CVE-2026-93990: libexpat Improper Handling of Unicode Encoding
Vulnerability summary
CVE-2026-93990 is a high (CVSS 8.7) Improper Handling of Unicode Encoding vulnerability in libexpat. EPSS estimates a 0.40% chance of exploitation in the next 30 days, higher than 33.1% of scored vulnerabilities.
Advisory lineage
Upstream 0 · Downstream 7 · Related 4Timeline
- 19-09-2026PublishedVulnerability first disclosed.
- 28-09-2026Last modifiedVulnerability information updated.
Description
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.
CVSS metrics
9 scores · 6 sources · 2 versions- OSV · AlpineEcosystem advisory · calculated8.7HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
- OSV · UbuntuEcosystem advisory · calculated8.7HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N
- OSV · DebianEcosystem advisory · calculated8.7HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
- NVDNIST8.7HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
- OSV · Red hatEcosystem advisory · calculated7.5HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- OSV · UbuntuEcosystem advisory · calculated7.5HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- NVDNIST7.5HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
EPSS trend
Probability
0.40%
Chance of exploitation in the next 30 days
Percentile
33%
Versus all other CVEs
30-day change
Change unavailable
Change unavailable
Known exploited
Compare KEV catalogsNot in any of the 3 tracked catalogs.
Public exploits
0 foundNo public exploits indexed.
Weaknesses
CWE · root causeAffected systems
Ubuntu16 products
- ayttmAll versions (No fix recorded)
- cableswigAll versions (No fix recorded)
- cadaverAll versions (No fix recorded)
- coin3All versions (No fix recorded)
- expatAll versions (No fix recorded)
Redhat3 products
- expat< 0:2.8.4-0.2.hum1
- expat-devel< 0:2.8.4-0.2.hum1
- expat-static< 0:2.8.4-0.2.hum1
Alpine1 product
- expat< 2.8.5-r0
Debian1 product
- expatAll versions (No fix recorded) | < 2.5.0-1+deb12u4 | < 2.9.0-1~deb13u1 | < 2.8.4-2
Libexpat1 product
- libexpat< 2.8.5
5 vendors · 22 products
References
17 links- github.com/libexpat/libexpat/releases/tag/R_2_8_5Release Notes
- blog.hartwork.org/posts/expat-2-8-5-released/Vendor Advisory
- www.vulncheck.com/advisories/expat-through-2.8.4-malformed-utf-16-acceptance-via-unchecked-surrogateThird Party AdvisoryARTICLEREPORT
Show all 17 references
- images.redhat.comARTICLE
