Expat before 2.8.5 fails to validate that a high surrogate in UTF-16 input is followed by a low surrogate, allowing malformed UTF-16 sequences to be accepted. Attackers can supply UTF-16 encoded XML containing lone high surrogates that consume the following code unit, causing Expat to pass unpaired surrogates to applications built with XML_UNICODE and to silently replace input characters in other builds.
Weakness
The product does not properly handle when an input contains Unicode encoding.
Affected Software
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Red Hat Enterprise Linux 10 | RedHat | expat-0:2.7.3-1.el10_2.5 | * |
| Red Hat Enterprise Linux 8 | RedHat | expat-0:2.5.0-4.el8_10 | * |
| Red Hat Enterprise Linux 9 | RedHat | expat-0:2.5.0-6.el9_8.5 | * |
| Red Hat Enterprise Linux 9 | RedHat | expat-0:2.5.0-6.el9_8.5 | * |
| Red Hat Cost Management On-Premise 1 | RedHat | costmanagement/costmanagement-ui-rhel10:1791460851 | * |
| Red Hat Hardened Images | RedHat | expat-main-2.8.4-0.2.hum1 | * |
| Red Hat Update Infrastructure 5 | RedHat | rhui5/cds-kubernetes-rhel9:1791978534 | * |
| Red Hat Update Infrastructure 5 | RedHat | rhui5/cds-rhel9:1791978528 | * |
| Red Hat Update Infrastructure 5 | RedHat | rhui5/haproxy-rhel9:1791978528 | * |
| Red Hat Update Infrastructure 5 | RedHat | rhui5/installer-rhel9:1791964013 | * |
| Red Hat Update Infrastructure 5 | RedHat | rhui5/rhua-rhel9:1791978693 | * |
| Matanza | Ubuntu | devel | * |
| Matanza | Ubuntu | esm-apps/focal | * |
| Matanza | Ubuntu | esm-apps/jammy | * |
| Matanza | Ubuntu | esm-apps/noble | * |
| Matanza | Ubuntu | esm-apps/resolute | * |
| Matanza | Ubuntu | jammy | * |
| Matanza | Ubuntu | noble | * |
| Matanza | Ubuntu | resolute | * |
| Matanza | Ubuntu | stonking | * |
Potential Mitigations
- Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
- When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
- Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
Related Attack Patterns
References
- https://blog.hartwork.org/posts/expat-2-8-5-released/
- https://github.com/libexpat/libexpat
- https://github.com/libexpat/libexpat/commit/ff6e1d7e750bbe245178f51a47a965dc8342861a
- https://github.com/libexpat/libexpat/pull/1282
- https://github.com/libexpat/libexpat/releases/tag/R_2_8_5
- https://www.vulncheck.com/advisories/expat-through-2.8.4-malformed-utf-16-acceptance-via-unchecked-surrogate