buffer of 1000 bytes is defined.
The value of the xmlHeaderEnd attribute is copied into this buffer.
No bounds checking is performed before copying.
If the supplied value exceeds 1000 bytes, adjacent stack memory is overwritten.

Because this occurs on the stack, critical data such as:

Saved frame pointer
Return address
Stack canary (if present)

may be corrupted.

If stack protection mechanisms are enabled, the program typically aborts. If protections are disabled or bypassed, execution control can be hijacked.

Affected Environments

This vulnerability impacts:

Media processing pipelines using GPAC
Automated transcoders
Media ingestion servers
Streaming backend services
Applications parsing untrusted NHML files
Desktop tools like MP4Box if handling malicious input

Systems running hardened builds (stack protector, ASLR, NX) reduce exploitability but do not eliminate crash impact.

Exploitation Details

Attack Scenario

A crafted NHML file is created.
The xmlHeaderEnd attribute is filled with a long string (greater than 1000 bytes).
The file is delivered via:
- File upload
- Remote media ingestion
- Email attachment
- Web application upload endpoint
GPAC processes the file.
Stack buffer overflow occurs.
Process crashes or code execution occurs depending on environment protections.

Payload Characteristics

A typical proof-of-concept file contains:

Excessively long string inside: xmlHeaderEnd=”AAAAAAAAAAAA…(repeated)…”
May include:
- Pattern offsets
- ROP chains
- Encoded shellcode
- Stack pivot attempts

Public PoC examples demonstrate crash conditions under AddressSanitizer. Code execution has not been broadly weaponized but theoretical feasibility exists in non-hardened builds.

PoC material is publicly available for educational and research purposes only.

Impact Assessment

Confidentiality Impact

If exploited for code execution, attackers may read sensitive media, configuration files, or tokens accessible to the process.

Integrity Impact

Arbitrary commands could be executed under the application’s privilege context.

Availability Impact

Most realistic outcome in hardened systems is process termination. Media services may repeatedly crash upon receiving malicious files.

Exploitability Analysis

Condition	Result
Stack protector enabled	Process abort
ASLR + NX enabled	Harder to exploit
No memory protections	Possible RCE
Service auto-restarts	Repeated crash loop

The vulnerability requires only that malicious input be processed. No authentication is needed if exposed upload functionality exists.

Detection Strategy

Detection should focus on:

Abnormal NHML attribute lengths
Process crash indicators
Stack corruption messages
Suspicious file uploads
Unexpected child processes

Host-Based Detection

Linux Journal Logs

Query:

journalctl -xe | grep -E "segfault|stack smashing|gpac|MP4Box"

Syslog Monitoring

grep -i "stack smashing detected" /var/log/syslog

Auditd Rule Example

Monitor execution anomalies:

-w /usr/bin/MP4Box -p x -k gpac_exec_monitor

EDR Hunting Query (Generic)

process.name IN ("gpac", "MP4Box") 
AND (process.exit_code != 0 OR process.crash = true)

Network Detection

Suricata Rule

alert http any any -> any any (msg:"GPAC NHML xmlHeaderEnd Overflow Attempt"; 
flow:to_server,established; 
content:"xmlHeaderEnd=\""; 
pcre:"/xmlHeaderEnd=\".{1000,}/s"; 
sid:900001; rev:1;)

Snort Rule

alert tcp $EXTERNAL_NET any -> $HTTP_SERVERS $HTTP_PORTS 
(msg:"EXPLOIT GPAC NHML Overflow"; 
flow:to_server,established; 
content:"xmlHeaderEnd=\""; 
pcre:"/xmlHeaderEnd=\".{1000,}/s"; 
classtype:attempted-admin; sid:900002; rev:1;)

Web Server Log Hunting

Apache / Nginx

Search for large request bodies:

grep "POST" access.log | awk '$10 > 50000'

Search for NHML references:

grep -i ".nhml" access.log

File-Based Detection

YARA Rule

rule GPAC_NHML_Long_Attribute
{
    strings:
        $attr = /xmlHeaderEnd=("[\x20-\x7E]{1000,}")/ nocase
    condition:
        $attr
}

Forensic Artifacts

If exploitation is suspected:

Core dumps
Crashed binary stack traces
Malicious NHML file copy
Network capture around upload
Application restart logs
Memory dump (if RCE suspected)

Mitigation

Immediate Actions

Upgrade GPAC to patched version.
Restrict NHML file ingestion from untrusted sources.
Apply upload size limits.
Enable application sandboxing.

Hardening Recommendations

Compile with:
- -fstack-protector-strong
- -D_FORTIFY_SOURCE=2
Enable ASLR
Run service with minimal privileges
Use container isolation

Official Patch / Upgrade

Upgrade to a version containing the official fix commit:

Official Patch Commit:
https://github.com/gpac/gpac/commit/9bd7137fded2db40de61a2cf3045812c8741ec52

It is recommended that the latest stable GPAC release be deployed rather than manually patching older builds.

Risk Rating Justification

The vulnerability is rated High because:

No authentication required
Remote delivery possible
Stack corruption involved
Potential RCE under weak protections

However, modern OS mitigations reduce widespread weaponization likelihood.

Conclusion

CVE-2026-27821 represents a classic unsafe memory handling issue in a widely used multimedia framework. While many environments will experience crash behavior rather than full exploitation, the presence of a stack overflow in file parsing code must be treated seriously.

Any system accepting external media files and using GPAC should be patched immediately. Detection controls should be implemented to monitor large NHML attribute usage and unexpected process crashes.