CVE-2026-43623 Overview
CVE-2026-43623 is a stack-based buffer overflow [CWE-121] in the microtar library through version 0.1.0. The flaw resides in the raw_to_header() function in src/microtar.c. The function uses strcpy() to copy 100-byte ustar format name and linkname fields that may lack null terminators, writing up to 355 bytes into a 100-byte destination buffer on the stack. Attackers can trigger the overflow by supplying a crafted TAR archive processed through mtar_open(), mtar_find(), or mtar_read_header(). Successful exploitation can corrupt adjacent stack memory and lead to arbitrary code execution in applications that link the vulnerable library.
Critical Impact
Attackers can corrupt stack memory and potentially execute arbitrary code in any application that parses untrusted TAR archives using microtar 0.1.0 or earlier.
Affected Products
- microtar library through version 0.1.0
- Applications that statically or dynamically link microtar to parse TAR archives
- Downstream projects bundling the unpatched src/microtar.c source file
Discovery Timeline
- 2026-06-01 - CVE-2026-43623 published to the National Vulnerability Database
- 2026-06-02 - Last updated in NVD database
Technical Details for CVE-2026-43623
Vulnerability Analysis
The vulnerability stems from unsafe string handling when microtar parses ustar-format TAR headers. The ustar specification defines name and linkname as fixed-width 100-byte fields that are not required to be null-terminated when the full field is used. raw_to_header() copies these fields into fixed 100-byte stack buffers using strcpy(), which keeps reading until it finds a null byte. When attacker-supplied fields lack terminators, strcpy() continues into adjacent header fields, writing up to 355 bytes into the 100-byte destination.
The overflow is triggered through any of the public entry points that read headers: mtar_open(), mtar_find(), and mtar_read_header(). Any application that opens an untrusted TAR archive with microtar is exposed.
Root Cause
The root cause is the use of strcpy() to copy non-null-terminated, fixed-width binary fields. The function trusts the in-archive layout as a C string instead of using a bounded copy such as memcpy() of exactly 100 bytes followed by explicit termination. The destination stack buffer has no guard, so adjacent saved frame pointers, return addresses, and local variables can be overwritten.
Attack Vector
Exploitation requires the victim application to process an attacker-controlled TAR archive. Delivery vectors include archives received over the network, uploaded to a service, or extracted from a downloaded package. User interaction is required to open or process the malicious file. No authentication is needed to craft the archive.
The vulnerability is described in prose only. No verified public exploit code is available at the time of writing. See the VulnCheck Advisory on Microtar Buffer Overflow for additional technical details.
Detection Methods for CVE-2026-43623
Indicators of Compromise
- TAR archives containing name or linkname fields that fill the full 100-byte width without a trailing null byte.
- Unexpected crashes, stack canary violations, or segmentation faults in processes that invoke mtar_open(), mtar_find(), or mtar_read_header().
- Child process spawns or outbound network connections originating from a parser process immediately after archive ingestion.
Detection Strategies
- Inventory binaries and packages that statically link microtar or include src/microtar.c and flag any at or below version 0.1.0.
- Apply YARA or hash-based scanning to identify the vulnerable raw_to_header() implementation in third-party software.
- Inspect TAR archives at gateways and validate that fixed-width header fields contain a null terminator within the declared width.
Monitoring Recommendations
- Enable Address Sanitizer (ASan) or compile with stack protectors in development and QA builds to surface overflow conditions before release.
- Alert on process crashes correlated with archive parsing activity in endpoint and application telemetry.
- Monitor file-write and process-creation events in services that accept user-supplied archives.
How to Mitigate CVE-2026-43623
Immediate Actions Required
- Identify all applications and container images that bundle microtar and treat any version at or below 0.1.0 as vulnerable.
- Block or sandbox processing of untrusted TAR archives until a patched build is deployed.
- Rebuild affected binaries with stack protection (-fstack-protector-strong), FORTIFY_SOURCE, and position-independent executables to raise exploitation cost.
Patch Information
No official patched release of microtar is referenced in the advisory at publication. Maintainer discussion is tracked in GitHub Issue #28, GitHub Issue #29, and GitHub Issue #30. Downstream vendors should apply a source-level fix that replaces strcpy() with a bounded copy of exactly 100 bytes followed by explicit null termination, and validate field contents before use.
Workarounds
- Pre-validate TAR archives with a hardened parser that rejects headers whose name or linkname fields lack a null terminator within the 100-byte field width.
- Run archive parsing in a low-privilege, sandboxed process such as a seccomp-restricted container or a separate user account with no network access.
- Replace microtar with a maintained TAR library where untrusted input must be processed in production.
# Compiler hardening flags for downstream rebuilds of microtar consumers
gcc -O2 -D_FORTIFY_SOURCE=2 -fstack-protector-strong \
-fPIE -pie -Wl,-z,relro,-z,now \
-o myapp main.c microtar.c
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
