CVE-2021-22898

What it actually does

The option parser for CURLOPT_TELNETOPTIONS mishandles NEW_ENV variable pairs, causing uninitialized stack buffer contents to be transmitted to the TELNET server over clear text.

Why it matters to the business

Sensitive internal process memory or configuration data may leak to an external party during telnet-based operations; the flaw affects long-supported curl releases still present in many distributions and build pipelines.

How an attack unfolds

An attacker supplies a crafted -t/--telnet-option string containing NEW_ENV variables to a curl client configured to connect to a malicious or compromised TELNET endpoint; the uninitialized buffer contents are sent verbatim, exposing whatever data happened to be on the stack at that moment.

How to fix it

Upgrade curl to a version containing commit 39ce47f219b09c380b81f89fe54ac586c8db6bde or later; Debian 11/12/13 users should install c-ares 1.16.1-1 or newer packages that incorporate the corrected libcurl build.

How to confirm the fix worked

Confirm the installed curl binary reports a version newer than 7.76.1 and that the git hash or package changelog references the listed fix commit; re-run any telnet-option test cases with memory sanitizers to verify no uninitialized data is transmitted.

Typical attack path

1. Identify or lure a target into invoking curl with the -t option against a TELNET destination under attacker control
2. Craft a NEW_ENV variable pair that triggers the parser flaw
3. Receive the leaked stack bytes in the clear-text session
4. Inspect the received data for credentials, tokens, or other sensitive material

How to detect it

Search outbound network logs and process execution history for curl invocations that include the -t flag or CURLOPT_TELNETOPTIONS together with TELNET destinations; inspect packet captures on port 23 for unexpected binary or high-entropy payloads immediately after the NEW_ENV exchange.

What your security team should do

• Prioritize patching curl in all CI/CD agents, containers, and developer workstations that still ship versions 7.7-7.76.1
• Audit scripts and automation for use of -t or CURLOPT_TELNETOPTIONS and replace with safer alternatives where possible
• Add curl version checks to software bill-of-materials pipelines given the high ecosystem prevalence of the library