CVE-2026-8829: HTML::Entities Use-After-Free Information Disclosure

Technical summary

The vulnerability stems from unsafe pointer caching in the XS (C) implementation of _decode_entities. The routine stores a pointer (repl) into a SV (scalar value) retrieved via hv_fetch from the entity2char hash. When the input SV equals a value SV in that hash and contains a recursive entity reference, a subsequent call to grow_gap() reallocates the SV's PV (pointer value) buffer, invalidating the cached repl pointer. The copy loop then reads repl_len bytes from the freed allocation, violating memory safety. CWE-416 (use-after-free) is the root cause. The exposure is read-only; no memory corruption or code execution is possible.

Business impact

This vulnerability poses a confidentiality risk in deployments processing untrusted HTML or entity data. If an attacker can control entity input to an application using HTML::Entities, they may craft payloads that leak adjacent heap memory—potentially containing sensitive data such as cryptographic keys, authentication tokens, or other process memory contents. The impact depends on application context: web services that decode user-supplied HTML are at moderate risk, while server-side templating engines or email processing systems handling adversarial input face higher exposure. No availability or integrity impact exists.

Affected systems

HTML::Entities versions prior to 3.84 are affected. The distribution is maintained as oalders/html-entities. Perl applications and frameworks (including Catalyst, Mojolicious, and other web systems) that depend on HTML::Entities for entity decoding are in scope. Systems running Perl 5.x with this library installed should be evaluated. Verify exact version deployed via `perl -e 'use HTML::Entities; print $HTML::Entities::VERSION'` or check META.json/Makefile.PL in the installed module directory.

Exploitability

Exploitation requires the ability to supply or influence HTML entity input to the application—a network-reachable attack surface with no authentication or special privileges required. Crafting a reliable exploit is non-trivial; the attacker must engineer an input payload that triggers the exact condition (self-referential entity in a value that matches the input) to reliably read freed memory. Once triggered, the leaked data lands in the output or is available to subsequent processing, making extraction possible but not guaranteed. The CVSS:3.1 score of 7.5 (HIGH) reflects the ease of network access and lack of user interaction, tempered by the low practical likelihood of successful, repeatable exploitation and the confidentiality-only impact.

Remediation

Upgrade HTML::Entities to version 3.84 or later. For Perl module management via CPAN or a local mirror, update using `cpan HTML::Entities` or your preferred package manager (apt, yum, homebrew, etc., depending on how Perl modules are provisioned). Verify the upgrade with the version check command noted above. No interim workarounds exist; patching is the only reliable mitigation. Review application logs to identify any suspicious patterns in entity decoding inputs if you suspect prior exploitation.

Patch guidance

HTML::Entities 3.84 and later contain the fix. This is a maintenance release in the stable line. Deployment is low-risk; the patch corrects internal memory management without API changes. Test in a staging environment to confirm no dependency conflicts, then roll out to production. For embedded Perl environments (e.g., mod_perl, embedded interpreters in applications), ensure the updated module is installed in the correct @INC path. Verify patching with: `perl -e 'use HTML::Entities; die q(outdated) if $HTML::Entities::VERSION lt "3.84"'`.

Detection guidance

Monitor for attempts to decode malformed or excessively recursive HTML entities in application logs, especially user-controlled input. Network-based detection is limited; look for HTTP requests with unusual entity sequences (e.g., nested self-referential entities or patterns like `&EntityName;` appearing recursively). Endpoint detection can monitor for unexpected heap memory disclosure patterns in process memory forensics, but this requires advanced EDR capabilities. Application-level logging of entity-decode errors or warnings may surface suspicious inputs. SIEM rules should flag any crashes or memory errors in Perl processes handling entity decoding.

Why prioritize this

This CVE merits prompt patching due to the HIGH CVSS score (7.5), easy network exploitability, and potential for information disclosure in any internet-facing service using HTML::Entities. While not on the CISA KEV list, the attack surface is broad across Perl-based web frameworks and templating engines. Organizations processing untrusted HTML (e.g., content platforms, email systems, web crawlers) should prioritize this upgrade. Patch urgency is moderate-to-high; plan for deployment within 2–4 weeks, sooner if the application is public-facing or processes sensitive user data.

Risk score, explained

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N = 7.5 (HIGH). Network access (AV:N), no authentication or special conditions (PR:N), no user interaction required (UI:N), and a high confidentiality impact (C:H) justify the score. Attack complexity is low (AC:L) because triggering the flaw requires only crafted input, not exploitation of a race condition or OS-specific behavior. The absence of integrity and availability impact prevents a CRITICAL rating. The vulnerability is serious enough to warrant immediate action but falls short of emergency-level threat status.

Frequently asked questions