CVE-2026-75589: Net::OAuth Perl Timing Attack Vulnerability

CVE-2026-75589 Overview

CVE-2026-75589 is a timing side-channel vulnerability in the Net::OAuth module for Perl in versions before 0.33. The module verifies HMAC-SHA1, HMAC-SHA256, and PLAINTEXT signatures using Perl's eq operator, which performs non-constant-time string comparison. An attacker who can submit OAuth messages and measure server response times can recover valid signatures one byte at a time. Under the PLAINTEXT signature method, the comparison target is the signature key itself, meaning successful exploitation leaks the consumer_secret and token_secret. This flaw is classified as an observable timing discrepancy [CWE-208]. RSA-SHA1 verification is not affected because it validates through an RSA key object rather than string comparison.

Critical Impact
Attackers with network timing access can recover OAuth signatures and, under PLAINTEXT, extract long-lived consumer_secret and token_secret values.

Affected Products

Net::OAuth for Perl versions before 0.33
Applications using HMAC_SHA1 signature verification via Net::OAuth
Applications using HMAC_SHA256 or PLAINTEXT signature verification via Net::OAuth

Discovery Timeline

2026-08-19 - CVE-2026-75589 published to the National Vulnerability Database
2026-08-19 - Advisory posted to the OpenWall OSS-Security list
2026-08-19 - Last updated in NVD database

Technical Details for CVE-2026-75589

Vulnerability Analysis

The vulnerability resides in the verify routines of the Net::OAuth::SignatureMethod::HMAC_SHA1, HMAC_SHA256, and PLAINTEXT modules. Each routine compared the received signature against the locally computed value using Perl's eq operator. The eq operator returns as soon as the first differing byte is encountered, so the time to reject an invalid signature grows with the length of the matching prefix.

An attacker who can submit crafted OAuth messages and record precise response timings can iterate one byte at a time. By identifying which candidate byte produces a marginally longer processing time, the attacker reconstructs the full signature without exhausting the entire signature space.

The impact varies by signature method. For HMAC_SHA1 and HMAC_SHA256, an attacker recovers a valid signature for a specific message, enabling message forgery for that request. For PLAINTEXT, the comparison operand is the concatenated secret material itself, so the attack directly discloses the consumer_secret and token_secret.

Root Cause

The root cause is the use of an early-exit string comparison operator for cryptographic material. Constant-time comparison functions process every byte regardless of match position, eliminating the timing oracle. Net::OAuth previously relied on the standard Perl eq operator, which is not designed for cryptographic use.

Attack Vector

Exploitation requires the ability to submit OAuth requests to a verifier and observe response latency. Remote network attackers with a stable timing channel can perform the recovery, though practical exploitation typically requires many requests and low network jitter. No authentication or user interaction is required against endpoints that accept unauthenticated OAuth verification attempts.

text

// Patch excerpt from Net-OAuth Changes file
- HMAC_SHA1 and HMAC_SHA256 signature keys cannot be blank.

+ HMAC_SHA1, HMAC_SHA256 and PLAINTEXT signature verification now use
+ constant-time comparison (CVE-2026-75589).

// Source: https://github.com/vurtdev/Net-OAuth/commit/a1a16b58add85668ef4fcda642a486ceed098eba.patch

text

// MANIFEST diff introducing the shared SignatureMethod base module
 lib/Net/OAuth/Response.pm
+lib/Net/OAuth/SignatureMethod.pm
 lib/Net/OAuth/SignatureMethod/HMAC_SHA1.pm
 lib/Net/OAuth/SignatureMethod/HMAC_SHA256.pm
 lib/Net/OAuth/SignatureMethod/PLAINTEXT.pm

// Source: https://github.com/vurtdev/Net-OAuth/commit/a1a16b58add85668ef4fcda642a486ceed098eba.patch

Detection Methods for CVE-2026-75589

Indicators of Compromise

High volumes of OAuth verification requests from a single client using near-identical messages that differ only in the signature field
Repeated requests targeting the same consumer_key or token value with incrementally varying signature bytes
Client behavior that pauses briefly between requests, consistent with timing measurement loops

Detection Strategies

Inventory Perl applications and CPAN dependencies to identify installations of Net::OAuth below version 0.33
Enable access logging on OAuth verification endpoints and alert on abnormal request rates from individual IP addresses or API clients
Correlate authentication failures against the same consumer_key or oauth_token to identify signature brute-force patterns

Monitoring Recommendations

Track OAuth failure ratios per client identifier and rate-limit clients exceeding a defined threshold
Ingest web server and application logs into a centralized analytics platform for retrospective hunting on signature-verification anomalies
Alert on any use of the PLAINTEXT signature method, which should generally be disabled unless required by legacy integrations

How to Mitigate CVE-2026-75589

Immediate Actions Required

Upgrade Net::OAuth to version 0.33 or later from CPAN
Rotate any consumer_secret and token_secret values that may have been exposed through vulnerable PLAINTEXT verifiers
Disable PLAINTEXT signature verification if it is not strictly required
Restrict OAuth verification endpoints to authenticated or rate-limited clients where feasible

Patch Information

The fix is available in Net::OAuth 0.33, which introduces a shared Net::OAuth::SignatureMethod base module implementing constant-time comparison for HMAC_SHA1, HMAC_SHA256, and PLAINTEXT verification. See the GitHub Security Advisory GHSA-g8xr-69p3-gw56, the patch commit, and the MetaCPAN release notes for details.

Workarounds

If upgrading is not immediately possible, apply the upstream patch that replaces eq with a constant-time comparison routine in the affected SignatureMethod modules
Rate-limit repeated OAuth verification attempts from the same source to make timing measurements infeasible
Add jitter to error responses on OAuth verification failures to reduce the fidelity of any remaining timing channel
Disable the PLAINTEXT signature method in application configuration to eliminate the highest-impact recovery scenario

bash

# Upgrade Net::OAuth to the patched release
cpanm Net::OAuth@0.33

# Verify installed version
perl -MNet::OAuth -E 'say $Net::OAuth::VERSION'