CVE-2026-57432: Perl Buffer Overflow Vulnerability

CVE-2026-57432 Overview

CVE-2026-57432 is an integer overflow vulnerability in Perl versions through 5.43.10. The flaw resides in the S_measure_struct function used by the pack and unpack built-ins. When Perl calculates the total size of a template structure, it multiplies each item's size by its repeat count without validating for overflow. A crafted template with a large repeat count wraps the signed SSize_t total to a negative value, allowing the @, X, and x position codes to advance the buffer pointer out of bounds.

The vulnerability results in an out-of-bounds heap read [CWE-125] that can leak adjacent memory contents to the caller.

Critical Impact
A pack or unpack template derived from untrusted input can read heap memory past the allocated buffer, exposing sensitive process data to attackers.

Affected Products

Perl versions through 5.43.10
Applications embedding Perl interpreters that process untrusted pack/unpack templates
Linux distributions and platforms shipping vulnerable Perl builds

Discovery Timeline

2026-07-13 - CVE-2026-57432 published to NVD and disclosed on the oss-security mailing list
2026-07-14 - Last updated in NVD database

Technical Details for CVE-2026-57432

Vulnerability Analysis

The defect lives in S_measure_struct inside pp_pack.c. This helper walks a pack or unpack template and computes the total byte size needed for the operation. For each item, it adds len * size to a running total of type SSize_t, a signed integer. No arithmetic overflow check exists before the addition.

When an attacker supplies a template containing a very large repeat count, the multiplication and accumulation wrap around and produce a negative total. Downstream code that handles the position opcodes @, X, and x compares the computed length against buffer bounds using signed arithmetic. A negative value satisfies the guard and causes the buffer pointer to move outside the intended allocation. Subsequent reads return heap contents adjacent to the buffer.

Root Cause

The root cause is missing overflow validation on signed integer arithmetic when tallying structure sizes. Signed length comparisons in the position-handling code then trust the corrupted total, converting an arithmetic bug into an out-of-bounds heap read.

Attack Vector

Exploitation requires the target application to pass an attacker-influenced template string to pack or unpack. This pattern appears in Perl programs that build templates from configuration files, network protocol descriptors, or user-supplied form fields. The attack vector is local because it requires code execution within the Perl process context, but the impact spans confidentiality, integrity, and availability.

// Patch from pp_pack.c adds explicit overflow guards
        break;
    }
}
+        if ((size > 0) &&
+                ((len > SSize_t_MAX / size) ||         /* detect overflow of len * size */
+                 (len * size > SSize_t_MAX - total)))  /* detect overflow of total + len * size */
+            croak("Pack template structure size is too large");
        total += len * size;
    }
    return total;

Source: GitHub Perl Patch Commit 5f7eb6b

Detection Methods for CVE-2026-57432

Indicators of Compromise

Perl processes crashing or aborting with Pack template structure size is too large after patching, indicating attempted exploitation attempts
Unusual memory contents returned from Perl scripts that consume external template data
Log entries showing pack or unpack invocations with abnormally large numeric repeat counts sourced from untrusted input

Detection Strategies

Perform static analysis of Perl codebases to identify calls to pack and unpack whose template argument is derived from user input, network payloads, or file contents
Enable Perl warnings and monitor stderr for pack-related diagnostics on production hosts
Deploy runtime memory safety tooling such as AddressSanitizer builds of Perl in test environments to surface out-of-bounds reads

Monitoring Recommendations

Track installed Perl versions across the fleet with software inventory tools and flag any release at or below 5.43.10
Monitor process telemetry for Perl interpreters exhibiting unexpected crashes, segmentation faults, or abnormal memory access patterns
Alert on outbound data flows from Perl-based services that contain patterns consistent with leaked heap memory

How to Mitigate CVE-2026-57432

Immediate Actions Required

Upgrade Perl to a fixed release that includes commits 40754ed and 5f7eb6b from the upstream repository
Audit application code that passes attacker-controlled data into pack or unpack templates and add server-side validation on any numeric repeat counts
Apply distribution security updates as vendors ship rebuilt Perl packages

Patch Information

The fix is delivered in two upstream commits. Commit 40754ed reworks size calculations for the B, b, H, and h format codes to avoid additional overflow paths. Commit 5f7eb6b adds an explicit SSize_t_MAX overflow check in S_measure_struct and introduces the Pack template structure size is too large diagnostic. Additional discussion is available on the OpenWall OSS-Security list.

Workarounds

Sanitize and cap repeat counts before passing them to pack or unpack, rejecting templates whose computed size exceeds a safe application-defined threshold
Refactor high-risk code paths to use fixed, hard-coded template strings rather than dynamically assembled ones
Run Perl-based services with reduced privileges and process isolation to limit the value of any leaked heap contents

bash

# Verify the installed Perl version and confirm it is above 5.43.10
perl -v | head -n 2

# On Debian/Ubuntu, apply distribution updates once available
sudo apt update && sudo apt install --only-upgrade perl perl-base

# On RHEL/Fedora derivatives
sudo dnf update perl