CVE-2026-9499: Out-of-bounds read in QTextCodec::codecForName() in Qt

Severity: medium (CVSS 6.3 v4.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-07-21 · updated 2026-07-23
Weakness: CWE-125

Description

An out-of-bounds read (buffer over-read) vulnerability exists in QTextCodec::codecForName() in Qt. When the function is called with a QByteArray that is not NUL-terminated (for example, one created with QByteArray::fromRawData()), the codec-name matching routine reads past the end of the supplied buffer. In most cases this results in an incorrect text codec being selected; in the worst case, if the over-read reaches unmapped memory, the process crashes (denial of service). The over-read is bounded by the length of the longest codec-name candidate, and the out-of-bounds bytes are only compared internally against Qt's fixed list of codec names, so no data is disclosed to an attacker. Applications that do not pass non-NUL-terminated QByteArrays to QTextCodec::codecForName() are not exposed. The affected code resides in the Qt5Compat module from Qt 6.0.0 onward, and in Qt Core (qtbase) in Qt 4.x and Qt 5.x.

Affected products

Qt: from 6.0.0 before 6.8.8; from 6.9.0 before 6.11.1; from 4.0.0 before 6.0.0 — fixed in 6.0.0, 6.11.1, 6.8.8

How to fix

Upgrade to a fixed version: Qt 6.0.0, 6.11.1, 6.8.8. Check the vendor advisory for mitigations.

References

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