CVE-2026-6656: Perl Crypt::Password Timing Attack Flaw

CVE-2026-6656 Overview

CVE-2026-6656 is a timing attack vulnerability affecting Crypt::Password versions through 0.28 for Perl. The check_password method uses the built-in eq operator to compare password hashes. This comparison exits at the first differing byte, producing measurable timing differences that attackers can exploit. Remote attackers can use these timing discrepancies to progressively recover the underlying hash value byte by byte. The flaw is classified under CWE-208 (Observable Timing Discrepancy). The vulnerability affects any Perl application that authenticates users through the Crypt::Password module.

Critical Impact
Attackers can recover stored password hashes by measuring response time variations during authentication, undermining the confidentiality of credential data.

Affected Products

Crypt::Password Perl module versions through 0.28
Perl applications performing authentication using the module's check_password method
CPAN distributions bundling Crypt-Password-0.28 or earlier

Discovery Timeline

2026-07-20 - CVE-2026-6656 published to NVD
2026-07-20 - Openwall OSS-Security discussion published
2026-07-20 - Last updated in NVD database

Technical Details for CVE-2026-6656

Vulnerability Analysis

The vulnerability resides in the check_password method of the Crypt::Password module. The method compares a computed hash of the user-supplied password against the stored hash using Perl's built-in eq string equality operator. The eq operator performs a byte-by-byte comparison and short-circuits at the first mismatch. This behavior creates a side channel: comparisons that share more leading bytes take measurably longer than those that fail early. An attacker with network access to an authentication endpoint can submit crafted candidate hashes and measure server response times. By iteratively refining guesses based on timing feedback, the attacker recovers the stored hash one byte at a time. The recovered hash can then be attacked offline with dictionary or brute-force techniques to obtain plaintext credentials.

Root Cause

The root cause is the use of a non-constant-time string comparison for security-sensitive data. Cryptographic hash comparisons must run in time independent of input content. The eq operator does not satisfy this requirement. See the affected source lines in the MetaCPAN Crypt-Password Source.

Attack Vector

Exploitation is network-based and requires no authentication or user interaction. An attacker repeatedly submits authentication requests with chosen inputs and records precise round-trip timings. Statistical analysis over many samples isolates the timing signal from network jitter. The attacker infers matching byte positions in the stored hash and extends the known prefix until the full hash is recovered. Further discussion is available in CPAN Ticket #180162 and the Openwall OSS Security Discussion.

Detection Methods for CVE-2026-6656

Indicators of Compromise

High volumes of failed authentication attempts from a single source targeting the same account, with request patterns consistent with timing measurement.
Repeated authentication requests carrying systematically varied credential values against endpoints backed by Crypt::Password.
Elevated latency measurements or precision-timing probes recorded in web server access logs.

Detection Strategies

Inventory Perl deployments and identify applications importing Crypt::Password at version 0.28 or earlier.
Instrument authentication endpoints to log request timing and flag clients issuing high-cadence, low-entropy credential submissions.
Correlate authentication failure rates with source IP reputation and behavioral baselines in SIEM platforms.

Monitoring Recommendations

Alert on sustained authentication attempts exceeding a rate threshold per account or per source address.
Monitor for anomalous distributions in authentication response times, which may indicate active timing probes.
Track CPAN package inventories and flag any regression to vulnerable Crypt::Password versions during deployments.

How to Mitigate CVE-2026-6656

Immediate Actions Required

Identify all Perl applications using Crypt::Password and confirm the installed version through cpanm --info Crypt::Password or equivalent tooling.
Replace calls to check_password with a constant-time comparison such as Crypt::Util::constant_time_eq or a manually implemented XOR-based comparator.
Apply authentication rate limiting and account lockout controls to reduce the number of timing samples an attacker can collect.

Patch Information

At the time of publication, no fixed release of Crypt::Password is listed in the referenced advisories. Track CPAN Ticket #180162 and the Openwall OSS Security Discussion for upstream fix availability.

Workarounds

Wrap check_password in application code that performs constant-time hash comparison before returning results to the caller.
Introduce a small randomized delay on authentication responses to increase noise in timing measurements, while acknowledging this is a partial mitigation only.
Front authentication endpoints with a reverse proxy that enforces per-client request rate limits and blocks high-frequency credential probing.

bash

# Example: constant-time comparison replacement in Perl
# Replace: return $stored eq $computed;
# With a constant-time comparator:
use Crypt::Util;
my $cu = Crypt::Util->new;
return $cu->constant_time_eq($stored, $computed);